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Hey guys! It has been a while since I posted a question, because I fear the answer... I changed the oil last time to Rotella T as I have read much about it on this site. I was driving in July on a very hot day and the car began to knock about one block from my home. I parked the car and haven't started it since. The car was not overheating and it had normal oil pressure as far as the dash gauge showed. Anyone have any thoughts?? :(

If you suspect a rod bearing, start it up and, one at a time, disconnect or short out the spark plug wires to see if removing one wire makes the knock go away. If so, it may be time to drop the pan and check further.

PS - did the knock occur when the engine was under load, or at an idle? Light metallic knock or a heavy, thud type of sound? Have you ever had the pan down to clean it and the oil pump intake screen? IMO this is something that should always be done when first putting a car into service unless you know it was recently done.

PPS - might be helpful if you could post an audio file of the sound.

Paul,

Start it again and drive it a mile or so around the block. I've had mystery noises come and go in various cars. The knock on my 327 is still there, i may have a line on a ready to go rebuilt engine, otherwise, likely sending this one to ross...Of course it starts knocking after i get the whole car back together!

Had a rod knock on my 34 Eight many years ago, and about 100 miles from home. Found the offending rod by shorting the plugs one at a time, and then left that plug wire off for the drive home to save the crankshaft. Poured babbit bearing of course, but at about 83,000 miles I couldn't really complain.

Paul - also possible it's a wrist pin knock, generally most noticeable at idle or low speeds. Takes the form of a "double knock" with a pause inbetween before the next double knock.

And of course it could be lots of other things, for example fuel pumps have been known to make such sounds.

It has been a while since I heard the knock... (only because I have refused to start the car again) though it seems that it was while driving on a flat street and in the garage as well. I am fairly certain that it wasn't a double knock. The odometer shows 18,000 or so miles. The car is super clean, but I have to think it is 118,000 being 61 years old. I will try the spark plug thing tomorrow afternoon. If that helps, then its a biggish deal?? I will have it fixed rather that changing the engine if it comes to that...

Assuming for the moment it's a connecting rod knock, it doesn't have to persist very long to cause damage to the crankshaft, the result of which most folks would call a "biggie".

Do the plug check, and let us know.

Piston slap is another possibility, although the sound is quite different. More like a slight 'ringing' accompanying the knocking noise. And a stethoscope will isolate it to the top end.

Can't be certain but I'd think that disconnecting a plug lead would also eliminate piston slap noise, meaning that if it is slap and not a big end, you might think you've got a bottom end problem rather than the much less serious issue of a worn piston skirt or similar.

I had a V8 (Chev engine) that I drove fairly hard for over ten years, with noticeable piston slap from day one. Initially I suspected it was bottom end noise but it wasn't. Ideally I SHOULD have fixed it but it never got much worse in around 100,000 miles so I left it alone. But a bottom end problem would need immediate attention as Owen stated. Matter of fact, it might be worthwhile slipping in some Wynns or other slip additive while you are even doing your checking. It might help prevent damage which could happen within seconds of starting (if you were very unlucky, that is).

Dang! What would cause such a thing? Could I have loosened some 'crud' with the Rotella T?? Very depressing...

Asusming again it's a rod knock or piston slap, or similar, such things are usually caused by either (1) neglect or poor maintenance, (2) abuse, or (3) high mileage.

I know nothing about Rotella so maybe you have a working idea. Many years ago when I got my first V8 Packard, was dumb and ignorant & followed some very bad advice to use high detergent oil and a pint of kerosene to get the crud out. No dropping the pan first or hardly anything else done that many have recommended on the forum as preliminaries.

The detergent concoction did as advertised and loosened all the crud so well that it clogged the oil intake screen. After about a 5 mile trip, the engine started knocking severely. In my case, not the end of the world because I had planned to do the engine anyway--just not that soon.