No idea what's wrong with my 1997 Chevy 2500HD

[Donaven361]

I have a 97' Chevy 2500HD 5.7 L31. The truck runs fine at idle and under low load around town. No misfires, and starts right up. Only thing is when you actually press on the gas > ~30% throttle the engine puts out no power, and jerks around like it's stalling out or misfiring. Whens it finally stops jumping out of its mounts it starts increasing speed very very slowly. There is no CEL or codes in the history at this point.

Here's all I've replaced and tested.
- new air filter
- cleaned MAF
- new fuel pressure regulator
- cleaned/inspected injectors
- new fuel filter
- new fuel pump
- new distributor, cap and rotor, wires, and plugs, gapped at .060.
- new EGR valve
- new PCV valve
- new ECT sensor
- new TPS
- compression test satisfactory, highest 207psig lowest 195psig
- fuel pressure within specs
- no vacuum leaks
- everything is good on diagnostics computer fuel trim, MAF, etc. (OBD2)

No idea what it could be. Anyone got any ideas ?

 

[Donaven361]

Also replaced the camshaft position sensor too.

 

[stutaeng]

How are you checking and testing things? All those parts were bad?

What is your fuel pressure when it does it? Can you post snapshots of your livestream data from your scanner, again, while truck is acting up?

What's the history with the truck? Ran fine before? Original owner or just bought it?

 

[Donaven361]

How are you checking and testing things? All those parts were bad?

What is your fuel pressure when it does it? Can you post snapshots of your livestream data from your scanner, again, while truck is acting up?

What's the history with the truck? Ran fine before? Original owner or just bought it?

I bought it not too long ago, it was running horribly before I got my hands into it, couldn't make it around the block. Tested the fuel off the test connection on the fuel rail at idle, increasing RPM, and if it held the pressure when you turn the truck off. It didn't hold the 60 psi for even a minute that's how I knew the regulator diaphragm was bad. Few of the sensors were because of codes thrown and things like the EGR I did just because they're inexpensive and easy. But I don't have the scanner anymore, I had my mechanic in the passenger seat analyzing everything while I drove it and recreated the bad condition.

 

[stutaeng]

I bought it not too long ago, it was running horribly before I got my hands into it, couldn't make it around the block. Tested the fuel off the test connection on the fuel rail at idle, increasing RPM, and if it held the pressure when you turn the truck off. It didn't hold the 60 psi for even a minute that's how I knew the regulator diaphragm was bad. Few of the sensors were because of codes thrown and things like the EGR I did just because they're inexpensive and easy. But I don't have the scanner anymore, I had my mechanic in the passenger seat analyzing everything while I drove it and recreated the bad condition.

I see. That gives more to the story...

Did your mechanic see anything when he was looking at the livestream data?

Were you saying the fuel trims were ok. Was that under idle or when the truck was having issues? Have you checked the injectors electrically? I.e. injector balance test? Usually, faulty injectors will give lean/rich conditions, but I think they can fail intermittently, which is hard to diagnose.

 

[Donaven361]

I see. That gives more to the story...

Did your mechanic see anything when he was looking at the livestream data?

Were you saying the fuel trims were ok. Was that under idle or when the truck was having issues? Have you checked the injectors electrically? I.e. injector balance test? Usually, faulty injectors will give lean/rich conditions, but I think they can fail intermittently, which is hard to diagnose.

I haven't checked them electrically, but wouldn't that cause a misfire? Because the misfire counters stayed at 0 even when the condition occurred. He didn't see anything out of the ordinary, other than audible back fires when you come off the gas at high RPMs. But the backfires are what you'd see or hear when you tune a vehicle to backfire on purpose cause you think it's cool. Or you have leaky exhaust valves. The compression test ruled out the valves however.

 

[stutaeng]

I haven't checked them electrically, but wouldn't that cause a misfire? Because the misfire counters stayed at 0 even when the condition occurred. He didn't see anything out of the ordinary, other than audible back fires when you come off the gas at high RPMs. But the backfires are what you'd see or hear when you tune a vehicle to backfire on purpose cause you think it's cool. Or you have leaky exhaust valves. The compression test ruled out the valves however.

Misfire? Technically it would. I had a dead injector on my 5.7 and never got a misfire. I did have a lean code though, and sometimes an O2 sensor code. I replaced the 4 O2 sensors and problem didn't go away. Until I did an injector balance test the dead #7 injector was found.... hindsight, the O2 sensor code was the RESULT of the lean condition, not the cause.

What about a leak down test? I thought that was definitive on finding valve issues over a compression test? Sorry, I don't know much about that test. Others please chime on that.

O2 sensors waveforms looked then?

 

[GoToGuy]

An injector can get a fireing signal, so electric wise it looks good. But if mechanically the pintle is jammed or damaged = no injection with electric signal.

 

[Donaven361]

An injector can get a fireing signal, so electric wise it looks good. But if mechanically the pintle is jammed or damaged = no injection with electric signal.

This would throw a misfire code though right ?

 

[Schurkey]

I have a 97' Chevy 2500HD 5.7 L31. The truck runs fine at idle and under low load around town. No misfires, and starts right up. Only thing is when you actually press on the gas > ~30% throttle the engine puts out no power, and jerks around like it's stalling out or misfiring.

Test ignition coil output using a spark tester calibrated for HEI ignitions. A weak coil can produce all those symptoms.

HOWEVER, it should throw misfire counts.

Here's all I've replaced and tested.
- new air filter
- cleaned MAF

Verify MAF with scan tool.

- new fuel pressure regulator
- cleaned/inspected injectors
- new fuel filter
- new fuel pump

Verify fuel pressure under load with fuel pressure tester.

- new distributor, cap and rotor, wires, and plugs, gapped at .060.

Is the cam offset (adjusted by turning the distributor) within spec? If the cam offset is way wrong, it can cause misfire, AND screw-up the ability of the computer to identify which cylinder is misfiring.

Are these "fine-wire" plugs? Did you adjust the gap on them? (That's NOT recommended with fine-wire plugs.)

- new ECT sensor
- new TPS

Verify all sensors and computer outputs using scan tool.

- compression test satisfactory, highest 207psig lowest 195psig

Is the gauge accurate? Have you ever compared it to a known-good gauge? Those readings seem high to me. But they're nice 'n' even.

- fuel pressure within specs

Tested under load, when the engine is running rough?

- no vacuum leaks

Tested HOW?

What is your fuel pressure when it does it?

Yup.

I don't have the scanner anymore,

Need to fix that.

the misfire counters stayed at 0 even when the condition occurred.

That's scary. Connect a scan tool, watch the misfire counter when you ground a plug wire, deliberately inducing misfire on one cylinder. Does the scan tool show the misfire?

audible back fires when you come off the gas at high RPMs. But the backfires are what you'd see or hear when you tune a vehicle to backfire on purpose cause you think it's cool. Or you have leaky exhaust valves. The compression test ruled out the valves however.

There should be no backfires when you " come off the gas at high RPMs". That itself indicates a problem.

These are EXHAUST backfires, not INTAKE backfires...right?

Does this vehicle have an AIR pump? A faulty diverter valve could cause backfiring in the exhaust.

What about a leak down test? I thought that was definitive on finding valve issues over a compression test? Sorry, I don't know much about that test. Others please chime on that.

A leakdown test is better at finding leaky valves than a compression test. A rising piston can build compression faster than a barely-leaking valve can lose compression. A valve has to leak a lot to show on a compression test.