I recently purchased a 99 f150 with the 5.4 in it. It had 85,000 miles on it.

When i bought it there was an exhaust leak in the intermediate pipe which made the exhaust pretty loud. however after i fixed the leak i noticed an issue with my engine.

At around 1500 rpm or higher i get an engine knock. it honestly sounds like a hammer on metal that follows the rpms perfectly. It doesn't happen at low rpms at all. It is a very consistent noise. Its the same sound that is the same level of noise and the same patter to the sound (identical to the rpms) everytime To me it sounds like a woodpecker banging away, and under 30 mph can be easily heard with the windows down. It can be heard in gear, in neutral, in park it doesn't matter. It doesn't make any sound cold. repeat until the engine gets warmed up you can rev it up all you want but it makes no sound at all when cold, sounds like a nice beautifully running engine. only when it gets to normal temperature does the noise start.

I gave it brand new motorcraft plugs as a tune up and upon a couple of people's insistence an oil change. I drained the old oil which was really dark but no obvious shavings or anything and added 6 quarts of Valvoline conventional 10-40W and one 32oz container or Lucas Heavy Duty Oil Stabilizer. oh and a pureolater pure one filter from advance.

After 50 miles under this new setup i notice no difference in the noise whatsoever. its still there and still the same volume. I have no idea how the previous owner treated the truck.

I am an average mechanic that can handle almost any job, but i really wanna pinpoint the source of the problem and the cause before i waste time and money into it.

She runs fine, no issues with acceleration. every now and then she might idle slightly ruff but no big worries. It gets 13.5 mpg consistently.

 

Check to make sure the plugs are torqued properly, one of them may be loose and ready to launch out of the head. Check to make sure they are torqued to about 28lbs. The noise you describe may be the plug wobbling around in the head

 

I would switch to a 10w30 or a 5w30 , but definetly get that lucas out of there.

 

5/20 is the proper oil. Make sure it is not an exhaust leak you are still hearing. They were common to leak at the manifold on the passenger side and can sound like a knock. But... they are normally more pronounced when cold so I don't know. Get a mechanics stethoscope and pin point the knock. Could be a timing chain guide, bearing etc...

 

x2 on the exhaust leak. I've had simular deals turn to be small leaks. Another trick is to look at the exhaust system in the dark you can usually see the leaks.. Had a header leak in my Mach I once that I was sure the engine wast toast. Metal hammering sound.. BTW Lucus is crap..

 

Do NOT put 10W40 and/or Lucas into a Triton!