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Discussion Starter #1
Ok. I haven't been around the forums much and cannot figure out this issue and I really need some input.I've gotta 94 TA with a built lt1 under the hood. About a year ago the car developed a miss/shudder and I can't figure out a cause or solution. Its had three tunes, two optis, two coils, two icms. The plugs and wires are new as well. Throughout all the ignition parts being replaced this symptom has remained. Its a low end shudder and miss and not throughout the rpm range.

The engine only has 8, xxx on it. I'm running accel 30# injectors and have recently installed a new walbro 255. The fuel pressure was set at 43.5 but the car began to load up after driving for a bit. The mechanic recently dropped the fuel pressure to 42 and it seemed to make the issues worse. I apologize for the post being all over the place as its early. If anyone can throw any common issues and solutions out that'd be great.
 

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Have you put the car on a scanner that does real time sensor data, to see what sensors are telling you.

Have you tried dampening the plug wires and running engine at night to check for spark leakage.

Have you checked your engine grounds, especially the pcm grounds, as any extra resistance in those grounds will mess with the pcm's ability to control the air fuel mixture.

Have you checked the opti wire harness for, loose connectors, burnt and bent pins, bad wires. Also do a continuity check of opti harness to see if any extra resistance is in wires.

Have you downloaded the 94 factory service manual? In volume 2 it has a good pcm symptoms chart, which tells what happens when each pcm wire has a problem. Also volume 2 is dedicated to driveability, emissions, and electrical.
 

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Discussion Starter #3
Although not in front of me the datalogging showed no issues and no codes/failures detected. I've checked the wires and plugs in the dark, I've seen no signs of leakage with the wires. All the grounds have been checked. Double checked. Triple checked. The only thing from your list that hasn't been done is changing out the opti wiring harness. This thing is throwing no codes at all so that's making it more or a pain. Am I correct with the to being 43.5 @ prime and [email protected] idle?
 

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Yes, fuel pressure is as you specified. Have you taped gauge to windshield and taken her for a drive to see if fuel pressure maintains under various driving conditions.

Also double or triple check to make sure all plug wires are locted correctly. It's real easy to accidentally put one on the wrong cylinder.
 

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Discussion Starter #5
Sure have. Fuel pressure is keeping steady. I'm wondering if I'm looking at this from the wrong angle. The symptoms are similar to vapor lock in a carbed engine. Would an evap system issue such as a clogged can create these symptoms?
 

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I'm thinking more on the lines of egr. The egr valve could be stuck open and causing running problems.

Evap system is worth a try to look at, maybe something is hooked up wrong or lines with leaks, etc.
 

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Discussion Starter #7
Can't be the egr system as it was removed during the build since at the time i believed everything the engine builder said and tuned it out. Guess I need to get to work tonight looking for leaks or maybe issues with the cannister.
 

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evap system wouldn't do this kind of thing to you. if you're suspicious, plug the evap vac line to the intake and take a little drive with the gas cap loose if you want to rule the entire fuel venting system out. that rules out:

- evap sol. stuck open
- tank plugged
- inlet tank vent plugged

'the mechanic dropped the fuel pressure to 42', meaning you have an adjustable fuel pressure regulator? that's a hell of a lot of tuning to compensate for on this engine. i bet your AFR at low rpm is messed up.

we really need a realtime data log from when the shudder is happening at this point.
 

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Discussion Starter #9
Yea I know. If I get a log tonight I'll post it. Unfortunately this is my alternating weekend working the entire weekend. The logs kept showing a lean condition in the pass side. The vac was holding and there is no exhaust leak so I did research and found that the valve lash being off can cause these conditions to show on a datalog. So the valves were adjusted and that's when fuel pressure was dropped. I guess tonight I've gotta up the fp again.
 

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did whoever was doing the tuning know that you had an adjustable fuel pressure regulator?

adjusting fuel pressure is THE WRONG way to tune a car like this and there isn't much need to run higher than stock fuel pressure. set it to whatever it was tuned for

having only one bank reading lean is a concern for sure, but at least it gives you a starting point...
 

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Discussion Starter #11
In going to fix it tonight if I can get my hand back there. As much as I love driving my car its a pita having it sit more than drive. There's not a big lt group around here, not much gm at all, the overkill of foxbodies and imports has kinda screwed us fbody guys because nobody knows how to deal with these things.
 

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Discussion Starter #12
I checked the fuel pressure regulator and it checked out ok. Fuel pressure held. I know a tune can be off but what are the chances of the exact same issues after multiple tunes? The stutter/shudder isn't apparent during idle only under load.
 

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that's very common with ignition system related issues, for one, since spark happens much more easily when the engine isnt loaded

anything without a good datalog while it's stuttering is just grasping at straws at this point
 
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