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Discussion Starter · #1 ·
I'm having a heck of a time figuring out why my plugs keep fouling. I can't go more than 1000 miles without noticing the bad idle and poor performance. Here are the specifics:

84 CJ7
9,000 miles on rebuilt 258
Excellent compression... between 165 and 170 on all cylinders
Lose very little oil... about a quart every 1500-2000 miles
Platinum plugs and new plug wires

The plugs -- after about a 1000 miles -- always foul the same way. Half of the plug will have a black oily residue, the other half will have a dry, hard, chalk like residue.

I've tried different fuels, fuel additives, etc. I'm considering doing the GM HEI conversion so the plugs will burn hotter avoid the fouling problem. But, I'm concerned that would just hide the real problem.


Go easy! You're talkin' to a greenhorn.

1,905 Posts
Is the tailpipe black and sooty, does it puff black smoke? If so,it may be running rich on fuel. Carter BBD is know for that. You might want to check the timing also. I'd check the cheep stuff first; adjustments, airfilter, etc.
good luck


Discussion Starter · #3 ·
Nope, there's no black smoke or soot. I have had problems with the Carter in the past, but I just rebuilt it about 2000 miles ago. The mixture, metering rods, and idle are all set correctly. The filters are all new and clean and the timing is right on. When the plugs are clean, the beast runs like a beauty.

Good ideas though. Thanks.

Go easy! You're talkin' to a greenhorn.

7,409 Posts
The HEI helped quite a bit but I'm in the same boat.
I'm getting 3 to5k on a set.
Honestly I don't like the Bosch plugs at all although I've not
had any better luck out of Chumpion or autolite.
I've seen some threads that were talking about the new ac delco
something or other. They'll be my next shot. Hope somebody has an
answer because I could use it too.
Your symptoms sound like it was my jeep to a t.
Good luck & we'll keep changing plugs in the meantime.

85'CJ7 258 4" runnin33's

Discussion Starter · #5 ·
One quart of oil in less than 2000 miles?....on a rebuilt engine, that sucks. Normal usage (excluding leaks) is less than 1 quart every 5000. That and the fact that you see oil on the plugs tells you that its oil fouling. Who did your rebuild? I would be willing to bet that they knurled the valve guides instead of replacing them. This is one area where you can't take short cuts.....and you are seeing why. One good way to verify it is to let the engine sit over night and then pull the intake off. Look in at the intake valves....(yes, you will be able to see the tops of them)...if you see oil on them then guess what? On a good head, they should be nice and dry.

Let us know what you find.

John......southern CA
84CJ7,3"lift,32"BFG,4.11's,ARB,Solid Axle's

Discussion Starter · #6 ·
I agree with Raduckal. My bet is the timing.

A conclusion is simply the place where you got tired of thinking.

Discussion Starter · #7 ·
There is a easier way to check for valves leaking oil,pull the plug boots off the plugs,use a black marker pen and put a line on side of plug facing up
take the plugs out and i bet when you look at them with line on top,the oily side will be up as well.Bad sealing valvesThe amount of oil you say it uses
is way to much,i have rebuilt engines that had 1/8 side play in the valves at max open and with new seals they didnt do this. they did
replace valve seals didnt they?its been awhile,are they still using the nylon "hard" type seals in rebuildson a 258? they are easy to crack if
you arent careful putting them and the springs on over them.


Discussion Starter · #8 ·
/wwwthreads_images/icons/smile.gif You guys just are not lazy enuff. /wwwthreads_images/icons/smile.gif

Let the jeep sit over night then when you start it the next day
have someone watch the tail pipe for the dreaded blue smoke.
That will show if there is leakage from the valve stems seals.
I have run some real junk for long term and would say that for
you to foul plugs at 1000 miles I'm sad to say you have something
wrong. If there has to be something lets hope it is valve seals they
are cheap and easy to change.

Discussion Starter · #10 ·
I have the same thing happen to me. I have the same jeep. Do you loose power and it chugs after that 1,000 miles? Well, hopefully I fixed it but not sure yet. From what I gathered carbon (the cause of the black plugs) has been building up because of a faulty automatic choke on the Carter. I put a manual choke after changing the plugs, and she has been purring ever since. Hopefully this is helpful and correct.
Happy Jeepin'

'82 CJ-7 258 [/]] ~AMC

Discussion Starter · #11 ·
ChaseJeep7 -- Yeah, she starts to chug and lose power as the plugs foul...after about 1000. Let me know if switching to the manual choke does the trick.

DDawg16, LoneEagle, & gregg tanner -- Well, per the advice ya gave me, I checked for symptoms of valve trouble. I'm sorry to report that your suspicions were well founded. I did the two easy tests (like gregg tanner I'm lazy). Using LoneEagle's test, I discovered the oily sides on the plugs are facing up. There was also a small amount of blue smoke at start up.

I suspect this is also the source of another problem I've been having -- failed emissions tests.

As far as the quality and specifics of the rebuild, I can't really say. I bought the Jeep from my brother, who bought it from another guy, who inherited it from his father-in-law, who had the rebuild done (did you follow all that?). I'm beginning to think there may have been a very good reason I got such a "good deal" on the Jeep.

Thanks for the help everybody! Although the news was bad, it's a good feeling to find the likely source of the problem.

Go easy! You're talkin' to a greenhorn.

Discussion Starter · #12 ·
Well it still may not be that big of a deal,maybe who ever did the rebuild didnt put new valve seals in,like i said before i have put new
seals on some pretty worn valves and had no problem like this.And if this is the case,they can be replaced without removing the head.
using a air compressor to hold valve shut,and the proper valve spring remover.Heck we used to use cotton clothes line rope,feed it into
cylinder and hand turn motor till piston pushed up against the rope which held the valve shut.Do a compression test,if it is ok,i would least
try new seals,cheapest way to go..Unless you have the money to remove it and have it checked and whatever it needs done.IE:knurled
guides,new guides?(i dont know if they are replaceable)valve job,new seals,and gasket head set..

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