Funky low idle

[woody]

Wifey's 90 4.0

Sometimes when cold start, it idles really poor (300 rpm +-) adding throttle doesn't do anything, except make it sort of bog...doesn't stall or rev up. Once the Jeep decides that it's fully warmed up, the idle pops back to normal and it runs & drives fine.

I did the following seperately with no change in condition: disco'd & plugged the EGR valve line, unplugged and replugged the TPS and IAC. Cleaned TB and IAC, Replaced IAC with a spare, replaced coil wire with a spare.

I am tempted to swap out the coil and ICM with one of my spares, but I still think it's something fuelish or a sensor.

Any one have any clues?

 

[RookieJeeper]

sounds a lot like what was happening to me. I had a fuel issue and it would bog and miss like crazy. Turned out being a fuel pump issue. The sending line on top of the actual pump (inside the tank) became disconected, and would suck up the fuel and spew it out of the top. Leaving me with very little fuel delivery.

 

[woody]

At 24 years old/227k, it's very possibly a fuel pump (learned long ago not to let these Renix POS surprise me) but why would this issue consistently go away when the temp-gage gets up to normal/hot? And wouldn't a FP run poor (or not at all) under load?

Before it warmed up & ran fine, I tried a couple other things that DID kill the motor: Unplugged the MAP and unplugged the FP ballast resistor.

I think I need to add a fuel pressure test gage to my bag of tricks...

 

[GSequoia]

A 24 year old 1990

Have you checked the fuel pressure while this is happens, jsut for kicks? Also, how is the voltage during this process? Let me think about this, I had aproblem similar to this, but don't know what I did to fix it...give me some time it was awhile ago...

 

[SimpleXJ]

Woody, if you have a haynes or chiltons manuals try doing the sensor tests from those books to be sure about your MAP, TPS, etc. I don't think its the fuel pump either, the fuel pump doesn't care if the motor is hot or cold. Those pressure guages are expensive though. Let us know what the problem was when you find it.


SimpleXJ

 

[Neil]

Remember that the computer runs the engine in a closed loop untill it's warm. I would look for the problem somewhere in that circuit. HTH