Ive been having a hard time diagnosing and fixing my tj. It never did this in the summer,When i try to start it up in the morning sometimes it starts and sometimes it dont. only after sitting overnight. Got spark.battery good.I do have a 4.0 throttlebody,Would that have anything to do with it? Any info would be appreciated
if you already verified the spark when this occurs:
have you replaced the IAC valve+housing and the tps from your 2.5 t/b to the 4.0 t/b?
Also, would be a good idea to check the fuel pressure when it fails to start. As a quick test you can try to put the contact on and off a few times to prime the fuel line and see if that helps - if it does it could be that your fuel pressure regulator or check valve at the pump is slowly bleeding pressure out of the rail - that can also be verified by leaving a fuel pressure gauge connected over night and see if you still have pressure in the morning.
another thing that can cause this is the PCM temp sensor located at the rear (towards the firewall) of the cylinder head - if that is faulty it can cause the PCM to not deliver enough fuel when cold.
or in a more extreme case can be caused by too much fuel and your spark plugs get wet - try to take the plugs out and visually check them once this failure to start occurs.
there can be other reasons but these are a good place to start (basically if you have spark the reason is to much fuel, too little fuel or no fuel at all considering that the engine is mechanically sound).
