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Old 06-21-2021, 11:46 AM   #1
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1995 E350 "Class C" Ford 460 Idle challenges

I've been fighting a "stumble" type idle issue for a couple of years



Here is a little history:



it started as a rough "stumble" when topping a hill going form a load to a "light load" or "no throttle". After some research, I believed the issue was a "slow" EGR, replaced the EGR and the problem seemed to go away.



A few trips later, engine wants to idle VERY slow... won't idle blah blah - this time it appeared to be the IAC - replaced and viola idle is back to "normal" except it occasionally slows down then picks back up. If I'm in drive, the "slow down" will sometimes stall. I have noticed that if the engine is warm and it stalls, it usually has a "tough" start almost like the timing is to far advanced...



I have replaced fuel filter, IAC, EGR, MAP, and just recently replaced the fuel pressure regulator. No codes are present on the EEC KOEO and KOER are clear and "good", yet the idle refuses to smooth out.


I have pulled some run time data while the idle slow down occurs - it looks like it idles around 824 - 780 when things are "good" then drops to around 680 for a couple of samples. Also notice that the O2 drops to ultra lean when the slow down occurs.


I'm at my wits end as to what the heck is wrong here! Any pointers would be so much appreciated!!!

Code:
ENGINE  FUELPW1(IAC(%)  LOOP    MAP("HG)O2SEN   SPARK() ST      TP      TPS(V)  VSS(MPH)
     800     5.7      38Clsd        10.5    0.62      31      -2C/T          0.9       0
     808     5.6      38Clsd        10.5    0.49      24      -3C/T          0.9       0
     764       6      38Clsd        10.5    0.41      28      -1C/T          0.9       0
     764       6      38Clsd        10.5    0.41      28      -1C/T          0.9       0
     824     5.9      39Clsd        10.5    0.34      24      -1C/T          0.9       0
     824     5.5      38Clsd        10.5    0.31      24      -1C/T          0.9       0
     768     6.1      38Clsd        10.5    0.48      24      -1C/T          0.9       0
     824     5.9      39Clsd        10.5    0.28      24      -1C/T          0.9       0
     824     5.9      39Clsd        10.5     0.4      24      -1C/T          0.9       0
     808       6      38Clsd        10.5     0.4      24      -1C/T          0.9       0
     792     5.8      38Clsd        10.5    0.65      26      -2C/T          0.9       0
     792     5.6      38Clsd        10.5    0.63      30      -2C/T          0.9       0
     776     5.6      38Clsd        10.5    0.63      30      -2C/T          0.9       0
     824     5.9      39Clsd        10.5     0.7      24      -2C/T          0.9       0
     780       6      38Clsd        10.5    0.73      30      -3C/T          0.9       0
     780       6      38Clsd        10.5    0.73      30      -3C/T          0.9       0
     820     5.6      39Clsd        10.5    0.73      24      -3C/T          0.9       0
     820       6      39Clsd        10.5    0.73      24      -3C/T          0.9       0
     820       6      39Clsd        10.5    0.73      24      -3C/T          0.9       0
     812     5.2      39Clsd        10.4    0.74      25      -3C/T          0.9       0
     804     5.5      38Clsd        10.5    0.71      28      -4C/T          0.9       0
     804     5.5      39Clsd        10.5     0.6      24      -4C/T          0.9       0
     820     5.5      39Clsd        10.5     0.6      24      -4C/T          0.9       0
     808     5.6      39Clsd        10.5    0.23      24      -3C/T          0.9       0
     796     5.9      38Clsd        10.4    0.21      26      -2C/T          0.9       0
     796     5.9      38Clsd        10.4    0.21      26      -2C/T          0.9       0
     800     5.6      38Clsd        10.5     0.2      26      -2C/T          0.9       0
     772       6      38Clsd        10.5    0.19      26      -2C/T          0.9       0
     772       6      38Clsd        10.5    0.19      26      -2C/T          0.9       0
     768     6.3      39Clsd        10.6    0.15      30      -1C/T          0.9       0
     816       6      39Clsd        10.5    0.12      24      -1C/T          0.9       0
     816       6      39Clsd        10.4    0.12      24      -1C/T          0.9       0
     816     5.9      39Clsd        10.4    0.16      24      -1C/T          0.9       0
     816     6.1      39Clsd        10.3    0.38      24      -1C/T          0.9       0
     816     5.7      39Clsd        10.3    0.62      24      -2C/T          0.9       0
     820     5.7      39Clsd        10.3    0.62      24      -2C/T          0.9       0
     816       6      39Clsd        10.5    0.68      24      -2C/T          0.9       0
     688       6      39Clsd        10.6    0.68      30      -3C/T          0.9       0
     688       6      39Clsd        10.6    0.68      30      -3C/T          0.9       0
     764     6.2      40Clsd        10.6    0.08      30      -2C/T          0.9       0
     824       6      40Clsd        10.6    0.08      24      -1C/T          0.9       0
     824       6      40Clsd        10.6    0.08      24      -1C/T          0.9       0
     864     5.8      40Clsd        10.3    0.08      24      -1C/T          0.9       0
     864     5.3      40Clsd        10.3    0.09      24      -1C/T          0.9       0
     864     5.3      40Clsd        10.5     0.6      24      11C/T          0.9       0
     824     5.4      40Clsd        10.5     0.6      24      11C/T          0.9       0
     780     6.7      40Open        10.6    0.63      31      11C/T          0.9       0
     684     7.1      40Open        10.7    0.09      30      11C/T          0.9       0
     684     7.1      40Open        10.7    0.09      30      11C/T          0.9       0
     812     7.2      40Open        10.6    0.06      27      11C/T          0.9       0
     864     7.1      40Open        10.4    0.07      24      11C/T          0.9       0
     864     7.1      40Open        10.4    0.07      24      11C/T          0.9       0
     884     6.1      40Open        10.4    0.08      24      11C/T          0.9       0
     860     6.1      40Open        10.5    0.26      24      11C/T          0.9       0
     860     6.1      40Open        10.3    0.26      24      11C/T          0.9       0
     876     5.9      40Open        10.3     0.5      24      11C/T          0.9       0
     896       6      40Open        10.3    0.09      24      11C/T          0.9       0
     896     5.9      39Open        10.3    0.07      24      11C/T          0.9       0
     892     5.9      39Open        10.3    0.07      24      11C/T          0.9       0
     860       6      39Open        10.3    0.06      24      11C/T          0.9       0
     816     6.5      39Open        10.5    0.06      27      11C/T          0.9       0
     816     6.5      39Open        10.5    0.06      27      11C/T          0.9       0
     840     6.4      39Open        10.4    0.06      24      11C/T          0.9       0
     852     6.3      39Open        10.3    0.06      24      11C/T          0.9       0
     852     6.3      39Open        10.3    0.06      24      11C/T          0.9       0
     836     6.1      39Open        10.3    0.06      24      11C/T          0.9       0
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Old 06-21-2021, 06:41 PM   #2
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Long list of things to check, but without righting a novel I would try to look at two things first.
1 vacuum leak at the intake, possible warped manifold, leak at gasket etc. High heat can make this occur intermittently. Therefore sometimes hard to find and diagnose.
2. Carbon build up (holding valve partially open) or perhaps leaks in the EGR system.
Just some ideas.
Hope you find it.
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Old 06-21-2021, 07:42 PM   #3
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Ahh yes Ford OBD I. Get yourself a code scanner and check and see if there's any stored codes in there. You don't need to see a check engine light to have a code.

