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 would try using almost all of the gas in you tank then trying a higher octane gas . allot of the times a 350 will knock on low octane fuel in the winter . that has bean my experience anyways . try it and let us know how it worked for you .

 

Go 1 step hotter on the plug (If at stock).
bump up the octane.

How many miles on it??

My '96 350 vortec (176000 miles) does(did) what you speak of. My low compression cylinder is 8.

Dont sweat it. Give it a tune up, change the oil every 2500miles, and take it easy on her,,,,she'll be ok.

Chevrolet quote: Brand new engine,,,,$2700, One year warranty,,,it doesnt matter who installs it.

 

Sounds like you're referring to the dreaded Chevy piston slap. It's a well known issue. Alot of people are complaining about Z-71s with piston slap. I saw where some guy was trying to start a class action lawsuit because of the slap in his new Z-71. I have a Z-71 but luckily I don't have the knock in mine. Chevy says it's not a problem but I know what piston slap causes in a Polaris....a shattered piston.
I think you're wasting your time trying to stop it because it's probably there to stay. Visit a few Chevy truck forums and you're sure to see someone talking about it.

 

I had what the called piston slap in my '97 F150. It only occured when it was cold. Ford ended up putting in a new motor but I started using Mobil 1 pure synthetic oil before the engine was installed and walla... no more piston slap!

 

The cylinder that you have the lower compression is the #6 cylinder. 144 psi is ok. Anything above 100-110 psi will still run smooth. Is your Z-71 a manual or auto trans. If it is a manual trans there is a fall out bearring in the clutch that can go bad and can knock at certain rpm's. Not saying that this is the problem but this will sound like an engine knock. Now does it sound definately like a knock or more like a ping. If the timing is off or the fuel is a low octane it will ping under a load and going up inclines. Try a high octane gas and maybe get some octane boost to see if that clears it up if it is a ping. You need to know if it is a knock or a ping. This will help. I am a certified ASE mechanic so i will help you as much as i can. It is hard to determine a problem without hearing it.

 

This is not detonation pinging.My dads old caprice did that under load and kept running after the engine was turned off. It had a rapid irregular sound when it was pinging.
This noise is a CONSTANT when ever the trucks engine is under load.Snap your fingers,now imagine that your finger are metal,that is the sound.Lound,definate and constantly in N'Sync(bad choice of words lol) with the engines rpm under load.It is one knock at a time,not a bunch of random overlapping nioses that happen from detonation(pinging)It is signifgantly less than engine rpm, but is speeds up and slows down in proportion the the engine rpm under load.The more load,the louder the noise, cruising the noise gets quieter,but still CONSTANT. Like I said before,revving in neutral,the engine makes no unusual noise.Gasoline grade changes nothing,I tried that. It is an 700R4 automatic,w/about 300 miles on the rebuild.

 

Bad lifter or cam lobe getting flat?