97 F150 4.2 V6. When it first starts from cold, it purrs fine. After it warms up, approx. 20-30 minutes, it starts to make a knocking or popping noise. The oil pressure guage fluctuates with the noise. If I take serpentine belt off, the noise tends to go away. Makes no sense. I changed the AC compressor, no change. While on the highway, drove 60 miles, no loss of oil pressure. Noise and oil pressure problem tends to happen only in idle and more noticeable when stting with foot on brake and in gear. I am thinking oil pump problem (gears hanging up or pressure relief valve sticking), but with belt off the sound tends to go away. Possibly crankshaft bearing, but i would think that would make noise all the time. Any advice and/or suggestions other than dynamite!! Thanks

 

How many miles?

Did you take care of the intake gasket that causes hydrolock?

How is the oil level?

 

Truck has 147k miles. Looks to be original intake gaskets. The oil level is great, no water in oil. Drained oil yesterday and it looks the way it should, had 2700 miles on the oil change. I woke up thinkin about the truck and it come to me that the intake gasket(s) could be leaking just a little. When it warms up, it starts putting antifreeze in the cylinder causing the piston to stop momentarily, which causes the oil pressure to jump. When the belt is off, it does not do it because the water pump is not pumping the antifreeze. After it sits and cools, it does not do it when started because there is no pressure.

Would anybodys educated guess be that the intake gasket is the problem?? Is the 4.2 V6 known for intake gaskets going bad??

I really appreciate any and all advice I can get, I am a hard working person trying to keep my head above water without living off the system. Thanks, have a good day!!

 

YES. 4.2's built before 1/15/98 had bad gaskets. You may be well on your way to a hydrolock.

 

I appreciate all the help!! I think I found the problem, intake gaskets. I have it apart and will know later if it works. Here they are so anyone else with this problem!! Thanks again!!

 

Replace the upper and lower plenum gaskets and the front cover gasket with improved gaskets from Ford, not a parts store.

 

How do you run the truck w.o the serpintine belt? My 02 has 1 belt i take it off and run it for 5 min my motors toast.

 

You don't run it long with the belt off - just long enough to see if the noise goes away.