whats wrong with my engine?

mbb1 · March 25, 2004

engine making odd sound. Kind of an exhaust sound like a bad cyl or something. dissapears over 2500 rpm to my ear. worse when cold.

new wires, dist cap, rotor, compression good, manifold pressure good. Not exhaust leak. Is this bad valve spring, or lifter, or something? 106K, mobil 1 5w30 always.

Oh yeah, only does it accelerating under load. wont do it if rev engine in neutral or park. Help. Hoping its something I can fix myself. No metallic tapping or anything like really.

chevroletmb · March 26, 2004

sounds like an exhaust leak

try reving the engine up in drive with foot on the brake so the

truck doesn't move. for some reson most exhast leaks

are only noticable under load.

citydriver · March 26, 2004

Pre ignition KNOCK maybe ....Check the base timing,if its to far advance it'l sound like marbles in th engine...ALSO check the exhaust flanges at the manifolds (as chevroletmb said) or take a good look at the flange area,any black marks are a dead giveaway of a leak

covvy · March 26, 2004

If it is an exhast leak it would be worst cold in the morns. Try this: Start the truck with the hood up in the morn then run around to underhood then rev it up by the throttle body and listen for the leak. This is the best way for as the engine heats up and expands then leak will become less prominant. If still not sure then get a helper and have them partially plug the exhaust with a rag and then listen for then leak. Ping usually gets worst as then engine heats up so I doubt that is your issue from your story. Good luck. (Oh and by the way, chev 350's are prone to cracking manifolds so don't be amazed if there is no evident leak at the flange. If you hear a leak and can't narrow it down then remove the manifold and check the underside for cracks. Mainly between the ports.)

Wingnut · March 26, 2004

Pre ignition KNOCK maybe ....Check the base timing,if its to far advance it'l sound like marbles in th engine...ALSO check the exhaust flanges at the manifolds (as chevroletmb said) or take a good look at the flange area,any black marks are a dead giveaway of a leak

If I'm not mistaken, the timing is adjusted on the fly by the computer...So messing with it is probably not a good idea in my opinion.

I had an anti-knock sensor go bad in my 92 (5.7 TBI) and it would detonate, ping, rattle, you name it if I got on it. I was thinking dead cylinder, cap/rotor/timing/etc, and was getting ready to pull the motor when a buddy with a code reader figured it out in about 5 minutes flat. I think I paid less than $5 for that part (salvage).

Is it throwing an SES light?

Cool Zr2 · March 26, 2004

If I'm not mistaken, the timing is adjusted on the fly by the computer...So messing with it is probably not a good idea in my opinion.

No, TBI's must be timed at zero degrees with the advance off. The 96 and up vortec are computer controlled.

mbb1 · March 26, 2004

Ill check again for exhaust leaks, restricting it sounds like an idea. Ive tried a stethoscope on manifold at idle and revving engine in park and dont hear anything abnormal. I thought I would hear something that way.

I checked base timing, its on 0. finding the connector was a pain , manual said it was in engine compartment, it was inside truck under glove box.

After sitting for a few weeks it ran really bad, undrivable actually. but I put in B12 chemtool and it cleared up and runs fine now, except for this issue. I figured that was a stuck injector. This sound has been going on since way before that happened so I think its unrelated.

wisconsinz71 · March 27, 2004

engine making odd sound. Kind of an exhaust sound like a bad cyl or something. dissapears over 2500 rpm to my ear. worse when cold.

new wires, dist cap, rotor, compression good, manifold pressure good. Not exhaust leak. Is this bad valve spring, or lifter, or something? 106K, mobil 1 5w30 always.

Oh yeah, only does it accelerating under load. wont do it if rev engine in neutral or park. Help. Hoping its something I can fix myself. No metallic tapping or anything like really.

I had a 92 that aftyer I put headers on I thoght had an exhaust leak. Would only be heard under the same circumstances you describe. Ended up being a noise from #7 cylinder ringing in the pipe. The way I found it was a really long 3/8 extension. One end on the pipe the other next to my ear. As soon as I did that I heard it right away. My guess is it is a valve leaking alittle. Was like that 2 years before I sold it and they guy who bought it is still running it--over 200k on it now.

mbb1 · March 27, 2004

sound seems to come from passenger side of engine

went out with engine cold and does make loud sound momentarily when revving exhaust with hood up, from passenger side.

On passenger side there is a metal shield around exhaust manifold, that a intake hose goes to to heat up some air on cold starts apparently, there is a thermostatic valve in the inlet duct for this. the shield seems to be riveted to the manifold, doesnt look removable.

underneath this shield ONLY my manifold is black, rest of it is pretty rust color. Im going to guess that the paint didnt just weather better under there. I may have a leak there. Is this a common place for a manifold to crack? can I remove the metal shield temporarily to check without destroying it? come to think of it , the last time I changed air filter, it looked "sooty" and not just dirty.