96 burb 7.4 misfire after 5.0 injectors

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mehcs

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Another update:
I loaned a fuel pressure gauge from AutoZone.
My fuel pressure barely gets to 10psi when I turn key no start. It then automatically drops.

When truck is running I’m at 18PSI.

I changed out the FPR with injectors. When I got to the original, I found the vacuum line was open to air.

Didn’t test fuel pressure prior. Could it be the the FPR is faulty?

please note: the truck ran fine other than slight hesitation from stand still. So I’m trying to rule out fuel pump.

Plus the previous owner “said” he changed the fuel pump.

Thoughts???

Fuel pressure doesn’t hold at all. Drops to zero as soon as I shut off truck. Don’t even need to release pressure in gauge…..
 

Schurkey

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This is a '96 Vortec 7.4L?

Fuel pressure is total crap. A sad fraction of what it should be.

First Guess: Defective regulator.

Second Guess: Plugged fuel filter and/or faulty pump.

Third Guess: Defective gauge.

No way to know what's truly at fault without further testing, although your fuel trims and O2 sensor readings tend towards lean--which indicates an actual fuel delivery problem.
 

mehcs

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This is a '96 Vortec 7.4L?

Fuel pressure is total crap. A sad fraction of what it should be.

First Guess: Defective regulator.

Second Guess: Plugged fuel filter and/or faulty pump.

Third Guess: Defective gauge.

No way to know what's truly at fault without further testing, although your fuel trims and O2 sensor readings tend towards lean--which indicates an actual fuel delivery problem.
Yes, 96 7.4 vortec…

I thought about the gauge as well but there’s literally no pressure at the test port when I screw in the gauge. Nor when I take it off. Just some fuel dripping out line after removed.

Also having hard start. Takes about 3 or four key turns to prime enough to get to turn over.

As soon as I shut off it instantly drops to zero.

I didn’t go cheap on the FPR, used ac delco.

Not sure what else to do but try another…

Also, fuel filter is brand new.
 

Schurkey

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The service manual set for your vehicle can be downloaded from the links in the sticky-thread section of the Engine forum. There's a diagnostic procedure for you to follow instead of replacing parts blindly.

Block the plumbing after the pressure port, see if the pump is capable of more than 18 psi. Service manual says to not allow more than 75 (!!!) psi or damage could result.

I don't know about '96, but the Fuel System Diagnosis for gasoline V8 is on page 6E-856 in the '97 C/K manual.
 

Orpedcrow

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The service manual set for your vehicle can be downloaded from the links in the sticky-thread section of the Engine forum. There's a diagnostic procedure for you to follow instead of replacing parts blindly.

Block the plumbing after the pressure port, see if the pump is capable of more than 18 psi. Service manual says to not allow more than 75 (!!!) psi or damage could result.

I don't know about '96, but the Fuel System Diagnosis for gasoline V8 is on page 6E-856 in the '97 C/K manual.
Aren’t the regulators on the return side? So if the pressure instantly drops when the pump turns off, wouldn’t that indicate a leaking/faulty regulator? Or am I thinking wrong?
 

Piratehunter

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when you replaced the FPR, did you get both of the old o-rings out of the housing and put both new ones on the new FPR? the little one is sometimes left behind when replaced or missed on installation.
 

Schurkey

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Aren’t the regulators on the return side? So if the pressure instantly drops when the pump turns off, wouldn’t that indicate a leaking/faulty regulator? Or am I thinking wrong?
Pressure can be lost via the regulator, via the injectors, or via the check-valve on the fuel pump.

Thus more diagnosis needed to determine where--and how many--leaks the system has.
 

mehcs

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when you replaced the FPR, did you get both of the old o-rings out of the housing and put both new ones on the new FPR? the little one is sometimes left behind when replaced or missed on installation.
hi pirate hunter. I’m 99% sure I did. But will find out soon.
 

mehcs

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Pressure can be lost via the regulator, via the injectors, or via the check-valve on the fuel pump.

Thus more diagnosis needed to determine where--and how many--leaks the system has.
Can rule out injectors since I jumped the fuel pump while plenum was off to make sure there were no leaks.
 

smokymtn65

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Sorry been away. Looks like good diagnostic info has been provided, something is definitely wrong in fuel delivery.
 
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