Idle Problem is back - 2002 4.0L, Auto Trans

[enjr44]

2002 4.0L, Autotrans, low idle problem in park.

Problem began with low idle when shifting from drive to park. It would drop to about 500 (or so) when warm. I cleaned MAF once (it looked good) and the IAC valve twice and each time the problem would go away for about 50 miles. I disconnected the battery each time I removed the valve. Cold start is good, about 1200 to 1500 rpm depending on temperature. The engine check light does not come one.

I replaced the Idle Air Control Valve. This time it worked correctly for about 100 miles. I got the new one from Napa. Again, I disconnected the battery before replacing the valve.

Currently when warm (after driving few miles), the idle is about 750 rpm in drive or reverse and nice and smooth. When put in park the idle drops to 500 rpm or less, stumbles around, and usually slowly recovers to about 700 rpm. Twice it has stalled before recovering. The old valve did not recover.

If you shift back into reverse or drive, it jumps back to 750 immediately. That sounds significant to me!!!!

So what is wrong with this thing???? What else could cause this problem? What voltages should I see if I back pin the IACV connector? Unlikely; but, could I have gotten a bad IACV from Napa since it does slowly recover most of the time and the old one did not? And, finally, why does it work correctly for a while after cleaning the valve? Could disconnecting the battery cause this?

I know this is a lot of questions; but I am stumped!
Thanks, Ed

 

[RonD]

Disconnecting the battery for 5 minutes will reset the computer, so it will use default settings until it relearns the new settings.

Try this, warm up engine to normal operating temp.
Unplug IAC connector
Engine idle should drop down to about 550rpm
If engine idles lower or dies, there is a throttle plate adjustment screw on the plate that the throttle cable attaches to, look under it.
Turn that screw one full turn and restart engine.
Adjust screw until rpm is 550 with IAC disconnected.
Turn off engine and restart, to make sure it stays at about 550rpm
Connect IAC, idle should jump up then settle back down to about 750rpm.

Disconnect battery for 5 minutes, so reset computer.

As to why your idle drops in Drive but not Reverse........??
Does idle go up when A/C is turned on?
PCM should have "target idle" settings for Park and Drive, but you need a scanner to see those.
The defaults settings are good since it works with those(after reset), but for some reason it is learning incorrect idle rpms.

 

[enjr44]

Thanks for your reply. The idle goes back to 750 in reverse or drive. When you turn on the AC with the fan on high speed the idle speed does not change in drive or reverse. The base idle speed is good.

"The defaults settings are good since it works with those (after reset), but for some reason it is learning incorrect idle rpms."
That the pcm re-learning the incorrect idle rpms is what is bothering me a lot. However, if the PCM had incorrect learned idle rpms, why does it slowly increase back to around 700 rpm if you just let it sit after shifting back into park? It takes about 15 seconds to do that, when it does it.

What I DO NOT KNOW about pcm contoled engines would fill the ocean. If the pcm is not supplying the correct re-learned idle rpm is there a way to fix it?

Ed

 

[BabyBronco89]

Vacuum leaks???