'98 Rough Idle and CEL when warmed up

I've searched and read.. Got a bunch of info, but didn't exactly find an answer.

Runs like a champ until I let it idle for 5-10 minutes, then drops to 500-600rpm, runs rough, exhaust pops (not backfire, but irregular) and finally CE light comes on.

Temp oscillates between 190 and 210 as usual, so no overheating problems.

Stepping on the gas clears it up and if I let it sit and cool for a while, it's back to running normal .. so i'm suspecting it's heat related.

I've read about the #3 injector, but that problem is described for '00-'01s, not sure if my '98 could potentially be having the same vaporization issue from overheating.

Also, I recently replaced the water pump. First time it threw a code was when i let it idle for 10 min after installing the pump. Originally I thought it had to do with the coolant level being too low and confusing the CTS and PCM, but I've since topped it off and have at this point ruled out the cooling system, since it seems to be working normal.

Next step is to check the IAC. I'm thinking it might be gunked up and running fine until the engine warms up, but them the PCM drops rpms and it doesn't allow enough air.

Also need to check the coil, rotor, wires and plugs to rule out ignition.


Any other suggestions?

Meanwhile, I need to pull the code. Does Autozone rent OBD2 scanners.

Not sure what the cause of your problem is, but in regards to OBD-II scanners at autozone, i'm sure they rent them....BUT they also will read the code for FREE. All you have to do is go there and ask them to pull the code off. I dont think they can turn the light off, but atleast you'll know what the problem is.

Hi, I have the exact same problem with my 2000 4.0!! If you find anything out please let me know...thanks!!

well ... the code came back >> "P0171 System Too Lean (Bank 1)"

I'm strongly leaning toward the O2 sensors at the moment. probably the upstream one. i happen to have a spare on hand, so i can swap it in.

again, the rough idle only occurs after engine is warmed up.
i'm guessing it's when the PCM goes from warm-up/Open Loop mode to Idle/Closed Loop mode.

Quote from the manual, re: Idle/Closed Loop mode:

"The PCM monitors the O2S sensor input and
adjusts air-fuel ratio by varying injector pulse width.
It also adjusts engine idle speed through the idle air
control (IAC) motor."

some good reading on the topic: http://www.naxja.org/forum/showthread.php?t=65367

Oh yeah, there's also some talk on other forums about K&N style (oiled) air filters gumming up the MAP and IAC. causing similar error code.

I can't say one way or another for sure .. though I did clean/reoil my K&N recently and and it's an easy one to check/clean. so i'll start there first and move on to the O2 sensors second.

That does sound like yout upstream o2 sensor is gone....given by the Bank 1 it is telling you.

Since you have a spare one handy, i'd go ahead and swap it in. Wouldn't hurt to have a new one in there anyways.