97 explorer xlt hard starting when warm

[tomrj]

Hi, my significant other's 97 XLT w/ 4.0 sohc is hard to start and has a rough idle after it's driven for an hour or so. It starts right up and idles fine when cold. I've cleaned the IAC , MAS and checked the air filter. Has anyone else had this problem and what did you do to fix it. It hasn't kicked any codes, thanks for your help and suggestions .

 

[tomrj]

ANYONE have any ideas

 

[2000StreetRod]

ECT sensor?

Do you have any way to read the engine coolant temperature (ECT) via the OBD II bus? Maybe the ECT sensor is defective and the PCM "thinks" the engine is cold when it is actually warm. The result would be a rich mixture. Is the exhaust black when the engine is warm started indicating a rich mixture? If you can't read the ECT via the OBD II bus then you can measure the resistance of the ECT sensor and compare it to the table below.

 

[tomrj]

thanks, I'll see if I can check that

 

[Flash]

You might have a leaking fuel injector that drips fuel into a cylinder when hot and under more pressure than when it's cold.

 

[tomrj]

used my code reader on the live data mode and it seemed like the coolant temp. rose in the expected manner, therefore I'm assumming that the coolant temp sensor is working. About the only item that raised a bell was the throttle position was reading 17.5 degrees @ idle and I thought that to be a little high, but don't know what the spec's call for.

 

[2000StreetRod]

TPS at idle

18% for the TPS at idle is typical. The PCM reads the TPS when the ignition is switched on and assumes that to be idle and stores it. The PCM periodically checks the TPS for a lower value and if one is encountered the old value is replaced and stored.

Can you read the long and short term fuel trims for each bank?

 

[masospaghetti]

My 2000 has the same issue. I'm guessing a leaky fuel injector as Flash said but haven't had time to diagnose it.

You can hook up a fuel pressure gauge to the service port when the engine is warm. Turn the engine off and see how fast the pressure bleeds down.