Speculation: Exhaust manifold gasket leak?

[cyberman]

I have a noise coming from the right side of the motor. It is not loud, varies with engine speed and only occurs under load, such as driving at 30 MPH (light load). I can't hear it at idle or when I rev the engine in neutral. It's hard to locate it exactly from the cockpit and it's too faint to hear when the top is down. Best location we can guess is top right of the engine.

Best description of the sound is crackling, rattling (low pitch) or perhaps popping, in other words hard to describe. My first guess would be a loose cam chain, but I assume a cam chain issue would be present regardless of engine load.

Other than an exhaust leak, any other ideas would be appreciated. Engine is up to 93k and recently had all rotating, belt-driven parts replaced except for the PS pump. No CEL and all readings are good.

[zellamay]

Check spark plugs. A few people have heard noises like that, that turned out to be loose plugs. Mine sounded like a broken head gasket. I found 1 plug very loose and another slightly loose. Tightened them ... noise gone.

[Mr Bingley]

Sounds like good old fashioned knocking (pre-ignition) to me. Could be a hot spot on a carbon deposit, or just low octane fuel. What octane did you last buy and how full is your tank?

[cyberman]

Check spark plugs.

I torqued them to specs. But these are new, and unfortunately, I had to take them out to check compression during my "mystery high idle" problem. I vaguely recall something in my distant past that one should replace the spark plug gaskets when they're removed. Is that still true, if it ever was?

Sounds like good old fashioned knocking (pre-ignition) to me.

I always put in 91 octane fuel, the highest readily available to me. I get it from a very large regional chain. But, I did top it up on the way back from Minnesota with 76 station (truck stop) gas. But, when I said "load", I meant low load, as steady driving 20-40 MPH, or moderate acceleration, and I associate preignition with heavy engine loads. Torque Pro is telling me timing is being advanced variably according to conditions.

Hot spot? Possible, when I checked the compression, I could read the stamped numbers on the top of the pistons and they looked very clean - but that doesn't mean there is no carbon buildup somewhere.

I'll keep poking around. Thanks for your responses.

[Mr Bingley]

But, I did top it up on the way back from Minnesota with 76 station (truck stop) gas.

Suspicious.

[cyberman]

Suspicious

I'll top it up again, that will leave about 1/4 suspect gas in the tank.

I found NKG does recommend replacing spark plug gaskets, here: https://www.ngk.com/learning-center/...vs-gasket-seat where they say:

Plug manufacturers recommend the use of a new gasket any time a plug is re-installed after inspection.

I'm not sure where I can find new gaskets, but I'll look. It may be easier to just buy new plugs because otherwise, I'll have to dig up the diameter and whether its flat or tapered. Funny, for the truly obsessive, I did find spark plug indexing washers for exactly positioning the electrode and wonder if anyone has dyno tested that idea: https://www.speedwaymotors.com/Spark...0aAk2oEALw_wcB

[zellamay]

When did this noise start? That will tell you where to look. Right after pulling plugs? Right after the 76 station? Right after an overheat? ........ It's hard to believe that the spark plug gaskets are leaking after 2 installs. I assume you cleaned the gaskets and seats. ......I've heard pinging, bad head gaskets, and loose spark plugs. Pinging is a clear sound compared to the others, which have a little "whoosh" to them. When I've had pinging, I could turn it off and on with the gas pedal. Both of the others made the sound while idling. ..... I wonder if octane booster would give you some indication as to whether it's pinging..... I remember indexing spark plugs, don't know whether it made any difference. Also, I thought the idea was passe due to 4-valves and modern spark plugs.

[BladeRunner919]

I have never, ever, replaced a spark plug gasket and have never had a problem. Get a bit of hose, stick one end in your ear and probe the engine bay with the other - it acts as a stethoscope and may help you isolate the source.

[cyberman]

I could turn it off and on with the gas pedal.

I can too, but only under load. When it happened, I put in neutral and revved the engine back up to 2000-3000 RPM and couldn't hear the noise.

Get a bit of hose, stick one end in your ear and probe the engine bay with the other

Old trick, one I remember from back in the 70's although my uncle, who was a mechanic had an actual stethoscope. So, at idle in the shop, er garage, I tried that and couldn't locate the source. I'll try again with the spark plug trim cover removed and see if I hear anything. But will all the advice, I'm thinking it was pinging, either due to bad gas, or a super hot engine bay. The noise showed up when we returned from a 3-hour drive, at the end, with the outside temp a muggy 88 deg. and the AC on.

Looking at aviation service bulletins, they have to replace the spark plug gaskets each inspection - they buy boxes of copper gaskets. They say if you don't the end of the plug can extend a bit into cavity, get hot and cause preignition. So, like you, I've never replaced a spark plug gasket in my life and never had an issue. I consider that very low probability. Even though the gaskets compress and it's possible on the second torqueing the plugs seated a bit further.

Oh well, I'm off to top up the tank and pay a visit to a leather worker. I need him to fabricate a patch I can glue on to the driver's seat to keep foam from coming out until I can afford new seat leather. That may be a cold day in hell the way the covid economy killed my little business, and they way this little puppy likes to drink cash. She's a thirsty old gal.

[zellamay]

"I could turn it off and on with the gas pedal.
I can too, but only under load. When it happened, I put in neutral and revved the engine back up to 2000-3000 RPM and couldn't hear the noise."
Right: only under load.
You didn't say when it started, but it seems like pinging. I say try some octane booster, as diagnostic tool.
((( I had to re-enter this post. The thread just suddenly discarded my previous post! This is the second time it's happened since the new format was introduced. Something is wrong !!! )))