If I let the car sit for a full day or more the car drives beautifully for about 30+ minutes. Once the engine bay gets nice and heat soaked the car starts having random misfires that you can see on Autotap. You can also feel and hear the misfires. Of course you can feel and hear it most below 2k rpm. The coolant temps are the same for a heat soak condition and not.

Now I have been fighting this problem for about 6 month and replaced all sensors and electrical components. I also have LT1 Edit and currently am running in open loop all the time so we don't have to consider the PCM learning or O2s for this problem. This problem started after about a month when I completed the hotcam and LT header install.

Does this symptom sound like anything you guys have heard of?

The car ran 112 mph at the 1/4 which is 6 mph higher before the mods. That is what you would expect for hp gains. The car has good power so I think there is no mechanical issue. The compression is normall as well.

 

When you say you changed all the electrical components we assume you mean wires, coil, opti, TPS, MAP and the like.

Did you replace the lifters when you did the cam change? If not, how old where they? Not sure if original lifters can take the lift of the hot cam but for now let's assume that is not the poblem.

Have you tried randomly heating certain items with a blow dryer while the engine is cold to isolate components that may be breaking down when hot? Also use freeze spray to absolutely isolate the culprit by immediately cooling it back down.

What about fuel? Remember in open loop, everything is richened up. then in closed loop, things go back to normal.

Maybe your mods require more fuel and dirty original injectors can't deliver enough. Maybe you need a new fuel map.

Just thought I'd throw out some ideas to get you thinking.

 

Is it a misfire or a stumble?

Does it misfire near the redline when hot at WOT?

If not prolly opti or coil or coil wire.

If its a stumble it could also be EGR related.

 

Thats sound very similar to the problem I had after my cam install. The car would miss only after about 10 minutes of driving. It turned out to be the ignition module soaking up too much head from the LH cylinder head plus all the heat sink compound had dried up.

Heres a writeup on how to fix it

http://csce.uark.edu/~jgbertr/rb/4th...ml#ICM_cooling

 

Thanks for the replys. LT4s have no EGRs, BTW. I am forcing the PCM to run in open loop all the time using LT1 Edit because of fried O2 circuitry. It may be running a tad rich that way because I am not done tweaking it.

Last week I thought I discovered the problem because at night I was looking for spark plug wire arcing and I found some so I replaced the plugs and wires. I also noticed that there was a arcing around the coil and ICM. It looked like it was between the two. I have been running with the ICM cooling mod for serveral months. Maybe the dielectric grease did dry up anyway. The Corvette engine bay gets really hot! It seems hotter than the fbodies I have had.

The misfire is only detectable while crusing or idle. I put in new lifters with the cam. I think I answered your questions.

Over the 6 month period I may have had more than one problem. As you can see below I have been through a lot.

I have replaced
* plugs (three times) they all look clean and the same, NGK TR55, and Autolites 106, QEM.
* plug wires (Three times)
* optispark (twice)
* coil (twice)
* ICM
* O2s (two sets) Bosch and Delco not including the originals
* PCM (an Ed Wright and a PCMforless version)
* Fuel filter
* MAF
* MAP
* Both coolant temperature sensors
* Fuel pump
* Injectors

I have cleaned
* throttle body
* IAC

I have checked
* plugged all vacuum lines to test out the idle (before intake reseal)
* Compression
* inspected and re-adjusted rockers/valve springs/pushrods
* Resealed the intake
* Fuel pressure is at 42lbs with key on and engine off. Now have adjustable one.

The way it cuts makes me think it is electrical. It cuts out so hard sometimes while cruising that it bangs the drive train. I have an aluminum flywheel, BTW

 

wow, i'm so jealous. It looks so much easier to get to all the engine crap in a vette than in a Z or TA. It sure would be cool to see an f-body hood do that...... Wow , I am so off topic, any way good luck.

 

I would say that everything is easy to get to except the front of the engine. For example the frame is directly in front of the crank pulley. It was more of a pain to do the cam job than an fbody.