I have this loud knock coming from the drivers side of the engine compartment. The thing is that it doesnt knock until the engine is completely warmed up(ten minutes or so). I tried to flush the engine with Gunk engine flush but it didnt fix the problem. This knock is pretty loud like I have a diesel engine instead of a 302. Any ideas on how to fix or diagnose the problem? Thanks Folks :-(


91 Bronco XLT 302

 

It's tuff to diagnose a problem such as this without actually hearing the sound. Is your oil pressure good as measured from a real oil pressure guage? Is it truly a knock is it more of a tick? If it is more of a ticking sound and is originating from a valve cover then it is more than likely a lifter problem. A lifter/valve ticking will usually only be heard once every 2 engine revolutions (unless you have multiple valve/lifter problems).
Diagnosing a knock is harder. A main bearing will have a real deep knock. A rod bearing or piston pin will have a louder sharper knock than a main and you will hear it 2 times per engine revolution.
I know this makes it about as clear as mud but is the best that I can do. A mechanic's stethescope may help but if you don't have one of those then a long screwdriver with the handle stuck to your ear and probing around engine with the other end will work. Just don't try listening to the fan blades!!!
Good Luck,
DannyP

 

One other thing could be detonation. You find this by adjusting your timing. Mark the original position of your distributor. Then make small adjustments rotating the distributor until the knock goes away. Also how many miles are on your engine. Could your valves be slapping your piston tops? This happens when the timing chain jumps a few teeth from being excessively worn out. Did you feel for exhaust leaks around the gaskets? Both the block to manifold and manifold to pipe. These are all posibilites that are hard to figure out without hearing it. I hope this helps some what.

 

Get an oil pressure guage like the other poster said, and if the needle jumps every time it knocks, you've probably got a rod bearing out.

 

take the fan off and watch the balancer to see if it wobbles as it runs. if it wobbles in rythem with the knock, it could be the mains. My 351 did the exact same thing... it had a broken crank. Ran great when cold... sounded like hell when warm...

 

Give this a good read. It has helped me in the past.

www.autosite.com/garage/encyclop/ency01l.asp

This section is about finding knocks.

Roger Lane