Alright, I have a 92 Cherokee 4.0. I just got done with a complete engine rebuild. Only thing that was not replaced was the block,pistons and rods. Everything else brand new. I went back with new Champion plugs, wires, cap and rotor button. I have a new coil from autozone because during the rebuild I found it to be cracked. I replaced the fuel pump today (bosch) because I had extended crank issues. Fuel pump took care of extended crank issue.

On the the idle issue. When I start the truck it idles great then starts to get a missfire like roughness. As soon as you give it gas it smoothes out and runs great. The other day it did it for awhile, the. Stopped and idled great for a couple of days. Then today I jump in it and I was afraid I was not going to make it to work it would start to Idle so bad. I have no CEL. Checked all vacuum lines all are intact. I believe my exhaust manifold is cracked but not sure if it is going to cause the problem. No engine noises and has plenty of power with no signs of trouble while driving at speed. Where is the best place to start chasing this issue at? Can the O2 sensor cause it to do this? Its just frustrating because I want get it running perfect before I move on to other stuff on the project list. Thanks guys I appreciate any information

 

Might start by checking the TPS voltage, not sure if 92 is still adjustable.. Operate the throttle slowoy by hand a couple of times, make sure the voltage increases smoothly as the throttle opens and returns to a stable, consistent value when it is closed. (engine off, key on)
Also have a peek at the IAC and make sure it isnt' gummy or sticking.

 

Edit..Double post

 

I'm pretty sure the TPS is not adjustable beyond 90. Like Radi points out the IAC is a common cause of idle problems. (since we no longer have an idle speed screw, it more or less does that). If you click on my signature, there is a Write-up Cruiser did for cleaning the earlier, which I'm pretty sure are the same.

 

TPS is non adjustable. IAC motor has beeb cleaned as well as the throttle body and all of the passages in the throttle body. I did that when I was rebuilding the engine. I also did pull my plugs. Checked for any cracks, no cracks and all appear to be burning fine. No wet plugs or black plugs. No smoke from the exhaust either.

 

My issues started when my intake cracked. Replacing the manifold and MAP sensor solved it.

 

I was thinking MAP Sensor. I will smoke test the intake to see if there are any cracks.

 

Try disconnecting the MAP sensor. Mine was driveable that way until I got my new one.

 

Ill give it a try.

 

A cracked exhaust manifold can let (suck) air in pre-O2 sensor, royally throwing it off. especially at idle because the exhaust doesnt have as much pressure behind it. it could cause rough idle and bogging down on acceleration. especially if it only happens after it warms up and the engine goes into closed loop.

just my two cents

 

My 98 is doing this too... about the same situation as the OP too. I'm thinking it my valve springs.. (there is a TSB for 4.0 engines about this)

 

?????????

 

Soo, today I tried to fire it up had a really long crank. Only other thing that would cause that in my opinion would the fuel pressure reg on the rail. NAPA had one. Put it in fired it up and ran for 15 minutes fine. Shut it off and left it sit for and hour. Went back out and it fired after first try. So that is solved let it idle a few more minutes and turned the a/c on. Then the rough idle started. I noticed that when ever it would start running rough the a/c compressor would turn off. Like the computer would shut off loads to try to correct idle. I turned the a/c off Idle returned to normal. Off an educated guess, i figured that the idle air control valve could not keep up with the loads on the engine. So NAPA had one of those. I threw it in, pulled the battery cables for about 30 minutes. Fired back up and ran for an hour with a/c on without a hiccup. I am not saying it is fixed but, right now seems o.k.

 

I was curious if the TPS output voltage was holding steady at idle, or jumping around. It appears it doesn't matter, lol. IAC FTW!

 

No..I mean I hope you didn't get me wrong Chuck. , mentioned the TPS CAN f with the idle. Both the OP and trucken, didn't post anything on checking that is why the ???

Whoops again!

Might be that Cruiser also mentioned a TPS can affect idle. Not being an adjustable TPS on a 90 or earlier Renix, doesn't mean it can't be checked.