New to the forum and have searched but would appreciate your comments!
1990 coupe with 135,000 miles. Runs well but I have a "knock" that gets louder and faster as I accelerate. Sounds like coming from pass side valve cover area. Search seems to indicate a problem with Egr. Also have code 32. If I slowly accelerate, I hardly hear it.

Ideas, tests, etc would be greatly appreciated!

 

I'd say you have 2 problems. Not hearing it, I'd go with a valve lifter. You need to determine if the noise is coming from the upper engine or lower engine. So as not to scare you with a lot of hypotheticals, you need to isolate the exact area of the noise. As far as the code 32, Google 90 corvette code 32, one of the things that comes up is this tutorial....http://tech.corvettecentral.com/2012...egr-diagnosis/

 

if you have the time, and space to do this safely, jack the car, make sure you have multiple stands and / blocks and start the cart. get under it with a mechanics stethiscope. you can get them for 7 bucks at the local auto parts store. be careful under the car and ONLY DO THIS if you have it mega blocked up.

use the steth and hold it do the bolts on the oil pan. then also use it up top as zr1fred said and see if you can isolate the noise. cheers. VT.

 

Also use the stethoscope by each injector. Sometimes they get noisy as they pulse.

 

Thanks for those ideas!

I forgot to say that this car was driven from California to Texas two weeks ago after I bought it! Changed engine oil and no metal. Even cut open the filter. Clean as a baby's rear! Could this be an exhaust manifold leak? Again, not present at idle. Only hear it at around 2000 with median acceleration.

 

Could be many things. If its underneath, you have spun bearings, cracked flex plate, or toasted tc.

If you can, try the steth trick.

 

Upload a YouTube video So it wasn't make the noise till you changed the oil?

 

I had an exhaust manifold leak on my 1991. When you really hear it is when you put your right foot into the go pedal. It gets louder. If you are coasting down a hill the noise almost goes away. The manifold bolts were all loose on my right side manifold. Really hard to get a wrench or socket on them, but check them and make sure they are tight. Best wishes. The people on this forum are great and will help you a lot.

Steve

 

 

x3 sounds like an exhaust leak to me dont tear your motor apart

 

I agree that a YouTube might help us a lot. But, is it more of a knock or a tick sound under load?

An exhaust leak is higher pitched than (rod) knock. Pulling plug wires is not so easy on these cars (except for LT5s), but pulling one plug at a time might help ID which cylinder(s) are involved. But, before you do that, as suggested, a stethoscope will tell you if it is internal or external almost immediately. If it is a leak, make sure the exhaust manifold bolts are tight. You might get lucky and not have to replace gaskets...unless you have MY luck!

(Hint: sometimes pulling the probe off and just using the open rubber hose can sometimes work better to isolate the source of some sounds. Try it both ways. You might be surprised with the difference in the effectiveness of the two methods.)

Conversely, a mechanical source, e.g., a mechanical (internal) noise will come through the stethoscope "loud and clear".

A faint knocking sound at idle (heard w/o the stethoscope) could be piston slap, but that is not usually loud, i.e., subtle knock or almost a ticking sound. And, you don't usually hear the faint piston slap it when driving or under load. Just when idling and the hood is up, and (from my experience) piston slapping quiets down as the motor comes up to operation temp.

Mechanical "knock" is a lower pitch - more guttural(?) - than an exhaust leak. Most common (in no particular order) are damper failure, rod knock, flex plate, torque converter, FW bolts.

However, because it occurs under load, AND (you say) increases w/ speed and all but disappears when coasting/decelerating...is a classic symptom of rod knock...but not always. (I'd need more info before concluding anything at this point. Use that stethoscope and see if you can determine if it is external or internal first, and we can proceed from there. OK?)

Even if it IS a rod knock, IIRC the pan on a C4 SBC can be dropped w/o pulling the motor, yes? I have been able to re-bearing a few motors from below w/o pulling the motor - tho personally I'd rather NOT do a motor that way. (I only mention it as an alternative "down and dirty" method. Pulling the motor is not that much more work, and once the motor is on an engine stand, the whole sheebang can be overhauled, e.g., bored if necessary, new rings, valves ground, fresh bearings, crank and valve seals, new cam, lifters...a fresh motor(!) while it is out.)

OR...do I dare suggest an LS conversion instead of overhauling? Just a thought.... But, assessment FIRST. Decisions LATER!

Please forgive me if I'm way off the track you might rather have in mind. It comes from decades of hot-rodding SBCs, and in terms of work, I (like many of us here) enjoy the prospect of modding as long as I'm in there anyway, or at least leaving nothing to chance (e.g., bores needing honing or maybe even boring?). After all, it IS a Corvette, and performance and reliability are essential, right?

I'll shut up now...

 

Is this noise discussion when hood is down, in all posts? Just curious.