Mechanics

NEEDS THE #3 SPARK PLUG CHANGED EVERY 2 MONTHS

1999 Ford Explorer • 6 cylinder 4WD Automatic •

I have a 99 ford explorer with a 4.0L OHV engine that needs the #3 spark plug changed every 2 months or The following symptom occurs every 2 months.

The following symptom occurs every couple months then goes way completely after the spark plug is changed. On start up the engine will have a noticeable miss then after about a minute it will clear up and run fine. When driving 40-50mph light acceleration or cruse the engine will start bucking( missing) seems to clear if down shifted or accelerated. I tried ignition parts coil, wires.

The compression is fine. Ford changed ignition parts and intake manifold gaskets 8yrs ago and70K miles. Like stated before a new plug always makes it run good for while. No fouling or abnormalities on plug. Any ideas? Thanks, J
Avatar
Jwhit
April 29, 2012.




Ok your PCM has 3 diff outputs to the 3 coils the coils share 2 plugs each. Since only 1 plug is acting up that points to the plug wire for the suspect cylinder. Does the plugs color on that cylinder match the other same coil plug?

Tiny
Fixitmr
Apr 29, 2012.
I have already traced the plug wire. Ohmed it out to confirm and even swapped the physical wire from cyl 2 and three. Still made no difference. The plug in cylinder 3 looks the same as cyl. One time I swapped plugs from cyl 2 to 3 and the problem went away for 2 months.
Thanks,
J

Tiny
Jwhit
Apr 30, 2012.
Have you compared the spark strength/quality on that cylinder?

Tiny
Fixitmr
May 1, 2012.
Yes, and the cylinder relative to other cylinder seems fine. I have heard there where issues with the cyl head cracks etc. I tried removing the plug and re-torquing it. That did not work. No signs of head gasket issues. Any ideas?

Thanks,
J

Tiny
Jwhit
May 2, 2012.
Try a hotter heat range plug? I had plug problems with a tempo and cured it by going to hot plug.
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Tiny
Fixitmr
May 4, 2012.
Same car, same problem. Only difference - cracked ceramic insulator on the spark plug #3.

Tiny
Pawel
Jun 1, 2012.
I have changed this spark plug at least 9 times since I've owned the vehicle. I torque the spark plug and ensure the gap is correct each time.
Explorer starting to act up again. I think I'll try the hotter plug idea. Will be repairing a valve cover leak soon, so I think I'll change the inject in that cylinder too. Can't hurt

Tiny
Jwhit
Jun 2, 2012.
I chased this problem for several years. You have a small crack in the head between the intake valve and exhaust valve. This is very common, perhaps inevitable on a 98T head. Coolant leaks into the cylinder when the car sits and it runs rough for 10 to 20 seconds after a cold start. After 2000 to 3000 miles the coolant contaminates the ceramic on the plug and it becomes electrically conductive and causes a misfire. You change the plug and you are good another 2000 to 3000 miles. This occurs on the #3 cylinder (back passenger side) and #4 cylinder (front driver side). K& W block sealer may plug the leak a bit and you can get more than 2000-3000 miles out of a plug. The only real fix is an aftermarket head that has more metal at the point where is cracks. That's what I did for mine. Google "Ford 4.0 ohv cylinder head crack" to see pictures. Good luck.

Tiny
Jimschirle
Jan 17, 2013.

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