With the age of the RV I would start looking at vacuum lines, i bet some have perished and need to be replaced.

Then you could have a bad IAC motor, especially if it was an aftermarket one
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Old 06-21-2021, 08:37 PM   #4
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Your datastream output looks like it comes from a scanner. Which brand is it? Some of these scanners provide flawed data.


Your PCM obviously permits idle in Closed loop control, although this doesn't always happen. Towards the end of your data trace it changes to open loop fuel control. Can you post photos of the data decals on the door jamb, or from the 60 pin data connector on the PCM? That might allow me to discover how the engine is programmed to idle.


Ford's idle speed control system uses a fast response control channel in conjunction with a slow response one. The idea is that if a load is suddenly applied to the engine while idling (like a power steering load or the AC compressor clutches in) then the fast response channel will change applying an almost instant correction( within 1 or 2 engine revolutions), and then over a few seconds, the slow response control will take over, and the fast response correction is backed out, leaving it is reserve for a second load.


The IAC solenoid is the slow response control, and the ignition timing is the fast response control.


Your data trace shows that the ignition timing is being actively driven to stabilize the idle speed, and it also shows that the IAC is really not playing much of a part in this control. If there were no loads varying on the engine, then I would expect most of the control to be done by the timing.



Was the AC enabled or was the power steering moved during this data gathering?


The thing to note is that the ignition timing listed in your data trace is merely what the PCM would like to happen. However the PCM doesn't directly control the ignition timing, it tells the ICM (Ignition Control Module) to set the timing it wants. So you may have an ignition timing problem. The correct module for 460s of this era is the CCD (Computer Controlled Dwell) module, which is colored Black. Unfortunately many of the Ford parts books copied by aftermarket suppliers list the Gray colored module, often referred to as the Push-Start module. The module will be on the fender well, I believe mounted on an aluminum heatsink.


If you can find that, tell us what color it is. You may have the wrong one installed.



Lastly, you say that the mixture at some points goes ultra lean. That may be true, but you cannot tell that from the O2SESsor voltage. It is a Narrowband O2 sensor, and it can only say if the mixture is rich or if it is lean. No information or richness or leanness is given.



That's the first chapter of my novel, and there are more, but that's probably enough to be getting on with.
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Old 06-22-2021, 05:53 AM   #5
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mpaton: Thanks for the massive feedback! First off, I forgot to mention I did replace the rotor, plugs, and wires, fuel filter, as well as hunted for vacuum leaks. While I have found a few small leaks and replaced several hoses that were "questionable" (basically fall apart when touched), none of these repairs changed anything.


Code scanner is an Actron CP9690. I spent some time looking for a reasonably priced device that would give me live data similar to OBD II - mostly because I was out of ideas and really need to see what the computer sees.


Since my other work (EGR, IAC, and MAP all more than 1 year ago), the only stored code I had was a DTC 334 EGR closed valve voltage high - which turned out to be an out of spec EGR sensor (EVP) - tested 50ohms where it was supposed to be > 100ohms. Replaced that and now no codes stored or otherwise.



The data was collected with the motor just idling, no AC and not messing with the power steering.


I have attached a couple of pictures - one of the "BLACK" and I believe original ICM and one of the calibration tag in the door... I was a little surprised to not see the typical tags in the door. I also shot a picture of the vehicle data under the window in the bunk. I have owned this rig for 14 years, and it ran GREAT for 11 of them, so if there are "wrong" parts in there I would think it was something I did, and I haven't replaced the ICM or the EEC


Thanks for the O2 feedback. I had replaced the O2 sensor, interestingly I put the old one back in for some testing. The code reader seems to show the old O2 sensor working but much slower (aka staying at .89v much longer, only dropping to .08v for short periods). The newer O2 sensor seems to respond faster and give more intermediate voltages. I'm guessing the old one is still "working" just more slowly?!? Either way, motor idle issues remain with either sensor.



I have been suspicious of spark/timing - as it almost seems like it maybe PIP missed #1 cylinder for a rev or maybe missed a pickup... so the ICM and PIP may be involved here?



Additionally, yesterday I finished replacing the Fuel pressure regulator - I have tested the fuel rail and pressure stayed at 35psi - vacuum or no vacuum, so I was hoping it was the FPR - but nothing changed. I'm a little suspect of the fuel pressure gauge as it appears to change pressure just by squeezing the pipe. I have a second gauge to try just to confirm pressure is 35PSI. I was thinking 35 is high for idle and low for WOT, maybe it just can't keep fuel mixture right with that fuel pressure? BUT I don't have power issues driving down the road, just the idle challenge.


Maybe the next move is a fuel pump (after confirming pressure)?
How would I verify PIP and ICM function? I did disconnect the SPOUT jumper - the engine runs differently, but still has the cyclic idle hesitation (aka the computer can't advance the timing, yet it still drops RPM).


Thanks for your help and ideas - it's getting frustrating!
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Old 06-22-2021, 05:25 PM   #6
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Maybe this video / audio will help generate some ideas... it almost sounds like it's backfiring out the intake (aka too lean?)


https://youtu.be/h9mZB3Ym704
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Old 06-22-2021, 06:32 PM   #7
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Have you tried the (maybe overly simplistic) old standby of spraying short blasts of carb cleaner or a weak stream of propane all over the top end of the running engine to see if the idle speed varies? I ask because it does sound just like my 460 did when a vacuum line cracked (and so could be any intake-passage leak on top.)
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Old 06-22-2021, 08:35 PM   #8
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The 4th photo you posted in post #5 shows a decal with the calibration on it. It used to be that Ford would let you look up details of the calibration on Motorcraft.com, but it seems that now that isn't free. If you run across the PCM, it will have a gray socket on one end where the large connector goes into, and on one side of that there should be 4 characters in a font 3 times bigger than all the other characters on the socket. IF you find them, post them, as it's possible I MAY have copy of the binary code from a long time ago, and I could find out some useful data. But not a big deal.


Your data stream shows that the PCM believes it is changing the timing, from the 22-24 deg BTDC at high rpm to more like 30-32 deg BTDC at low rpm. Are you able to put a timing light on the engine, and see that the timing actually is changing? On my F-53 I could never get to see the timing marks with the engine running; the fan was a bit too close, and the air pump was in the way. So I removed the cover plate at the bottom of the bellhousing intended for accessing the torque converter nuts, and transferred the timing marks to there. I'd imagine the van is also difficult.


I see that you changed plug wires. I did that once, and it ran worse, so I put the old ones back. Are you able to let the engine idle when it's dark? If the plug wires are jumping across wires, that would be very visible at night.


I know you changed the EGR position sensor, but I'm wondering if you ever took the valve apart and cleaned it. I gather it's possible for the valve seat to get dirty and hold the valve open, so that EGR happens when it isn't supposed to, like at idle.


Most of the pressure gauges used to measure fuel pressure, aren't very accurate, but are reasonably consistent. As you say, 35 is a bit high for idle pressure, and is definitely too low for heavy load. The pressure is supposed to be 39psi above whatever the pressure is in the intake. I'm not sure I believe what the Actron is telling you for manifold pressure (and I don't believe mine either) but atmospheric pressure is 29.92" of mercury or 14.7 psi. If it is idling at 10.5" absolute, then that is

14.7 * 10.5/29.92 or 5.15psi absolute or about -9.5psi gauge.


That would mean that idle fuel pressure should be 39 - 9.5 or about 30psi, and of course 39 under full load.
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Old 06-23-2021, 07:47 AM   #9
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mpaton:


OK - after removing the air filter I could see the connector: TOM0(?)


EGR - I replaced the valve 2 years ago for the original "stutter" cause by what I believed to be a slow EGR - I may pull it to be sure it's clean, but the original 20 year old one was VERY clean, don't really expect the new one to plug up in 2 years.. but ya never know.


Timing light: I tried to pick up the marks with a timing light a couple of days ago, couldn't see the marks - reading online, I think I need to clean the hub and then mark the 5 or 10 degree marks with a white paint pen to see them - It was VERY difficult to point the light at the "pointer", but I did get it in there with the air filter removed, just couldn't see the marks.


As far as the pressure - Thanks for the full explanation. I wasn't thinking about the absolute vs relative pressure. What has me worried is the "sitting over night/no vacuum", turn key on, jumper pump test - where I believe the pressure should go up to ~40 or 42psi - I'm only getting to 35 (although the gauge could be off). Then running, it doesn't really move around like I would expect (idle vs WOT etc)- almost like there is a one way valve in line with the gauge. If I release the pressure at the gauge it drops and fuel comes out of the test line... but not much. I also tried pumping fuel into a cup from the test line while pump jumped and the flow was super low - maybe the shrader valve restricts flow? I guess you don't need much flow to check pressure...


Bobby F:
Thanks for the "easy" test... I will give that a try later today. I think a vacuum tube leak would be the BEST/easiest scenario if I can find it.. that's the big IF
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Old 06-23-2021, 02:07 PM   #10
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A little more observations from this afternoons testing:


- No engine speed change with propane running over the manifold (around the injector area & down the transmission hoses) - was really hoping this would point to an easy fix.



- While idling in closed loop, the Short term fuel % (ST) starts around 20% (and is rich according to the O2), and slowly works down to a modulating -2 to +2 and the O2 moves around as expected. As soon as the engine stumbles, the ST jumps back to 20% & rich, then starts working its way back down. After some period, the EEC decides to go open loop, sets the ST% to 11% and it mostly stays there - occasionally 12% - and the O2 reports LEAN the entire time. Engine stumbles in open loop has not change to the ST% and O2 continues to report lean (.08v).


What's weird in my mind is that the ST% was -2 to +2 in closed loop and O2 modulated rich/lean... but when it went open loop @11% fixed, it was lean 100% of the time... maybe the ST% is different in the different modes?
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Old 06-23-2021, 03:03 PM   #11
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Quote:
Originally Posted by mkmer View Post
mpaton:


OK - after removing the air filter I could see the connector: TOM0(?)

TOM0 it is. Thanks for finding that. Unfortunately, I don't have that in my coillection, so won't be able to discover any E-350 specific things.


Quote:
Originally Posted by mkmer View Post
Timing light: I tried to pick up the marks with a timing light a couple of days ago, couldn't see the marks - reading online, I think I need to clean the hub and then mark the 5 or 10 degree marks with a white paint pen to see them - It was VERY difficult to point the light at the "pointer", but I did get it in there with the air filter removed, just couldn't see the marks.

Remember to make marks corresponding to the data coming out of the scanner at idle, approximately the 20 to 32 BTDC range, as well at the 10 you need to set the timing.



Quote:
Originally Posted by mkmer View Post
As far as the pressure - Thanks for the full explanation. I wasn't thinking about the absolute vs relative pressure. What has me worried is the "sitting over night/no vacuum", turn key on, jumper pump test - where I believe the pressure should go up to ~40 or 42psi - I'm only getting to 35 (although the gauge could be off). Then running, it doesn't really move around like I would expect (idle vs WOT etc)- almost like there is a one way valve in line with the gauge. If I release the pressure at the gauge it drops and fuel comes out of the test line... but not much. I also tried pumping fuel into a cup from the test line while pump jumped and the flow was super low - maybe the shrader valve restricts flow? I guess you don't need much flow to check pressure...

It does sound a bit like you may have an extra schrader valve in there. Sometimes the pump hoses have an extra one, but there is supposed to be a small clip to hold it open,, but that clip is the first thing to disappear when the hose comes apart. At least that's how it's been for my 2 Harbor Freight gauge sets.


Remember that the pressure will be at zero gauge after you've connected it, then it should go up to about 40 after you turn the ignition on (atmospheric pressure in the intake) and then go down after it starts. So if you did have a schrader not held open, that might explain what you're seeing.


When you first turn the ignition on, the fuel pump will run for 1 second and then stop. If the fuel rail pressure went to 0 gauge overnight, then that 1 second may not be enough to get the pressure to 40. Once the engine is cranking at 400rpm or more, the pump comes on again. Or you can cycle the key off and on again to get a second of pump several times to see the pressure come up.



Quote:
Originally Posted by mkmer View Post
Bobby F:
Thanks for the "easy" test... I will give that a try later today. I think a vacuum tube leak would be the BEST/easiest scenario if I can find it.. that's the big IF

I looked through some of my 11 year old test runs, and found that my IAC duty cycle at idle was usually in the 45 to 65% range. However I didn't note whether this was in gear or not, or whether the AC was on, both of which would raise it. Either way, this doesn't mean that your data of around 38% is wrong, as there is a huge amount of control law math going on in the PCM when the engine is idling. But it could mean that you are getting more air than I am that is NOT coming through the IAC, which could be an intake leak, or something as simple as someone fiddling with the throttle stop screws, or the canister purge valve sticking open.


It's important to note that even if you do have a manifold leak, this will not cause a lean mixture in the engine. That only happens when the leak bypasses the air metering device, be it a carburettor or a Mass Air sensor, neither of which you have. It's not possible to bypass the MAP sensor that is your air metering device, so any manifold leak air will reflect in the manifold pressure and have matching fuel supplied. I suppose it's possible you could disconnect the MAP sensor, which would then not sense any air at all. However the PCM would then detect that and use the throttle position sensor as a backup, and also set a code, and you don't have any of those.
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Old 06-24-2021, 05:30 AM   #12
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Well....


Fuel Pressure:
I found my neighbors fuel pressure gauge - much better gauge. It's statically at 40 psi, and is dropping to 32psi at idle as it should be. The strange thing is, his was doing the same "high hold" behavior - once it got to 40 it stays there until vented. Luckily, his gauge had a vent. I'm suspicious of the fuel rail valve being pressed in far enough, but it doesn't make much sense that it acts like a one way valve.


EVAP valve:
After reviewing the scanner data, I notice the EVAP was ON at idle... seemed strange, so I checked out the valve. Apparently ON = closed, OFF = open. There was no vacuum until the scan tool reported OFF... and it only does that intermittently while reving the engine. Probably turns off on the road under load - but either way, the valve is operating (too bad because that would have been another "easy" fix)




As for leaks: You were saying it wouldn't run lean (maybe), but it is running RICH and it appears to keep adjusting the Short term fuel to a "big number" aka richer, every time the motor stumbles. Then, I suppose because it's rich, starts moving toward 0% until another stumble. When it does run long enough without a stumble, ST% floats between -2 and +2 and O2 looks good with a rich/lean toggle...



What's not clear to me is WHY in open loop, ST @ a fixed 11% the O2 reports LEAN (.08v) and doesn't move, while in closed loop 0% is essentially perfect....


All that said, much mussing here for a guy that hasn't worked with the older Fords:



I think I'm down to a spark/timing issue? One thought I had was the PIP sensor is missing the #1 mark every now an then? Maybe it's a timing mark which keeps spark (maybe a normal mark is shorter causing EEC to be back on #1 early), but causes the SFI to be out of wack? OR maybe it just misses a mark all together, again tossing the SFI out of wack for a bit? Otherwise, distributor basically forces the right spark plug to fire, but not necessarily the correct injector... Seems like this behavior would be hard to identify under load/open throttle with much more air flow "covering up" the occasional incorrect injector firing and "re-vaporizing" the fuel?



Is there a good place to connect a scope to the PIP? Is the SPOUT a good spot?

I'll also try again to find the timing marks an verify...


vacation is coming and fixit time is running out... I suppose it runs "good enough" for now.
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Old 06-24-2021, 06:40 AM   #13
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Well....

As for leaks: You were saying it wouldn't run lean (maybe), but it is running RICH and it appears to keep adjusting the Short term fuel to a "big number" aka richer, every time the motor stumbles. Then, I suppose because it's rich, starts moving toward 0% until another stumble. When it does run long enough without a stumble, ST% floats between -2 and +2 and O2 looks good with a rich/lean toggle...

What's not clear to me is WHY in open loop, ST @ a fixed 11% the O2 reports LEAN (.08v) and doesn't move, while in closed loop 0% is essentially perfect....
I agree it looks like that from your data, and I had wondered that also. However I seem to have lost confidence in the ability of the Actron scanner people to get it right, and one of the things to get right is to make sure that all the information they show on one line is actually true at the same moment. They may well have collected fast changing data (like spark advance) at say 10 times a second, and slow changing data ( like evap control) only every 1 second, and just copied the same data cross multiple lines. We don't even know how fast they sample the data.


So I just distrusted that anomaly. I know that isn't much help to you right now.


I haven't worker out how to make this a good test yet, but you could clear out the fuel trims by disconnecting the battery for a while (opinions seem to vary from 10 seconds to 20 minutes). I think it would take a minute or two for them to regenerate themselves.



Quote:
Originally Posted by mkmer View Post
I think I'm down to a spark/timing issue? One thought I had was the PIP sensor is missing the #1 mark every now an then? Maybe it's a timing mark which keeps spark (maybe a normal mark is shorter causing EEC to be back on #1 early), but causes the SFI to be out of wack? OR maybe it just misses a mark all together, again tossing the SFI out of wack for a bit? Otherwise, distributor basically forces the right spark plug to fire, but not necessarily the correct injector... Seems like this behavior would be hard to identify under load/open throttle with much more air flow "covering up" the occasional incorrect injector firing and "re-vaporizing" the fuel?

I'm not sure how reliable (or not) these are. The PIP sensor is actually inside the distributor, and it's like a cylinder with 7 wide teeth and one narrow one to mark #1.
So I guess you could take a look and see.


I don't know that this affects your issue, but you don't actually have SFI (Sequential FI), you have MPFI (Multi Port FI). So you do have 8 injectors, one for each cylinder, but the PCM fires one batch of 4 injectors at one time, and then 360 crank degrees later, the other 4, all at one time. This saves on parts cost, and wasn't seen as that important on a truck. The idea is for the injector to always fire against a closed intake valve, and the injectors in one batch have their spark plugs fire in sequence, so that up to a certain combination of RPM and pulse width, all 4 injectors actually will fire against a closed valve. Above that, some of the valves will be open, and mixture distribution isn't as good as with SFI.


It also has the side effect that if somebody removes the distributor and puts it back a tooth or 2 off, and then "fixes" that by moving the plug wires around the distributor cap, then the engine appears to run fine, but the injector batching system is now messed up, and at low power the injectors won't all be injecting against a closed valve. I wouldn't have thought that his could contribute to your problem, but as the cap is marked for the #1 plug lead, it's easy enough to check that the #1 lead actually goes there.




Quote:
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Is there a good place to connect a scope to the PIP? Is the SPOUT a good spot?
I'm afraid I don't know the answer to that one.

Quote:
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I'll also try again to find the timing marks an verify.


The PCM varying timing to control idle is a pretty powerful control, and I'm surprised that it looks like it isn't having much effect. So that's why I'm suggesting you check and see if you can see the timing vary at idle.[/QUOTE]
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Old 06-24-2021, 12:40 PM   #14
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Does the 1995 460 have vacuum-driven distributor advance?
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