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Old 04-21-2016, 08:45 PM   #1
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What will OBD1 tell you?

I have a 1994 Itasca motor home with a ford 460 That is giving me fits. One of the biggest problems is I don't know what I'm doing but I have been keeping old engines running for a lot of years but to me OBD is something new, that should tell you how old I am. Any way my engine is not giving any codes I have had this MH for 12 years and when it developed a small miss I figured I'd change the plugs wires and cap/rotor fuel filter. (filter was clean) When I started it up it was the same as before I did it. I took it to the parts store for a reading and they say there is no code to read. No check engine light is on either. One man tells me to get a new computer. Went to a shop they too get no readings and Don't know what it is. Anyone with any ideas? What will the OBD1 tell you anyway? by the way fuel rail pressure was 40 on and 30 running and all the spark plugs looked the same when I took them out and vacuum was steady and 20 in. tail pipe seems to suck paper on the stumble and is erratic. G0ing down the road it pulls good and has power. seems to stumble the most at a constant spead.
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Old 04-21-2016, 08:56 PM   #2
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Welcome to iRV2.

Unfortunately OBD I was the " RED Headed Step Child " of useful info to diagnose any engine problem . You never know what it will tell you and if there is any truth to the info.
A good scope would tell you more .
I hate to ask ; have you run a compression test.
OBD II , will report/isolate a misfire , not OBD I; only about 2 dozen codes can be reported ; if memory serves.

BTW: Started at Chrysler when electronic ignition was starting to replace points and condensers.
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Old 04-21-2016, 09:04 PM   #3
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Lots and lots of things can cause a misfire. After covering the basics of a tune up, you will have to dig a bit further. If your misfire is at Idle/low speed, check for intake manifold gasket leaks.
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Old 04-21-2016, 09:26 PM   #4
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I don't know anything about the codes, but when the paper sucks back into the tailpipe, you have a complete miss on one cylinder. If it's not a plug, or wire, or the ignition, it's probably a stuck valve. Possibly an intake gasket as Blupill suggested. A head gasket would probably give a distinct sound, so that's not likely.


Sorry I can't help.


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Old 04-21-2016, 10:13 PM   #5
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I didn't do a compression at the time I had the plugs out because it didn't dawn on me I'd needed one. (to bad I didn't think hard enough) Bad compression should show up on Vacuum test too shouldn't it? same way with a vac leak I would think. The miss seems so sporadic and all the plugs look identical after 50000 miles. I doubt it always the same cyl. Although I haven't had the miss more than a few hundred miles.

She was running so good too. I appreciate you all jumping in and helping me think through this. Why doesn't check engine light come on? Is all that common? no codes and no light. Do I need a new computer? seems like it must be electrical. could this sound like sloppy timing chain? If it was, would there be any noticeable noise from the chain?
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Old 04-21-2016, 11:13 PM   #6
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CEL on OBD I system, would only come as a result , of a complete sensor failure, erratic vacuum would not trigger the light, a scratchy TPS wouldn't either.
I'll look for a list of possible OBD I codes , to refresh my memory.
There is a possibility given the erratic nature of the problem that oil residue build up on the back of an intake valve ; from a bad valve seal; could be the culprit.
Was the engine using any oil ?
Timing chain gears ; factory ; would have nylon coating on the cam gear , to control any noise.
Back to basics , vacuum testing , look for a flutter in the reading.
Compression test , more than 10% spread in readings
Like I said in my other post , a scope test with a GOOD tech at the controls could yield the cause if you don't get results yourself.
BTW: When I say good tech at the controls , I mean it , just before I retired , I was handed a work order for a valve job that was diagnosed at another shop with an automatic scope program. Truck was still under warranty, so it was brought back to the dealership.
Because the operator had set the scope to run the test and then walked away, without first examining the display ; and noticing 3 bad plug wires on screen; the auto function spit out a bad valves diagnosis , during the dynamic compression test.
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Old 04-22-2016, 12:33 AM   #7
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no oil usage. no blue or black smoke out the tail. anyone agree it needs a new computer? Thanks again
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Old 04-22-2016, 12:46 AM   #8
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Post your problem in the Ford Motor home Chassis Forum. Maybe Subford will jump in, he's an expert on OBD I.

http://www.irv2.com/forums/f23/
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Old 04-22-2016, 04:34 AM   #9
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I'm just an old shade tree, but this sounds like a valve issue.

My understanding of the computer may be off, but I believe if the check engine light doesn't come on, the reader is not going to find an error code. If the problem is a valve, it is possible that it won't trigger an alarm in the computer, and I have seen a couple of times where a sticking valve has fired off erroneous error codes which led this old shade tree chasing his tail.

I'm not sure a new computer would help anything, being that it really sounds mechanical. Sounds to me like you just need a good Ford mechanic.
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Old 04-22-2016, 06:37 AM   #10
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Not to simplify it even more, but did you try changing the plugs? I've had plugs (especially Champion) that look good visually, but have a crack that prevents them from sparking hence causing the miss.

Good luck!
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Old 04-22-2016, 08:44 AM   #11
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No computer needed yet.

Codes are counting light flashes after shorting wires in connector.

Check in the Haynes manual for specific instructions there.

Small miss under what circumstance?

If all plugs look same and good then no help there.

Given age of unit get a spray bottle and fill it with water and find a shady spot.

Start engine and attach vacuum gage.

Start by having engine at idle and gently wiggling every connector you can touch and watch gage and listen while doing so.

Next spray everything with water.

Hoses and manifolds.

They often leak along the valley area, our small block 89 van had a leak between valley part and riser that caused slightly higher idle.

Computer compensated for it and was a pill to diagnose.

Water spray the computer could not compensate and engine stumbled.

Tighten some bolts and all well.
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Old 04-22-2016, 11:22 PM   #12
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it'd fixed

just drove it 80 miles without a miss. All it needed was a Ign. control module. found it the old fashioned way. Seems to run fine I'll drive it a couple of days around here then I need to go to Washington state to pick up some farm equipment and come back. That's about 4500 miles round trip so l hope it's right. I hate when people start one of these and never come back and say what they did or how it was fixed. Kind of leave ya hanging. Thanks for all your pointers.http://www.irv2.com/forums/images/smilies/thumb.gif
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Old 04-23-2016, 12:52 AM   #13
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Thanks for the update. Glad you got it fixed. I also hate it when people don't come back to update their post.
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Old 04-23-2016, 10:54 AM   #14
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Note for you...

Yours may be like ours and have a "pip" plug near the distributor.

If I recall correctly you remove it and adjust timing for 10 degrees advance then insert it.

The computer delays timing as needed.

If you leave it out it will run constant at that but can be used to confirm if needed...may ping a bit.
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