98 Explorer, 5 liter, 63k miles - The symptom that I have doesn't match posts about piniging/knocking that I can find.

History and Symptoms:
Bought my explorer a month ago from an auto auction - no backgroung on repairs, although it was nearly spotless inside and out - no sign of any abuse. Check engine light was on all the time. The engine knocked steadily, when accelerating, but only during the first couple minutes after startup. Also, I see more water dripping out the exhaust than with other vehicles. If the engine is warm, the knocking only lasts about 30 seconds under load. It never knocks when idling or after it warms up, even when towing a 19 ft boat...... not a problem, once it warms up.

The first weekend, I pulled a plug and found some carbon and worn electrodes so I pulled and replaced them all. I didn't notice until after, that there was quite a noticeable difference in appearance between two plugs and the other six - metal deposits (pre-ignition?). But I'm not sure which two cylinders are in question. New plugs had no effect on the CE light or knocking.
Momentarily disconnecting the battery shuts of the CE light only for a few minutes of driving.
A week later, Autozone told me that the CE light was the front, passenger side O2 sensor. Still later that week a second Autozone test (different store) told me that it was definitely the front, driver's side O2 sensor.

Last week, first tried to clean (distilled water) the O2 sensor with no CE change. So I changed the O2 sensor and the CE light has not come on since.

I will change the original wires today and try higher octance on the next fillup.

I would check the intake manifold.......if I knew where it was.... my hanyes manual doesn't show this motor. I'm guessing that the 4.0's look different.

Something odd - the last time I disconnected the battery, then when for a ride, the CE light came on just as the knock disappeared.

Someone please help!

 

HMMMM, I've gotta say you'll have a little better luck over on the appropriate engine forum with this malady. Sounds like your timing is off for some reason or you've got a bad case of carbon build-up.

Good Luck.

 

thanks buzzard.... I did stumble across the problem while checking the lines connecting to the EGR valve. The 10" formed steel tube that connects to it was cracked. For some reason this crack has a much less obvious effect when the motor is warm. I was told that high temp epoxy won't hold up, so I replaced the tube. My guess is that the previous owner had some AC work done and that working near this areas created some stress and the crack...... problem is completely gone now.

 

I have a 1999 and have the same problem. It occurs only if left over night then taps for about 30 sec to a min then it is fine. No spark knock. It pulls a trailer fine no knock or tap. This only occurs after it has cooled down and sat for 8-12 hours. Hope there is a easy fix