tap tap tap

[dustybarn]

I've got a persistent engine noise that I'm having trouble diagnosing:

-Once per engine cycle (every other rev)
-Present at idle and every engine speed. It speeds up with the engine but does not change in basic character regardless of speed
-Does not change with engine load
-Shorting plug leads did not affect it

It sounds like a very loosely-adjusted valve, but I pulled both valve covers, checked the clearances, and found nothing amiss. While I had the valve covers off, I looked for signs that a rocker arm might be hitting a valve cover, and saw no evidence.

The ear pretty easily localizes it to the starboard cylinder bank; it can be heard most clearly through a stethoscope on the starboard valve cover and on the intake manifold.

Does anyone have any suggestions? I'm sorta stumped.

[Ron]

Small exhaust leaks can sound amazingly close to valve train noise.
Try this- With the engine cool, turn your water hose down to a small stream and run it around all of the exhaust system seams, especially the bottom of the manifold where it meets the head.
Or, pull your plug wires one at a time and see if the noise changes or goes away.

[opethmike]

Agreed on the exhaust manifold leak possibility. I've seen on several cars (not limited to just DeLoreans) where it REALLY sounds like a valve train issue, but turned out to be an exhaust manifold/gasket issue.

Try taking a look at your cylinder head where the exhaust manifold is mounted. Do you see soot/black dust in the area?

I know you live not too far from me. I could help you take a look at the issue at some point, if you would like.

[DMCMW Dave]

A worn rocker arm will do this. It will also seem to go out of adjustment very quickly as the hardened surface of the arm is gone.

The key to me is that he said it does not change under load, if that is true it is probably not an exhaust leak.

It's possible to loosen suspect arm adjusters and inspect the surface with a mirror. It should not be concave.

[dustybarn]

A worn rocker arm will do this. It will also seem to go out of adjustment very quickly as the hardened surface of the arm is gone.

The key to me is that he said it does not change under load, if that is true it is probably not an exhaust leak.

It's possible to loosen suspect arm adjusters and inspect the surface with a mirror. It should not be concave.

Hmm. I need to verify that the volume doesn't change with load. I have a loudish exhaust and the noise is audible at idle and all other load conditions...I'm wondering if it is in fact varying with load but its apparent loudness is more-or-less the same.

[Some guy]

I have that problem with my car. I know the valves need to be adjusted, but i used magic mystery oil to knock down a lot of the noise. The other problem was that one of the bolts on the catalytic converter is gone. So once i get the new cat i should have a mostly quite car.

[Ron]

Oops, nix the "...pull the plug wires" idea...I just noticed you shorted them all.

[dustybarn]

Thanks, everyone, for your input. I've gone back and done some more investigating:

It does not appear to be an exhaust leak.

I can hear it most clearly with the stethoscope probe against the side of the starboard cylinder head, between the #2 and #3 exhaust ports. It definitely sounds mechanical through the stethoscope, and does not respond to engine load. None of the valve clearances are loose.

I'm starting to think about all the possibilities; what things could be hitting other things at 1/2 crank speed? Could the camshaft be mistimed, causing a valve to kiss a piston? I know the engine has non-stock cams (I found a cam data sheet in the glovebox).

Any more ideas?

[Jonathan]

Hey Dusty,

Just for reference, here's a video of my car before an exhaust leak was fixed by replacing the gaskets:



And after waiting 30 seconds, I'll post the after video in a second video clip...

[Jonathan]

After exhaust manifold gaskets were replaced for your reference: