Rough idle.. 92 cherokee

Asked by Jul 24, 2015 at 03:22 AM about the 1992 Jeep Cherokee

Question type: Maintenance & Repair

OK; Jeep comanche, actually, but identical to cherokee except metal rearward of cab..
Down for about 1-1/2 years.. until I could do clutch job that turned into replacing tranny to be safe on one issue. Guess what? Wrangler AX15 bolts NP231 @ 10 degrees up over cherokee. This fix took longer.. but got it going.. ran fine.  Replaced leaky Oring on injector(s).. did all. Ran fine.. but after getting tank refilled ran crappy. Stalled and had towed. Eventually finding an open in CPS; verified problem 3 times before replacing with new CPS. This would also end with a brand NEW distributor (cap, etc).. and later, a second NEW distributor.  
Oh.. this is a rough idle that cycles.. almost dies then catches itself.. sputters, lags on opening throttle.. pops.. and even at higher rpm, same thing just higher rpm's. Replaced fuel pump. Replaced fuel filter.  Fuel pressure gauge indicates proper fuel pressure of 30-31 psi and runs up to 40 psi when vacuum hose disconnected to regulator. No gas our of regulator. Fuel pressure RISES when the engine is gunned. (no pressure/regulator issus).  Replaced spark plug wires with new premium. Replaced coil with extra. Then new coil. Still same. Swapped computer. Same. Swapped MAP, MAT, TPS, injectors,..  still no change. For the heck of it, replaced the fuel regulator. Big surprise.. no change.  Played around running spark plug wires up and down one terminal.. then it wouldnt start.  Back to where they go. Tried starter fluid over all vacuum lines.. no change. Removing from air filter makes no difference. It winds up, even if rough.. so dont think there is any back pressure to be of concern..
Spark plugs are Bosch platinum and look fine...  (Iv ran this brand of plug for over 100k and have them still look great..  I believe in them. Besides, the sudden failure of the engine isnt likely to be suddenly all plugs failing.  Will try.. but not holding breath!
Could be new fuel was bad..??  IN 45 years of auto maintenance Iv never actually seen that happen...
Man.. Im out of options!  After these two options.. Im going to pull all the sensors and test the wires between computer and sensors and controllers..
I have the jeep FACTORY manual.. and am WELL accounted with it, am an engine rebuilder, particularly the 4.0/4.6.. and was one of the first to build the 4.0 into a 4.6.. back in the early '90's (one reason my manual is well worn!).  I cant see the IAC causing this issue when it stays.. even at higher rpm's.. so ANY ideas, Anyone?????

1 Answer

18,810

I see you tryed a lot of things but did you clean the throttle body did not see you mention doing that in your question and also clean the IAC add fuel injector cleaner to gas tank. Google how to clean throttle body and IAC valve on your year, make and model of your vehicle.

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