Loud Metallic POP - Ideas ?

[Doug E]

My left ear has finally stopped ringing and my heart rate is almost back to normal. See below for what happened ... any ideas much appreciated. Tried searching but didn't come up with anything identical.

'81 SC with 115.7k miles. Pulled car out of hibernation today for 1st time since being parked in October (was stored in an enclosed garage). Started up after 2 twists on the key. Let it sit idling to warm up slightly while I checked the tire pressure then took her out to warm to 180 degrees so I could check the oil level.

Did about a 10 mile loop, pulled back in the driveway and let it sit idling while I checked the oil. No strange noises and once oil cap removed, idling dipped a little like it always does. Was a little low so started filling. Had about 1/4 quart into it when an very loud single PING rang out ... sounded like a gun shot ricocheting of a piece of metal. Nearly jumped out of my pants but managed to not spill the oil container and ran to turn the car off. My left ear was facing towards the top of the engine and took the full effect of the sound since I was contorted filling the oil.

Car has never done in that in 20+ years of ownership. Immediately thought the worst so checked underneath and no oil leaking anywhere or visible holes in the case. Got a flashlight and looked over the top of the engine ... no liquid anywhere or anything appearing to be out of place.

Removed the air filter to check pop-off valve and that was still intact. Other thing I thought about was a stud popping but I replaced all of them about 18k miles ago with steel studs.

After calming down I tried the key and it started right up and idled smoothly. No strange noises / grinding / rough idling, etc. I turned it off and there was no clanging / banging / grinding or loud noises.

I started it up again, backed it into the garage and shut it down. Again, nothing sounding out of the ordinary.

Was hoping the brain trust might have some ideas / other things to check.

Thanks in advance, appreciate it!

[jlex]

backfire?

[Doug E]

Thanks for the quick reply jlex. Hadn't thought about that possibility but it definitely didn't sound like any backfire I've ever heard before and it came from the engine compartment, not out the tailpipe.

[proporsche]

Do you have a hydraulic chain tensioners on your or still solid original ?

Ivan

[Doug E]

They are hydraulic Ivan. Were replaced prior to my purchasing the car.

[proporsche]

that makes me think..it was sitting and one of the tensioner was a little collapsed(due not having constant oil press) once it warms up it pumped up and that is what you have heard..other than this ..i would only do, drain the oil(at the engine only) and see on the drain plug for any debris..but again the engine sounds ok you said...

Ivan

[mrdstr]

Did you knock something into the fan or belts while you were leaning over it? Did an exhaust band break? it's possible the source came from outside the engine, not in.

[pmax]

Fan blade ?

[Doug E]

Thanks Ivan, hadn't thought about that possibility either and makes sense since it had just gotten to 180 degrees. One other data point relative to this possibility is that I never noticed the oil pressure gauge fluctuating or low while I was driving. Will check the oil drain plug when I have some time this week.

mrdstr - good questions. No, nothing went into the fan or belts and I didn't notice anything broken relative to the exhaust when I looked underneath with a flashlight but I'll double check.

[Jagshund]

A pop-back could have come from the engine compartment. I had a similar experience when accidentally running 2yo fuel but it came shortly after startup.

[Doug E]

pmax - no, sorry, I should have included that in my initial post ... all fan blades accounted for. I touched each one and none are loose or showing a visible crack.

[Doug E]

What's a "pop back" Jagshund? I always fill the gas tank right before parking it for the winter so it is possible there was some older fuel at the bottom of the tank but it would have been from last Aug/Sep, from the 2nd to last fill.

[pmax]

Any signs of impact on the fan blades ? Something that loud doesn't sound internal especially when it runs fine later.

[Jagshund]

Originally Posted by Doug E

What's a "pop back" Jagshund?

It's similar to a backfire except it exits topside, not out the exhaust. I've only experienced them with carburetors thus far.

[porsche930dude]

seems unusual for a backfire to happen after its warm like that and only once. Any evidence inside the airbox? The only thing i can think of maybe the clutch omega spring let go but i dont know how loud they are

[T6andT]

Well, here is one totally off the wall. When I did a bit of work on my 71' recently I was amazed at the number of acorns that had been gathered over the right bank. Wonder if while it was sitting a little nesting was happening, the drive moved things around and acorn or similar was launched through fan. If not I would go for a broken stud.

[MikeDL]

Even though new, I'd still check the head studs

[Doug E]

Jagshund - thanks for the explanation, mine is CIS.

porsche930dude - nothing outside the norm in the airbox. I checked the pop-off valve and that was intact.

T6andT - wish it was something like that but I've never seen evidence of any rodents taking up winter residence inside the single car garage (attached to the house) that she is stored in.

MikeDL & T6 - I also keep coming back to a stud but I remember when I had 3 broken that led me to replacing them all and the racket those were making. Didn't hear anything like hissing or missing when I restarted it twice but to be fair, that wasn't under load. Maybe I'll work my courage up after checking the drain plug and take it out for a short drive.

Crazy to think that if this had happened while I was driving I never would have heard it and probably would have been none the wiser, assuming it wasn't anything catastrophic.

[jac1976]

Missing any snaps or buttons on your clothes?
Is the engine kid release working properly since you heard that sound?

[Doug E]

jac1976 - no buttons missing and just checked the deck lid, release is working fine.