Sluggish 4.0L performance

I have a 1988 Jeep MJ with a 4.0L, an auto tranny and 31" tires with stock everything.LOL The motor is running rough. It idles erratically(it will go into a higher rev,no tack but im guessing 1200rpm, to a low 500rpm) and if I smash the gas from sitting still it sputters for a two or three seconds before it takes off. After it takes off it goes half decent. In my opinion it acts like its not getting enough fuel. I replaced the fuel filter and pump about 6 months ago. I also recently replaced the air filter. I also know its not a clogged cat. because I have a "highflow" cat. It also has a cherrybomb so its not anything w/ the exhaust. Any help would be great.

i have the same problem once and awhile..i'd like to know the answer too haha

I'd recommend cleaning the throttlebody and IAC... taking the TB off the manifold would be better, but I have always done mine installed, with some spray and a few odd brushes. I then trickle 1/3 - 1/2 can of Seafoam down through it while it idles & call it good.

It could also be a faulty or misadjusted TPS :dunno: Got multimeter?

Also insure the plastic hose that runs from the MAP sensor to the throttlebody is intact and connected... this will make a RENIX 4.0 run very poor.

HTH

<edit> I've toyed with five RENIX 4.0l engines of varying mileage, and ALL of them have and/or still give a wild (high) idle condition once in awhile. Mine seem to do it when first started... cold or hot<edit>

this is probably a dumb question but whats an IAC and i hear it all the time but what is RENIX

jeeppowermj said:

this is probably a dumb question but whats an IAC

Click to expand...

Idle Air Controller... it's a little conical plunger device that engages a seat inside the throttlebody. It attaches to the front with 2 small Torx bolts...looking at the TB from the front, it's on the passenger-side. The Throttle Position Sensor is the other gizzmo on the front (driver-side.) Both are connected to the wiring harness and like nice tight/clean connections.

I remove the IAC before TB cleaning, and clean it seperately (this allows beter access into the TB housing).

The MAP sensor is centered on the firewall (black plastic mushroom looking thing with a nest of wires & a 1/4" approx pipe that connects to the passenger-side of the TB.

Idle stepper motor should be the same as the IAC.
And if I understand it correctly, RENIX is the company that makes a lot of the componants, especially the electrical systems) in Jeeps prior to 91 (when Chrysler put in their own stuff). Frequently it's used to differentiate 87-90 4.0L stuff from 91+ 4.0L stuff.
Jeep on!
--Pete