Help! 1996 LT4 Engine Miss When Started 1 -1 1/2 hours after shutting off

[sgtrescue]

Here's my problem that started about 2 weeks ago. The engine cold starts normal, warms up properly and runs great until you stop driving and shut it off. Approximately 1 -1 1/2 hours later if I try to start the car it misses on 1 or 2 cylinders (extremely rough idle). No check engine light! If you let the car sit about 8 hours or overnight it will start and run great until you shut it off and try to start again after the engine has started to cool down. No smoke or smells! I'm getting ready to do some more extensive testing by warming it to temp and then shutting it off and trying restarts every 15 minutes while recording the engine coolant temps to see if I can determine when the trouble starts in the cool down cycle and (hopefully) when it clears itself. IDEAS/SUGGESTIONS?

 

[gmjunkie]

Here's my problem that started about 2 weeks ago. The engine cold starts normal, warms up properly and runs great until you stop driving and shut it off. Approximately 1 -1 1/2 hours later if I try to start the car it misses on 1 or 2 cylinders (extremely rough idle). No check engine light! If you let the car sit about 8 hours or overnight it will start and run great until you shut it off and try to start again after the engine has started to cool down. No smoke or smells! I'm getting ready to do some more extensive testing by warming it to temp and then shutting it off and trying restarts every 15 minutes while recording the engine coolant temps to see if I can determine when the trouble starts in the cool down cycle and (hopefully) when it clears itself. IDEAS/SUGGESTIONS?

Sounds like you have some injectors leaking down or the FPR diaphragm leaking,I'd start my checks with a FP gauge and a VOM!~!

 

[Hib Halverson]

When you start it up and it misses, does the missing continue for a short period then gradually smooth out, or does it start missing and continue to miss until you shut it off, again?

Also, have you run the fuel pressure tests in the factory service manual. If so, what were the results?

 

[94conv.]

Mine is doing the exact same thing, Ive been told its the injectors, the optispark pick up the ecm depending on who Im talking to. I called Gordon Killebrew today and he says its the pick up in the opti or the injectors breaking down when they get hot. Has anyone solved this problem?

 

[toptechx6]

Since your 96 is OBDII it has misfire detection, typically total misfires must exceed a certain percentage of total firing cycles before a code will set and illuminate the MIL. You do not mention the check engine lamp being on so it does not sound like that is the case with your vehicle, however using any decent OBDII scan tool you should be able to monitor misfire in real time.
As you watch the misfire monitor you will see misfire counts for each cylinder. Even though misfire monitor does not always work perfectly it should at least tell you what cylinder or cylinders are misfiring so you can test spark, fuel delivery and mechanical condition to isolate the root cause, hope that helps.