1999 Ford Explorer Intermitten misfires

MARTY0503

MEMBER
1999 FORD EXPLORER
6 CYL
4WD
AUTOMATIC
191,000 MILES

I just bought a '99 Explorer XLT with the 4.0 SOHC engine that has 191000 miles on it. I keep getting intermitten misfires, the engine codes say it's always cylinders 4, 5 and 6. I changed all the plugs and wires and this didn't seem to help. When I first start up in the morning it seems to be ok, then starts to miss after a few minutes of driving. It comes and goes, I can feel it bogging down during acceleration then it will leap forward when it starts firing right again. I've been told the coil pack probably isn't the issue. I also have a o2 sensor out but I'm pretty sure that is unrelated. Any help will be appreciated, thanks!

1 Reply

DAVE H

MECHANIC
13,383 POSTS

Check/clean or replace the CKP. I would not rule out a faulty coil pack !

Models Equipped With Coil Pack(s)
The EI system consists of a Crankshaft Position (CKP) sensor, coil pack(s), related wiring and PCM. The CKP sensor is used by the PCM to indicate crankshaft position and speed by sensing a missing tooth on a pulse wheel mounted on front of crankshaft. The coil pack(s) receives its signal from the PCM to fire at a calculated spark target. Each coil within the pack fires 2 spark plugs at the same time. The plugs are paired so one plug is fired on the compression stroke, and the other plug fires the mating cylinder, which is on the exhaust stroke. On the next cycle, firing strategy is reversed.
The PCM acts as an electronic switch to ground in the coil primary circuit. When the switch is closed, positive battery voltage applied to the coil primary circuit builds a magnetic field around the primary coil. When the switch opens, power is interrupted and the primary field collapses inducing high voltage in the secondary coil winding and the spark plug is fired.