This just started about 2 weeks ago. At idle while in gear if car sits for more than a minute the rpms will lower to 550-600 and stay there for a good 5-10 seconds then rebound up to normal and then repeat it self once or twice and then stay there. The RPMs were verified with my scanner and the battery volts drop also when this occurs. If it is not in gear at idle no problem as well as driving. I troubleshooted battery and alternator and both are good. I have a spare, known to be good, alternator and put that in and same problem. I cleaned the battery connections as well. I checked for vacuum leaks at idle out of gear and found none. The scanner shows normal 02 sensor swings and voltage so that sort of rules out a vacuum leak. Also no codes. What would cause engine to lower RPMs like that? This only happens when the engine is fully hot. Anyone else have this strange problem and if so what was it?

could be the engine just trying to go to the idle RPM that is has set in the PCM and then it doens't run well there so it shoots back up to keep the car running

lean or rich fuel condition, or Dirty throttle body or bad IAC. Can you check Long term fuel trim with your scan tool
Is Long Term Fuel Trim significantly in the negative range (Rich condition)?

Is Long Term Fuel Trim significantly in the positive range (Lean condition)?

I was watching the fuel trim and it was going down when the rpms dropped. I'll check it again and get the LT and ST values and report back. Might not be until next week as we have a monsoon moving in.

I'll check the FRP and also the MAF sensor as I think that may be the problem. I have been trying to find a slightly rough idle and off and on lean conditions and the only thing I didn't check was the MAF sensor. I'll update when I do the test.

In 97-99 I know the pcm has idle set at 600-650rpms. At the same time the pcm has the alternator cut off set for 550-600rpms.

If you have the AC/Defroster on, this could also be the clutch cycling.

Here it goes:

These are at idle and in gear

Evap solenoid seems to be on at all times hanging at 24%

LT Fuel Trim 149 Steady
LT Fuel Trim CL 0 Steady
ST Fuel Trim 160, 182 at idle out of gear
INJ Pulse 36

MAF 3 steady in and out of gear- it went to 4 once out of gear.

Disconnected MAF engine seemed to perk up, ran rough but would not drop RPMs while in gear. Oddly it stated it still was in a closed loop so I'm not sure if that is good. Here is the readings with MAF disconnected and in gear:

LT Fuel Trim 124
LT Fuel Trim CL 0
ST Fuel Trim 131

One odd thing was I reconnected MAF sensor, cleared codes and restarted. It would not drop RPMs at idle in park. Disconnected MAF sensor again and engine seemed to perk up again.


What do you think? MAF sensor and/or other issues?

First thing that controls idle should be the IAC, I forget if you checked that. Then maybe clean the maf?

I am going to replace MAF sensor and take it from there. I'll update after that.

You should test the sensor first. Replacing it is a expensive choice if the sensor is fine. Unplug the MAF, the engine may stumble a bit at first and then smooth out. If it runs about the same the MAF is not the problem. If you have a oiled air filter, the MAF should be cleaned. Cleaning the IAT and throttle body is way cheaper and is something that should be done anyways.