1987 Trans AM LG4 305 Auto

Can anyone help me figure out why my car will start up and run fine all day then after sitting for a while it will run really really rich like it is in limp mode. The MCS will not click. I have a new ECM and Hypertech PROM in the car. I have a new O2 senors as well as coolant temp sensor for the ecm. For some reason if I jump pins A and B on the ALDL port the SES light does not illuminate so I have no idea if I have any engine codes. I am stumped.

If I let the car sit overnight it will start and run fine all day. It's only after sitting a while ( like an hour or so) will it run in full rich mode and make the car run horrible.

Any suggestions would be appreciated!

 

Does the CEL illuminate as you are starting the engine? Does it always do it?

Just trying to narrow down if you have an ECM issue or just that somebody yanked the CEL bulb from the gague cluster (to mask multiple hard fault codes they didn't feel like fixing properly).

 

The ses bulb is good. It never illuminates at all though even at start up. I have a new ecm and prom in there already and still nothing promising. I'm going to have to check the wiring .

 

Yeah, I would think that would be the next logical step. A good CEL bulb but one that never illuminates typically means a bad ECM, but if your ECM is new (with the proper chip in it?) would tend to rule that out.

 

I do have the correct ECM and Prom but I just made a discovery that the SES Light driver module was missing and that is what was causing me to have no SES lightage and supposedly will kill possible communication with a scanner. When it comes in the mail Ill report back with my findings. If I can then finally be able to pull codes I should be golden and be able to determine why my car is running horribly.