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Plugs Fouling Bad???


NAXJA Forum User
I have had Bosch platinum 4 plugs for about 12,000 miles and about 2 weeks ago the jeep started running really rough and and missing horibally. It just happened all of a suddon, like in one day. I took the cap, rotor and plugs out and cleaned everything. I took the plugs to the wire brush and got them all cleaned up. I put it all back together and it started up perfectly and ran great like it had been previously. Then yesterday it did the same thing so I suspected the cap and rotor because it had been carboned a little and didn't think the plugs could have fouled already since I just cleaned them up 2 weeks ago. So I put a new cap and rotor in and it solved nothing. I pulled the plugs again and cleaned them and it fixed it again. i also noticed lately that in neutral, or ever since it started giving me thes problems, that the jeep has been reving at a constant 1200 rpm in neutral. i found an unplugged vacuum line and plugged it back in and it brought the idle back down. My question is, will the unplugged vacuum line and high idle cause the plugs to foul like they did?

I have never had good luck with plats, the 4.0L and the 2.5L both like the copper champion plugs...YMMV
An unplugged vacuum line causes a loss of vacuum. This makes the engine computer think that the throttle is open more than it really is, allowing more air into the engine, and concludes that the fuel mixture is lean so it enriches the mixture by adding more gas(keeping the fuel injectors open longer).

The extra gas that was being injected into the engine could have been the cause of your spark plug fouling.

If the problem disappears, you know that was the cause.

no no no don't you ever use bosche plugs again. that's the worst thing you can do to you rig. I know first hand bosche plugs are the worst especially the four pronged ones. Granted the vacuum leak didn't help but..... 12k miles and thier fouled. Think about it
I had the single prong boschs for 30,000miles on my first set and began having hard start problems so I ended up getting another set of plugs, what they had in the auto parts store, which just so happened to be Bosch plugs again. I installed those and put 65,000miles on them. I replaced those only because I was up for a smog test. They looked clean when I pulled them, the center electrode just had some noticeable wear. My third set are Autolite platinum. Still running fine..