Idle Fluctuating After Engine is Warm

[FordRanger1991]

Idle Fluctuating After Engine is Warm

so... I have a 91 2.9 liter engine that seems to have idle problem after it is warm. During this time the engine will fluctuate/jump from 400 to 900 rpms. Also, it is interesting that if I shut the engine off while it is having the idle problem, and then re-start it, the idle will be fixed and it will idle smooth.
This problem seems to happen when at stop lights and while in a drive thru. The truck has also recently started getting worse gas mileage...

The engine light comes on but will only come on after I let the truck idle for a while. When I pulled the codes I got a 41 R and 41 M during the KOER and KOEO test respectively. The tech library says...

(R) System lean - Fuel control
(M) System was lean for 15 seconds or more (no HO2S switching) - Fuel control

Seems like my problem must have something to do with heat as it only happens when the truck is warm. However, once restarting the vehicle it will then correct the idle. I am thinking it is some type of sensor...

The engine has about 15k after a re-build
I have cleaned the IAC
I have also checked the TFI for resistances and it looks like it is brand new.
It has a new engine temp sensor
New wires and plugs
New O2 sensor
Fuel pressure good and the FPR has no gas smell in its vacuum line
Timing is between 11 and 12
All the vacuum lines look great and I cant find a leak

Thanks for the time! I just don't want to take it somewhere and spend a lot of money. I will also try to get a video of the engine doing this and post it later.

 

[shane96ranger]

Have you looked at the MAP sensor? A friend of mine had an 88 Mustang that had an erratic idle (that would occasionally cure itself, but always return), and we cured it by cleaning the MAP sensor. Some people frown upon cleaning the MAP sensor, but I personally have had good results with it. We used carb cleaner to clean it, and let it completely dry. Using something more sensitive to electronics may be better, such as MAF cleaner.

 

[FordRanger1991]

Ya I was thinking it could be something involving the MAP sensor. I had no idea that you could clean them though. I guess I will give it a shot for kicks and giggles. Thanks for the idea

 

[shane96ranger]

Ya I was thinking it could be something involving the MAP sensor. I had no idea that you could clean them though. I guess I will give it a shot for kicks and giggles. Thanks for the idea

I just sprayed it in the port where the vacuum line connects.

 

[RustedRanger]

The IAC could still be bad,sounds kinda like what my truck did when it was bad.

 

[FordRanger1991]

So last night I ended up changing the map sensor for fun and I reset the computer ( left the battery unplugged for a half hour or so). This morning I went on a drive in the truck to see if the Map changed the idle problem and it would appear that it did because I could not duplicate the fluctuating idle problem.
I also ran the engine with and with-out the IAC plugged in. Without the IAC the RPMs dropped about 200-300 so I am assuming the IAC is good.
However, when I first started the truck this morning it instantly died and when I re-started it it seemed to not have the higher idle speed like it usually does in the mornings when it is cold. Usually it idles at around 1400 for 30 seconds then will drop to about 1000 until it is warm.
I am wondering if the initial die this morning was because of the new MAP and the computer resetting.
I guess I will just have to wait till tomorrow to see if it dies on the initial cold start.
I will also be driving my truck later today to see if I can produce that idle problem.
Thanks for the comments! I will get back to you all when I know for sure if my problem is fixed.