My car develops a problem once it warms up - it sputters at high RPM under hard acceleration (between 5000-5500 RPM). It is fine when the engine is cold.

With the air-flow mods that I have done (see sig), I suspect that it is running lean when the problem occurs. Which leads me to question whether my fuel pressure is adequate.

At idle, fuel pressure is 45 PSI. Under hard acceleration (when the sputtering occurs), the pressure is between 42-43 PSI.

Do I need to install an ajustable fuel pressure regulator and up the pressure?

 

I would say no to the fuel pressure reg.

Sounds like you have one of a ton of other possible problems. You'll get some tips.

 

I will put my vote on one of the O2 sensors. Is the SES or SYS light on?
That fuel pressure is fine for a LT1. Did you test the injectors for a fall in fuel press. after the engine is shut off?

 

I'll put money on a bad EGR valve...something simple to check BEFORE you start throwing $$$$ at it

 

I recently replaced both front o2 sensors but did not replace the one "down-stream". Do you know what that one does? Someone told me that GM put it there to keep people from removing the cats. I haven't been able to find any info online to explain its purpose.
The SES and SYS lights have not come on and it is giving me no trouble codes that would relate to this problem.
As for the injectors, no, I didn't test them. They were also recently installed and are a "matched set" that came with calibration papers, so I assume that they are fine.

Edit -
It sounds like the down-stream o2 sensor is to warn you when the cat is going bad (odd that it is only on one side). Thank you Google

 

I would think that the regulator is fine, but that possible the fuel pump is starting to go.

 

My fuel pressure numbers are less than yours and the car now runs great since I replaced the PCV valve and the top cover gasket on the throttle body. Gasket was stretched out of shape, sagging down, looked like from vacuum pressure acting like a diaphragm.

The car would stumble on mild or hard acceleration after warmed up, even worse when I had made several stops (engine cut off), and would not idle without acting like it was randomly missing (real rough idle) but not all the time. Plugs and wires were changed making no difference.

I had replaced, or tested almost everything related to electrical, fuel and air flow to no avail. I'll never figure out how the top gasket on the throttle body works but it is the clear air intake that pushes air into the right valve cover eliminating crankcase pressure and works with the PCV valve that goes into the intake manifold. After the car was running like it should I cleaned the MAF sensor and the t-body and it made a big difference in acceleration.

The gasket has two holes in it other than the six mounting holes for the cover. Those two holes are sealed from each other by the cover and gasket separating the two ports (holes) that go down into the t-body somewhere. Under the gasket is just a void as far as I can tell that goes to a hose going to the valve cover.

Sorry for being so longwinded.

Edited: No codes ever showed to give a clue as to what was wrong.

 

My '92 was doing this and I found the cause to be a dirty fuel filter. It was the original and the car had about 105k on it. If you don't know the history of the filter it is easily changed and at $8 it's cheap.


Dave

 

Anytime I read about a high RPM miss, I start to think "Optispark." But that's just me.

 

No need to apoligize, I appreciate you taking the time to share your experience. I suspect that you may have hit the nail on the head in suggesting the top cover throttle body gasket. When I (recently) installed the air-foil, I had to remove the top cover. I noticed that the gasket looked stretched out of shape and thought to myself, I should probably replace this. Alas, I did not, and it hadn't crossed my mind until I read your post. I will definately replace that bad boy immediately and let you know if that does the trick.

L71Maynard,
I recently installed a fuel filter. In the last month, I have spent mucho dinero trying to get this car running correctly.

Besides the fuel filter, I have replaced:
Opti-spark (distributor), cap & rotor, plug wires, spark plugs, ignition coil, ignition module, air filter, pcv valve, & all (8) fuel injectors.

The distributor, at least, did need to be replaced, as it was rusty inside (water pump went out on previous owner). The rest of the items, not so much.

Thanks again guys!

 

I ordered a new top throttle body gasket (also known as the clean air lid gasket) from gmpartsdirect.com. Then I looked at their delivery schedule - "usually ship within 7-10 days". Ugh, I couldn't wait that long to know if that was the problem, so I made my own gasket with some black permatex. Innitially, I thought that did the trick. Throttle response is FAR better than it used to be. It also cured a part throttle hesitation that I had grown used to. Unfortunately, the high RPM stumble is still there, growing progressively worse as the engine warms up. Sometimes it actually backfires.

I thought that maybe there was an air leak somewhere in the air intake ducting so I put all of the stock pieces back on (removed open air lid, power couplers, & aftermarket MAF housing). With everything back to stock, the problem has not gone away.

From idle-5,000 RPM, the car runs better than it ever has. Once it reaches 5,000+ RPM, it sputters and sometimes backfires. Fuel pressure tests show no problems. I would suspect a spark problem, but I just replaced the opti, wires, plugs, coil, & ignition module - what else could it be?

 

^^^ ditto....ttt