Okay, my buddy was towing a trailer with his 94 grand cherokee, and all of a sudden it started missing. This happened under load, trailer was about 2,000 pounds. He pulled the spark plugs, and on the number 3 plug, it looked like these pics. One side kind of reddish brownish, the other side looked like it had oil caked on it. I was thinking maybe a bad valve guide, causing oil to drip down onto the plug, and then the plug would heat up, and bake the oil onto the plug? Do 4.0's have problems with valves/guides, etc. very often? Any suggestions or ideas would be appreciated. P.s. sorry, the pics didn't turn out very well.
Jeep 4.0 weird engine prob.
- Thread starter spiderman
- Start date
Spork
Tin Foil Hat Equipped
- Location
- Saratoga Springs, UT
Other than the 1.3 option 
How long have the plugs been in?
How long have the plugs been in?
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pokeyYJ
Guest
With a spark plug discoloring, maybe the car overheated? I too would try and replace the plugs and maybe check the wires for too much resistance, there should be EXTREMELY low resistance in plug wires.
Question? Isn't redish brown indicating the plug was too hot?? Who's got a Haynes manual???
Question? Isn't redish brown indicating the plug was too hot?? Who's got a Haynes manual???
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pokeyYJ
Guest
After looking at the pictures closer and consulting with a coworker I think that maybe you were getting some pre-detonation, look at the second picture and see the small white portion on the electrode area, looks like it got chipped. Could have been cause by too low of a grade of fuel.
I think that maybe you were getting some pre-detonation, look at the second picture and see the small white portion on the electrode area, looks like it got chipped. Could have been cause by too low of a grade of fuel.
crimsonride
huh?
- Location
- Murray
pokeyYJ said:After looking at the pictures closer and consulting with a coworker
Especially under towing, although, your computer should advance/ your timing and acount for this. You also should have been able to hear detonation, seems like the most likely problem to me though.
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pokeyYJ
Guest
crimsonride said:
This is true, but too a point. The computer can only adjust the timing to specified values and no farther. This is esp. true with pre-OBDII vehicles, and, if I am not mistaken, the pre-OBDII Jeep 4.0's were distributor contoled spark management, thereby limiting the available adjustment even more. Vehicles with distributorless ignition have more control over timing advance, but again only to certain preprogramed limits.
Sorry for the delay, haven't been at the computer for a while. Did a compression test, and the cylinder with the bad plug tested 10 pounds lower than any of the others, but none were very high. No preignition/detonation that the owner is aware of. He always puts in some octane boost crap. But the new plug is starting to do the same thing. I am leaning toward a valve guide or something like that. Oh, well.
rustybronco
Flat Land Offroader
- Location
- Illinois
Sometimes detonation can't quite be heard.