My truck has a pronounced "knock" from the engine.I tried to follow the noise with my ear but the sound seemed to come out from the tailpipe.I got a new exhaust system and the knock is still there.The knock is none existant if I race the engine in neutral.The knock is only present under acceleration.The knock lessens in intensity when cruising,and increases in intensity if the engine is laboring( ex. overdrive at 60mph up a mild incline).The knock is always w/in 1/4-1/2 engine rpm,cannot count exactly. I just did a compression test w/ a screw in the spark plug hole type gauge and 7 cylinders were b/t 162 psi and 168 psi.However,the cylinder that is one the trucks left side, 2nd from the firewall measures only 144 psi.Over all the engine is strong and although it knocks constantly runs quiet and smoth at idle.Oil pressure at 2000rpm is 30psi,but it drops to 10-15 at idle,except for a cold start it is aroung 45 psi at idle.

350 TBI Chevrolet Z-71, Any clue as to what and how to fix?

 

I am no mechanisc but it sounds like spark knock. Maybe you got some old/bad/wet gas ? You can try running a little higher octane fuel or get your fuel somewhere else than where you usually buy it if you haven't already done this. Maybe a bottle of dry gas and/or fuel injector cleaner in the gas. Make sure they are safe for fuel injected engines. How many miles since the last engine tune up ? A tune up may possibly help. How old or dirty is the air filter? Check that also. Just a few suggestions. Good luck.

 

Is it knocking or is it more of a ticking sound? If it is knocking/ticking at an idle, it most likely isn't Bad Gas. Bad gas causes more of a Chattering sound and is generaly most noticable under a load. Your Oil Pressure is fine. Without hearing it in person, it would be hard to do anything but guess. If it is coming from the top end of the motor, you may have a Lifter going bad. If the lifter is bleeding off, it will cause your rocker arm to be out of adjustment causing a ticking sound. Exhaust leaks can make some pretty strange noises too. I know you said that you replaced your exhaust, but did you replace the exhaust manifold Gaskets? Like I said, without hearing it these are all just possabilities.

Good Luck.

 

I was thinking it might be in the valvetrain somewhere near the cyl w/lower pressure.It is not the noise caused by detonation.I have plenty of experience with a car doing that .The caprice my dad let me drive in high school would keep on running for about 5 seconds after I turned it off.The noise is hard to hone in on but when I got a new exhuast system,I had it routed out the opposite side ofthe truck than the orignal,and the noise seemed to follow.The noise is a harsh metallic noise.

 

Here's my 2 cents...

Probably not a lifter...that noise would b there all the time. Probably not a fuel problem...all cyls would b makin the noise. Probably not a rod brng...that would rattle (knock) big time if u held the rpm bout 2k with no load. Sounds to me like u got a cracked piston skirt or a loose piston wrist pin. The skirt will make more noise cold. The Pin will make more noise warm...when revin the engine to bout 2k with no load...as it revs up...it will knock...as it revs down it will b quiet. To figure out which cyl it is...disconnect and ground the plug wires...one at a time...till the noise is gone. Either way...next time buy a Ford!!!! heehehe

Good luck!