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2002 no start when warm.

biggman100

NAXJA Forum User
Location
NY
I have a 2002 4.0l automatic that has been having the oddest issue. Most, if not every time its cold, say after sitting overnight, it will start, but after it warms up, if you shut it off, it wont even try to restart, unless i use a test relay in place of the fuel pump relay. After testing the pins in the PDC where the fuel pump relay goes, i get ground, constant 12v, another ground that becomes positive once the key is turned on, but, i dont get anything on the energize pin, not a quick blink, or anything else, so, i am wondering what could be turning off the pump and not energizing the relay once its warmed up. i have tried unplugging both temp sensors to see if it would start in limp mode, with no luck. With it hooked to my scanner, with the bypass relay installed, i get good readings from all the sensors while its running (for example, 29.2 psi oil pressure, 194 degrees on the temp sensor, 670 RPM's) and all the other sensors seem to be reading within the correct range. The CPS and CKP are both new, and i have made absolutely sure the CPS is indexed properly. I have even tried swapping the ASD and fuel pump relays with new ones, and still get the same results. It seems to me that it has to be something that the ECM only looks for once it has warmed up, but, other than the temp sensor, im not sure which other sensors would come into play at that point, except possibly upstream O2 sensors, but i dont see how they would cause a no start. The only code i have found is for a downstream O2, which i doubt has anything at all to do with it not starting. I even swapped the CPS from my personal 01, that i know has no issues (since i drive it everyday), and with my CPS in the 02, it still acted the same, so i am beyond stumped at this point. Any advice would be greatly appreciated.
 
What do you have on the relay? 85 and 86 should have power and by grounded by the PCM to energize the relay. The other 2: one should have key on power and the other should go to the fuel pump. If you have a ground changing to a power I'd start with that circuit.
 
I found out that a wire under the PDC was broke, fixed that, and now the pump runs, but, i still get either a hard start, or no start, but, it has progressed to half the time not even starting when cold, so, im honestly leaning towards bad gas, since, no codes are present, and going through and unplugging various sensors (temp, MAP, IAT, IAC, and every other one i could think of), and it still didnt make any kind of difference on whether it started or not.
 
It actually turned out to be a bad fuel pump, even though it was replaced with a new one just over a month ago.
 
Gotta love new parts that fail...

mac ‘hard to rule anything out’ gyvr
 
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