HELP! Phantom knock


Vehicle: 1991 Ford Explorer, 4WD, V6, 400,

167+K miles

Usage: Daily driver; 50+/- miles per day at 45 mph average

History of problem: Parked vehicle one evening after work; no problems noted. Used my back-up vehicle for a few days to give it some run-time. A few days later, the temperature had dipped to the low 20’s so I decided to drive the Explorer.

Problem surfaced immediately upon starting.

Problem: An interesting tapping/light knocking sound. It sounded a bit like a valve train rattle but was just a little louder. Increased rpms after a few seconds warm-up to a little over 1200 and the sound disapeared. Knock is present at idle but will “float-out” at higher rpms as long as vehicle is sitting in the drive but will continue knocking at higher rpm if the engine is under any load at all.

The noise seems to come from “everywhere” and “nowhere”. Listening without assistance and it seems to come from around the front center engine-or from the middle of the left(drivers) side-or from the middle of the right side but not from underneath. Try using a stethoscope, though, and it hides completely. Everything sounds normal no matter where the probe is placed. Try probing the oil pan and it doesn’t show at all.

Probing with the valve covers removed (engine runnning) and still nothing isolated as abnormal even though the sound was still “omnipresent”.

Heads have been removed. Heads look OK, valves look OK, pistons look OK, lifters look OK, pushrods are all straight. All wear patterns and carbon build-ups are uniform and do not seem excessive. No obvious slack was detected by looking at the pistons as the engine was rotated by hand but I have not yet put a couple of travel indicators on opposing pistons to see if they move in uniform amounts as the crank is rotated. Engine was run with belt removed and knock still behaved the same: there at idle and gone at rpm. Any good ideas before I put the top back on and see if it still sounds the same?

Thank you.



What viscosity oil are you running? Make sure you have your winter oil in (if you run something lighter in the winter). In my GLI, it was 20W50 in the summer, 10W40 in the winter, while my current cars use the same oil year-round (5W20 and 10W40).


I’m just guessing here but it might be a wrist pin in one of your cylinders.


Replace the lifters no matter how they look. You don’t want to use the old ones.


Valvoline, 10/40 for high mileage vehicles, synthetic blend.
Have been using it for seveal years in both vehicles year round with no problems.
Oil presure stays about mid-range winter and summer.
Thanks for the suggestion, though.


Try rocking the pistons by pushing one side on the top and then the other side. I’d do it with each piston at the top, center and at the bottom of the stroke not knowing what to expect. I am wondering if you have piston slap. A little is harmless. I have a car with a little piston slap that sounds much like you describe except mine goes away when the engine warms. The car has well over 100,000 miles; has had slap during most of that mileage and the slap is not getting worse.


The roller lifters in this thing cost around $35 each. I don’t want to throw that much money at a “maybe” fix.
Thanks, though.


I’ll try that check.
Would it have started instantly, though?
Prior to the morning the noise showed up, there was no indication a problem may have been developing.