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Discussion Starter #1
My engine is a 1995 Z28 LT1 with EFI. This is lumped to a '76 Jag XJ12C. I am getting 36PSI at the fuel rail. Would this measurement eliminate a fuel delivery issue as the cause of a rough idle and a stumble on acceleration?
Thank you, Phillip
 

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Welcome to the forum.

Correct fuel pressure is around 43 psi @ key on prime. Mid 30's while @ idle. And at least 40 psi during other driving conditions.

A great tool for running tests on your car is an obd 1 scanner with real time sensor data capability. A running sensor scan can give good incite into the pcm & sensor activity of your 95.

95 uses obd 1 computer system, even though it has the 16 pin obd 2 aldl connector.

Since you have a lt1 engine, I recommend buying a $70 pc based oscilloscope. A scope is the only 100% way of testing the low & high resolution signals that optispark sends out to pcm. And bad optical section of opti causes running problems in many lt1 equipped cars.
 

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better if you can monitor your FP while doing a WOT run to see if it holds. 36 is on the low side. It should be 43.5 without vacuum to the FPR and 5-8 PSI less with the vacuum line connected

During a WOT pull to whatever your redline is the FP should hold steady, not slowly drop. If FP drops...you need a new fuel pump or wiring is compromised to it.

low FP will cause stumble on acceleration. Not the only cause but rule out fuel pressure before moving on to ignition
 

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Discussion Starter #4
OP

better if you can monitor your FP while doing a WOT run to see if it holds. 36 is on the low side. It should be 43.5 without vacuum to the FPR and 5-8 PSI less with the vacuum line connected

During a WOT pull to whatever your redline is the FP should hold steady, not slowly drop. If FP drops...you need a new fuel pump or wiring is compromised to it.

low FP will cause stumble on acceleration. Not the only cause but rule out fuel pressure before moving on to ignition
Thank you for the response. If I do need a new fuel pump, what would be a good flow rate? Perhaps you could recommend a specific brand/part#
Thanks again,
Phillip
 

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Discussion Starter #5
OP

better if you can monitor your FP while doing a WOT run to see if it holds. 36 is on the low side. It should be 43.5 without vacuum to the FPR and 5-8 PSI less with the vacuum line connected

During a WOT pull to whatever your redline is the FP should hold steady, not slowly drop. If FP drops...you need a new fuel pump or wiring is compromised to it.

low FP will cause stumble on acceleration. Not the only cause but rule out fuel pressure before moving on to ignition
Tested today:
WOT pressure was holding steady at 38PSI.
 

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Your FP looks to be holding. BTW the FPR is what keeps the pressure right with a good FP

Your stumble may be ignition related. Either Opti, coil or ICM. Unless any codes are being thrown, very hard to diagnose a Opti without a Oscilloscope. If Opti is not a AC Delco or Delphi that the off brand ones are highly suspect. Failing ICM can cause stumble but typically the are a no start or once car warms up engine shuts off and won't re-start...although I have had them just make the motor barely run but will start.

If your issue starts once car is warmed up....replace ICM

Do you have EGR on?

If so reach around back of it with engine running and push in the diaphragm. The engine should stumble, maybe stall. Does it?
 

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Discussion Starter #7
Your FP looks to be holding. BTW the FPR is what keeps the pressure right with a good FP

Your stumble may be ignition related. Either Opti, coil or ICM. Unless any codes are being thrown, very hard to diagnose a Opti without a Oscilloscope. If Opti is not a AC Delco or Delphi that the off brand ones are highly suspect. Failing ICM can cause stumble but typically the are a no start or once car warms up engine shuts off and won't re-start...although I have had them just make the motor barely run but will start.

If your issue starts once car is warmed up....replace ICM

Do you have EGR on?
/QUOTE]
If so reach around back of it with engine running and push in the diaphragm. The engine should stumble, maybe stall. Does it?


Thank you for your responses. My plan is to purchase an Oscilloscope and learn how to use it. I have never worked on a car that is run by a computer. My last car was a 1939 ford flathead so I have about 60 years of automotive technology to catch up on.
The issue with the stumbling begins almost immediately so perhaps the cause is not a bad ICM.


I have tried to check all the vacuum lines and hoses and I have not noticed any leaks.


I do not know what EGR is. I can not locate that in my Haynes repair manual. Could you please explain? I will then perform the test that you have suggested.


I have a notion to just purchase new ones of the coil and distributor and replace what's on the engine now. My hunch is that would solve the problem. However, I would feel much more satisfied if I can solve this with a proper diagnosis procedure including the use of the Oscilloscope.


Despite not being able to fix the problem yet, I am enjoying the diagnosis process so far. and I am learning a great deal about my LT1 engine.
Thanks again,
Phillip
 

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Discussion Starter #9 (Edited)
You might want to go to the following site and download the 95 service manual. Way better than Haynes. My Files
Ok, just did, all 4100 pages. Seems like it will be difficult to navigate through all the material. But, I'm up to the challenge. :smile2:Thank you.
I would like to find a hard copy of this.
Phillip
PS. Just found one on line, $119. It's on the way. Money well spent. Knowledge is power!!!!!
 

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EGR=Exhaust Gas Recirculation. It's bolted to the back of the intake manifold, drivers side. It has openings in the back where you can push your finger through to push the diaphragm. The car should immediately stumble, possibly stall, when you do so which means the EGR is working. If the EGR is not there than you would have a block off plate where it goes and the PCM would need to be tuned to eliminate it so a check engine light won't come on.

Given this is a transplant into a Jag I assume you have a Jag dash/instrumentation so not sure how the PCM would signal any CEL lights.

There is a free software program called scan9495 that is a OBD1 compatible program you can monitor your engine, read codes, etc. This may be a very helpful diagnostic tool for you


If you do not have a AC Delco or Delphi opti than there is a higher chance the no name one is defective.

ICM can exhibit issues on cold engine. Some auto part stores have testers for them. The testing can give false "good"


Not saying Opti and or ICM is your problem...but good suspects
 

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If you would go to same site as you downloaded manual from, then scroll down to the next to the last entry. It's "pdfill" the icon is blue. This set of apps will allow you to split the manual up into as many parts as you see fit.


Makes it much easier to navigate the manual. Once you learn how GM numbers each of the car's systems.
 

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Discussion Starter #12
Have not been able to get back on task. Hope to this weekend. Thanks again for all the help. I will do as suggested.
Phillip
 
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