Home Forums Gear Hunting Trucks and Four-Wheel Drives Chevy 350 engine noise

My 77 K10 pickup has started to make a nasty noise and has lost a lot of power. The noise is "like" a knock but seems to be ignition/timing related rather than a rod or valve gear. It is not as regular as knock usually is (not in sync to RPM) and sounds like it comes from under the intake manifold. The noise stops at certain RPM ranges. The truck has also started to backfire up through the carburetor when it the engine first starts. This is not the original motor it is a replacement(of unknown origin)with electronic ignition.

I thought about pulling the timing chain cover but A) that is a lot of work for a troubleshooting exercise and B) I read on the internet (so it must be true) that it is virtually impossible for the Chevy small block to "jump" the timing chain. It seems equally unlikely that the distributor somehow "jumped" a tooth but something happened.

Any ideas? Tricks for isolating the issue?

Its only at $1500 beater but my only transportation at the moment. So I want to get it fixed but I an not anxious to drop a new motor in it. I'd more likely start looking for another beater truck if it can't be fixed pretty cheaply.

Thanks

If you can get a hold of a timing light, it would make for an easy sanity check on the ignition timing. While it's unlikely that the distributor has jumped a cog, the screw for adjusting timing could have come loose. Along the lines of easy, get a can of starting fluid and check for vacuum leaks.

Did this start suddenly? I'd pull valve covers and check pushrods or rocker arms. Pushrod may have broke through the rocker arm tip. Simple check.

Wayne

An exhaust valve stuck closed will make a godawful thunking noise through the intake.

I pulled the valve cover off the side that seemed closest to the noise. The push rods and rocker arms look good. Everything looks like to goes up and down properly.

Use a stethescope to try to pinpoint the noise. If you don't have one, use a wood dowel. Hold the end against the engine and the other end in your ear. Don't let the fan hit it while it's in your ear.

Long screwdrivers work as well.

Or a piece of fuel line tubing.

A stethoscope will definitely help narrow down the noise.

The backfiring could be sign of improper timing, a sticky valve, or a worn or stretched timing chain and especially if it's a high mileage engine. The irregular knock might be the timing chain hitting the cover but I'm not sure about this. It's unlikely and I can't say from experience if this is even possible.

The easy way to check the timing chain is pull the distributor cap, put a socket on the crank, turn the crank back and forth, and watch the rotor to see how much slop is in timing chain. The should be some slop but not an excessive amount.

The next thing I'd do is check the ignition timing and then do a compression check on all cylinders.

This is a stretch but I'd add some Seafoam to the oil (6 ounces) to see if that might help free a sticky lifter or valve. It won't hurt but then again it may not do anything.

If you want some more expert advice go to fullsizechevy.com and join up. There's some very knowledgeable chevy gurus there. Here's the link ...

http://www.fullsizechevy.com/forum/forum.php

if you are backfiring through the carb you may have an intake valve issue, either chipped, bent stem or not closing well. Intermittent could be due to something like a broken valve spring which would have you relying on compression to slap the valve shut. Broken or weak valve spring could also cause some return lash (excessive) in the whole rocker, push rod, lifter, cam lobe motion delivery.

dig?

Pull valve covers and twist on each valve spring with valve in it's closed position. This will be the spring's most relaxed position, it should offer some preload on the seated valve, holding it closed.

A broken one will be obvious, weak springs could be random here or there caused by the spring fatigue or possibly a sunken valve.

I had an old suburban with a 350 where the lifters would start sticking and cause it to run cruddy and make a little noise. Every now and then about 100 mi. before oil change (that's when it was about a quart low too) I'd add a quart of Dextron to the engine oil. I was told years ago that it's really high detergent and it must have worked because the noise went away and the engine ran smoother. I can't swear that this is approved by anyone but it was my engine I did it to and I didn't have anything to loose.
Tom

All the advise so far is spot on. Not seen it said here yet, but could be a flat cam also. Intake lobe may open and close enough for idle and part throttle, but "pop" back through the intake when cylinder has a load. A broken spring could do the same and have more noises associated with it.

A broken valvespring and the noise doesn't go away. It gets worse and will hammer the crap out of everything in that cylinder.

could be double spring though, with one of the 2 springs broke.

Thanks to everyone for the wealth of information! I really appreciate it! I haven't had time to get out under the hood to try all of the suggestions but hope to get back on it tonight.

Thanks again!
SH

just get a big long screwdriver and stick the blade end against the valve cover over each cylinder with your ear against the handle . you can easily narrow it down to one cylinder. i've seen my mechanic do this.

It's amazing, the areas that a sound like this can come from.

I had an engine tick in recent years that drove me batty because it was coming from the low end and varied with rpm but engine load wouldn't change it. Wasn't crank or rod related.... it drove me nuts.


Until I figured out it was the mechanical fuel pump arm slapping off the back side of the crank lobe because the pump arm didn't have enough preload to ride firmly on the back side of the crank lobe. (335 series ford V8)

Small block chev would have a rod between this mechanical pump and is cam shaft actuated I believe.

You never know.....

(assuming it has a mech pump)

Good idea Dave, seriously, and I'm not knocking it but it wouldn't explain the loss of power. Of course that could be an entirely separate issue.

I like your thoughts on a broken valve spring though. That's something that I hadn't considered.

well, if something is leading to a short stroke or intermittent stroke on the mechanical fuel pump.... it could starve for fuel (power)

Also, lean pop (backfire through carb)




You just never know.

flat fuel pump lobe, worn pump push rod, weak pump return spring...

(I could do this all day)

Lol!

Some small block chevies had a nylon coated cam sprocket, this was to reduce noise. after a certin amount of miles it would begin to break off pieces of the nylon, which sometimes would get caught in the chain and would cause the chain to hit the cover at odd times when it came to a tooth that was still coated.