titchener

I've got a '94 Chevy truck with a 350/L05 with the 4L60E transmission.

To get it ready for my son to drive I totally rebuild the TBI unit with cleaned injectors and the truck has been running great for about a year.

Recently its began stumbling and missing under mid throttle. It will warm up and go in to closed loop fine, accellerate fine under part throttle but when you give it a little more in 2nd and 3rd gear to go up a hill or get on the freeway it starts stumbling.

I can't find the problem and its been driving me out of my mind.

Here's what I've done so far:

Replaced fuel pump and filter and verifed with a fuel pressure gauge temporarily taped to the windshield that its making a solid 11.5 psi when the stumbling occurs under load up a hill.

Yanked the distributor and rebuilt it with new pickup coil and new ignition model. It didn't have excess shaft side play or end play, put on a new cap and rotor.

Replaced coil.

Replaced the EGR valve and solenoid.

Replaced MAP and coolant sensor.

Replaced O2 sensor.

Replaced TPS.

Swapped in a different set of injectors that had just been cleaned.

Redid the ground wire for the ECM, added an extra wire from ECM ground wire directly to the battery.

Swapped in a different ECM.

Dumped all the gas and put in fresh gas with some stabilizer.

I put TunerPro RT on it and did some logs but its hard to see that anything is going wrong when the stumbling occurs, maybe the slow data rate makes that hard to see.

It doesn't show any knocks occurring.

I'm totally stumped at this point. I'm using TunerPro 4.0 and not seeing any knocks at all is making me a little suspicious so tomorrow I'm going to switch up to TunerPro 5.0 and try a later $0D mask interface file from moates.net, maybe the file I was using wasn't showing everything.

The truck runs fine under part throttle and in the middle of all this even passed a smog check with really low numbers.

When I swapped the ECM I had to reuse the same Prom module, can those Prom modules go bad and give these kind of symptoms?

Does anybody have any other suggestions on what could possible be wrong?

Thanks,

Paul T.

ex-x-fire

Soak the ignition wires w/ water & check for arching when its dark out.

titchener

Thanks for the suggestion on the plug wires, I had done a dark night view of them already and didn't see any arcing.

I finally got it running right again, although I did 2 things at the same time so I'm not 100% which one was the problem.

I had tested the center distributor wire with an ohmeter and it looked ok but today in case it might have been flaking out under vibration I put in a new one.

I also noticed that the cruise control wasn't working correctly and then looked in the service manual and discovered it shares the VSS signal with the ECM, so I also disconnected the cruise control module.

Its back to running great again, maybe in a few days I'll isolate it between those two things but for now I'm just going to chill for a while, getting up at 6:30am to drive him to school really sucked.