Strange Idle, No Codes, Looked Everywhere

[Kens1992mtxSHO]

I was sitting in my car listening to it idle (yes, I had nothing better to do) and the idle would drop just a little bit and rise to normal, then drop again.

Got no codes when I checked, and am wondering what could cause this. I have so far cleaned the MAF, cleaned the TPS and the problem persists.

Where do I go from here? I would hope a serious problem would throw some codes, but I know this isn't always the case.

 

[average sho]

IAC maybe

 

[cmeseadoin]

IAC maybe

What temperature was the engine? Had it been driven and gotten to operating temp or was this a "cold" start? I would hook up a scan tool and watch all my typical intake oriented sensors such as the throttle position, IAC, MAF, MAP etc. See what they are doing real time when the problem is happening. How much of a fluctuation are we talking (in rpm) and is there any work that has been done to the car? IE) coil, wires, plugs, intake, etc etc.?

I certainly would agree that it could perhaps be the IAC since that controls idle, but we'd need more data to know and diagnostic tests to prove it.

Also, if you cannot find anything at all and are sure you have exhausted everything once you get any codes from the pcm (you say there are none anyways). Pull the PCM fuse and leave it out for 15 minutes......reinstall it and force and Idle re-learn. See if that does anything!

 

[Kens1992mtxSHO]

The car was at normal operating temp. I'm not talking about a significant drop in rpm's (maybe 100) any more and it would stall, and I'm not getting an oil light at all. The car was driven about 200 miles the day before, all highway.

I have recently replaced the spark plugs, wires, seals (valve cover seal, spark tube seals, valve seals) and changed the oil. I switched to mobil1 synthetic this time around.

...where is the pcm fuse? I have disconnected the battery for several hours trying to get the car to relearn the idle that way...

 

[cmeseadoin]

...where is the pcm fuse? I have disconnected the battery for several hours trying to get the car to relearn the idle that way...

hmmmm, been a while since I have had to do that....I will need to look under my hood tommorrow and see if I can find it. I want to say it is in the fuse box at the base of the driver's window and under the hood right to the back of the driver's strut tower cover and between the firewall. I don't think it is on the inside of the car. I will look tommorrow in my free time and see if I can find it for you. I know it is there, ...but cannot recall right now. Perhaps someone else reading this might know exactly