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View Full Version : 88 XJ Misfire/hesitation....HELP!!!!!!



da9r00d
02-19-2010, 11:37 PM
i have a 1988 Jeep 4.0L, it misfires. It has a new coil (MSD blaster), MSD 6a Box, distributor cap and rotor and wires. still misfires, it was misfiring a while ago so i changed the CPS and CKS and it ran fine. but now im back to the same thing. is there any test i can do to find the problem. so i changed my Plugs... Even thoe they are new (2months old) from Bosch to Champion. big difference, runs way better idles better. But.... when i press the gas raising the RPMs slow it runs fine, but when i Smash the gas it starts to misfire. what could it be? coil? Wires? im gonna try and put the stock coil back on since it was new

W.O.R.P.Offroad
02-20-2010, 02:08 AM
could be your tps or the O2 makin it overload or run really lean. if its only when you really get on it leads me to thin more tps. not reading WOT right could cause to much or not enough advance causin it to cut out. you can try adjusting and cleaning the tps.

pvt.Tadpol xj
02-20-2010, 09:07 AM
How about your fuel filter? and have you tried putting the anti-oxidation compound around all of your plug-ins..put a little in and around and work the plug in and out a few times, but apply it sparingly. use some of the rounded type wooden tooth picks first inside your plug-ins to clean them out, but do not over size your plug-ins by forcing in the tooth picks...

xj287
02-20-2010, 10:19 AM
All of the above, also check simple things like valve springs, cmpression, plugged cat, dont forget the sensor teeth on the flywheel, too much crud and the crank sensor wont read right. Best thing to do is find some one with a Renix scan tool plug into the OBD data link and see what the computer in reading. With out reading the computers data stream, it's near imposible to diagnose an EFI engine effectivly.

No scan tool for Renix? Snap-on tools usealy has used MT2500's with AMC software and cables for around $250.

4.3LXJ
02-20-2010, 10:44 AM
If I remember correctly, the tps on the renix needs to be reset now and then. I have forgotten how, but Blue knows. Also, I agree with the O2 sensor thing. If it has been some time since it has been changed, change it. They do wear out before they throw codes.

da9r00d
02-25-2010, 12:44 AM
i went back to the stock Coil and ignition... did same thing. went back to the MSD, it runs fine now. i have know idea what happened but i works now.

Also how do I Adjust the Idle?

pvt.Tadpol xj
02-26-2010, 08:26 AM
You have to look very closely there at your fast idle cam. There where all of your cables attach to your throttle body. There is a set screw on the bottom of your throttle body. On some XJ'S there is just enough room to get a 3/32 allen in there to make the adjustment. After the motor is warmed up....:cool:

xj4life2
02-26-2010, 10:30 AM
You have to look very closely there at your fast idle cam. There where all of your cables attach to your throttle body. There is a set screw on the bottom of your throttle body. On some XJ'S there is just enough room to get a 3/32 allen in there to make the adjustment. After the motor is warmed up....:cool:

Thats the air bleed adjustment screw.

da9r00d
09-06-2010, 03:53 PM
im getting a misfire when i have the gas pressed only a little bit, does not matter what RPM, but if i hold the gas down it revs fine. i have everything new on the ignition and new injectors. new CKS and CPS sensors. gonna try and change the 02 sensor. and the intake/exhaust gasket.

da9r00d
09-28-2010, 04:11 PM
Still need help...... New knock sensor and new coolant sensor, NO CHANGE still misfires.

I think im gonna drive it off a cliff... but it would prob still be drivable after that.

im gonna take it into a shop and let them deal with it, ive already spend $1000 in sensors and ignition parts with no change

ice_cold
09-28-2010, 08:04 PM
Watch, it will turn out to be a plugged fuel filter or a pooched fuel pressure regulator.
Just a thought.

4.3LXJ
09-28-2010, 08:24 PM
Ever check those injectors?

da9r00d
09-29-2010, 02:13 PM
i have new injectors