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Old 08-08-2015, 04:27 PM   #1
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1992 Winnebago 454 TBI Fuel Pressure

Hello, on my 1992 454 motorhome, when i turn the key to on position the fuel pressure builds up to about 12 psi, I can hear the pump coming on then after two seconds roughly the pump turns off and the pressure drops back own to zero. I pinched off the fuel return line and it still does it. I pinched off the feed line after it builds up and it still drops after the pump turns off.

My sources say that pressure should stay up even after two hours. Well, the pump is brand new, the regulator is new and so is the fuel pump relay! The car runs fine. So my sources say there must be a leak either outside or inside the tank but if so why when I pinch off the feed line does pressure still drop?

Any ideas anyone? Thanks for your input ahead of time.

Kaweh
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Old 08-08-2015, 05:43 PM   #2
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If the car runs fine then whats the problem. Normal 1990's GM TBI fuel pressure is 12-14 PSI.
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Old 08-08-2015, 06:04 PM   #3
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If the car runs fine then whats the problem. Normal 1990's GM TBI fuel pressure is 12-14 PSI.
I believe i was pretty clear on what the problem is. Read it again!
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Old 08-08-2015, 06:43 PM   #4
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Well I don't understand the problem. If it runs good, who cares if the pressure drops off, maybe it is supposed to. There is a return line for any unused fuel.

I can't help you, maybe someone else can.
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Old 08-08-2015, 11:15 PM   #5
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The pump has a one way check built in it so fuel pressure doesn't bleed off. If it's not giving a hard start problem I wouldn't worry about it. If you used a cheap pump the check valve probably isn't sealing off. AC Delco pumps are the only way to go.
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Old 08-09-2015, 12:59 AM   #6
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The pump has a one way check built in it so fuel pressure doesn't bleed off. If it's not giving a hard start problem I wouldn't worry about it. If you used a cheap pump the check valve probably isn't sealing off. AC Delco pumps are the only way to go.


Yes it's a AC Delco. when i climb a hill the exhaust manifolds start to glow within 30 seconds. we cup the tbi shut restricting air flow in and the glow goes away so it's been confirmed that I have
a fuel delivery issue. no exhaust blockage, new catalytic. timing is good, engine has been rebuilt in fact with a new timing chain and new exhaust manifolds. no vacuum leaks and i put in a new
fuel pump, a new pump relay and the fuel pressure regulator has been rebuilt also. I am seriously amazed why this is happening but it cranks over right away good power. Even the computer
has been replaced.
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Old 08-09-2015, 03:33 AM   #7
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If you have a fuel starvation issue than the pcm will set an O2 sensor code for lean conditions. The exhaust manifold glowing is acually normal as under load the exhaust will get to about 2500 deg. you might check your egr system to insure lower combustion temps limiting nox in the exhaust and your manifolds may not get as hot. As far as the fuel pressure dropping off on shut down, don't worry about it unless you get a long crank time on startup. You can change the spring in the fuel pressure regulator in the throttle body to increase fuel pressure. Just look on line for the spring and instructions.
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Old 08-09-2015, 07:15 AM   #8
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So your problem is glowing exhaust manifolds while driving under a heavy load.

That is an indication of lean operation. Since you replaced everything in system and you say there are no vacuum leaks. Use a vacuum gauge and verify a steady vacuum.

Double check your base timing. (Don't forget to disconnect the PCM wire)

Or just retard your timing a few degrees and see if that helps.

Here is a lot of good info on TBI system.

http://www.73-87chevytrucks.com/tech...TBISystems.pdf
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Old 08-09-2015, 01:21 PM   #9
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Hi, I know you have checked your timing and believe it to be accurate.
I am an ase master mechanic, own a p30 and work at a chevy dealer.
I want you to consider the possibility that your harmonic balancer has "slipped"
It sounds like this is a reman engine. Is the harmonic balancer new? The balancers tend to slip or twist (inside at crank vs outside at timing mark) over time. This makes the timing marks inaccurate. Try adding a few degrees of advance to the engine and see if the glowing manifolds go away and you have more power.
If this is not the issue, what is the fuel pressure like when the problem is occuring?
Are there any codes setting in the ecm?
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Old 08-09-2015, 11:41 PM   #10
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The timing is correct. I have tried advancing it to 12 degrees and at 4TDC and it makes no difference. This is the original engine with only about 100k on it. no vacuum leaks. i put a new EGR in it because we found an ever so slight vacuum leak. o2 sensor is grounded and works fine. I hadn't considered the harmonic balancer. this all started because the pigtails to the injectors had been messed with before I owned it and the previous peeps had put electrical tape on it. Well, over the years with heat one wire became exposed and it caused a backfire. A code 42 was out. i fixed the problem and ever since then this is the new problem. This is not normal because the air hose to the hadley air horns ran next to the passenger side exhaust manifold. They melted twice. So that's how I know it's not normal. As i mentioned we cupped the tbi while going up a hill in first gear and glow was not visible at night. so it's starving for fuel under load. but again it all started as a backfire in the tbi (just to be clear not an afterfire in the exhaust). I rebuilt the engine it runs beautifully. as far as the harmonic balancer goes, i haven't opened anything there up and well, how can a backfire affect that? I mean that's totally different. just last night my injectors started to just leak and it smelled like crazy gas. i drive it with the cover off and so I had these other inectors that are 80lb rated for this rv, brand new and it fired right up. So we went on a test drive and the same thing is happening. The manifolds start to glow within i'd say 20 to 30 seconds of climbinb a hill. Giving it more gas only makes the glow more and if I keep it up (which I haven't now), the glow will go past the y pipe and down to who knows where!!!! It's not normal. I hope I cleared up some of the confusion allthough I am totally confused!!! LOL


One more thing: there are no codes. we just hooked up a scantool again and it's clear! no check engine light on either!!!
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Old 08-10-2015, 08:13 AM   #11
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Put your timing light on it and see if it advances with RPMs like it should. By the time you get up to around 4k RPM the timing should be around 30 degrees.

I know you said it was a new catalytic converter but can you take it off to test to see if it might be contributing to the problem.
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Old 08-10-2015, 01:19 PM   #12
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Ok, i will post more later, but, the only thing the slipped Harmonic Balancer will do is twist so the timing marks are not accurate. In other words, you will set the timing with a light to 4btdc, however since the mark on the pulley/balancer is spun around you may be at 15 after TDC. I had to set the base timing on my big block camaro to 33btdc(according to the wrong mark on the pulley) at idle to compensate for the slipped balancer, the actual timing was more like 12btdc. I have seen retarded timing cause the glowing exh. No way that is normal to glow like that. Hot is one thing, welding the manifold to the head is another.

If the cat were the fault, you would have lack of power all the time and you can almost hear the exh backing up with pressure.
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Old 08-10-2015, 11:19 PM   #13
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I wonder if the grounded injector wire could have damaged it. If you pull the air cleaner and look down the throttle body are both injectors spraying the same amount?
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Old 08-11-2015, 09:51 PM   #14
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I wonder if the grounded injector wire could have damaged it. If you pull the air cleaner and look down the throttle body are both injectors spraying the same amount?
Yes these are brand new inectors!
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