1999 Explorer with the 4.0l OHV. Run great for a few months then starts missing when first started and cuts out on light acceleration. Then a check engine light. Code 303. I change the sparkplug in cyclinder 3 and we start the cycle all over again.
The plug is not fouled in any way. It looks like new. I've swapped plug wires, and the coil pack. Compression is good. What confuses the issue is the new plug makes it run great, for a while. Once removed the plug from #2 and put it into #3 and it ran great again.
I am using anti-freeze ( But the timing cover is seaping a little) Pressure tested the cooling system and it held good. Using maybe a quart of antifreeze a month. No anti-freeze seemed to seaped in the cylinder. Performed test with hot and cold engine. I think perhaps and injector intermittently acting up, but why would swapping a plug cure it. The last symptom is when installing the plug I put a very fine amount of anti-sieze on the threads. When I remove the plug it seems to have built up a crust in the threaded area. I am not certain if all the plug do this. Any ideas?

 

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What kind of spark plugs are you using? Across several boards, folks have had issues with non-OEM style spark plugs causing driveability issues. If you have something like a multi-electrode plug you may consider replacing the spark plugs with OEM plugs.

Since swapping plugs seems to make it run fine for awhile, I'd suspect that you are not getting the plug wire, or swapped wires, seated correctly. Cylinder #3 is not a fun one to get to.

-Rod

 

I had the same issue with my 97 4L OHV engine. Had to pull the plug every 1000-1500k, finally out of desperation I ran a thread chaser down the hole..

It has now been 30,000k with not a miss. I found it easy access to run the front up on ramps and do the work from under the back of the engine.... Philip

 

This is no antifreeze in the oil. Running OEM type plug. Have used motorcraft,autolite and Bosch plugs. Already chased the threads and torgue the plug each time. I do feel a distinct click when installing the plug wire. Funny this is the only plug I am having trouble with.

Thanks for the feedback. Any other ideas
J

 

Can you get a cylinder leak down tester back there to see if you have a valve that isn't sealing properly? I actually ran in to a similar issue on my mom's 2005 Grand Caravan. The intake valve had a flat spot.

-Rod