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Random misfire under load?

C

chpmnsws6

NAXJA Forum User
Location
Springfield IL
Hey folks, I finally got the XJ back home. A new coil got it running agian, but its having a misfire that is prevalent under load. We were pulling round bails with it and it acted like a diesel pull truck when you are throwing too much fuel at em. It'd spit and sputter, then take off.

Ideas?
 
Dont know what killed the coil, but I'd change the plugs if you havent already in case they got carbon fouled from the coil misfiring. Change cap and rotor as well if ou havent. The connector on the coils gets loose often times leading to multiple coil replacement when the connector is the culprit. Wiggle the terminal going into the coil while running listening for RPM changes or complete die out.
 
Cap and rotor are new, but I'll check em again. CPS is new. I'll swap the plugs and wires (5k old on each) and wiggle the wires around. Thanks for the ideas!
 
i was gonna say cps, but if thats new, my next guess would be the pcm/ecu
if you know someone you could swap it out with, that'd be a good test

i was also going to suggest tps, but i think thats good since you dont have this problem under normal conditions. does it happen when you race the engine from idle?
 
What year/engine?

Test your fuel pressure and compare it to factory spec. Must be tested with a gauge; anything less is guesswork. Gauges can be rented from many of the big box parts stores.
 
Its a 97 4.0 XJ with a 231 case.

I'll check FP tonight. Was going to last night, but couldn't find my gauge.
 
I had a similar headache and it turned out to be the TPS, FWIW.

After testing the TPS and reading its erratic voltage signal to the ECU, it was apparent a replacement was needed on my rig.
 
I had to put a new TPS in it a few months back so it might be out again. It has 50psi of fuel pressure, distributor was tight, and wires all ohm out fine.

I'm going to pressure test the intake and see if it has any leaks. A new MAP might be in order, though I don't think its the problem. Its in the engine. When in park or neutral, it still acts the same.

This thing has me stumped, so I thank ya'll for the ideas!
 
Note from DJ
My first test would be to go out to yore Jeep after dark armed with a spray bottle filed with some water and adjusted to a fine mist.

Raise the hood have a dim background light on just bright enough to see what you are doing then start up the Jeep and carefully mist down the distributor cap, then each spark plug wire and spark plug.

And look for arcing.
 
No arching seen. Changed the plugs and wires but no change. How much play should the distributor have? It has a bit of side play and around 1/8th inch or so of play back and fourth.
 
Stuck the old OEM coil back on it and the misfiring slowed down IMMENSELY. With all the play in the distributor, I'm going to replace it just to sleep better, but I'm hoping it'll help.
 
You put the coil back in that originaly was dead with an improvement? Sounds like the primary input connector is crudded up. Did you try cleaning and crimping the primary terminals for better electrical connection?
 
To be real honest with you, I put a new coil on "just because" when I was doing the tune-up last time. The old one worked fine, with no corrosion noted. Can a bunk CPS cause this? It was replaced with a china special at the shop it sat at dead for a week out of town.
 
Switched the Crank position Sensor to a BWD unit and it purrs like a kitten!

So in the heat of the day on the highway, the truck smoked the ignition coil. The shop it was towed to put a cheap CPS to see if that'd fix it, which it did not. The next day I put a new coil on it which got it down the road, but misfiring badly. Putting a Borg Warner CPS brought it back to snuff, and the OEM CPS was probably still good.
 
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