1972 Mark Engine Knock

Hi Bill!
I have a question about my 1972 Mark IV with 28,000 original miles on it. I seem to have developed an engine knock. It only happens when I’m driving at about 40+ miles per hour and give it more gas. It seems to be happening in the 1500 – 2000 RPM range in my estimation. If I accelerate more, it goes away and I get pure, pulling “horse-pressure”. If I let off the gas, it goes away too. It seems to only happen at 1500-2000 RPM’s . At idle, when I rev the engine, I get no clicks, no ticks, no knocks, nothing. She’s super quiet. I change the oil and filter @ 3000 mile intervals and the oil is clean and slippery. 10W40/FL-1A. If I let the car sit for a week at a time, and start her up cold, I still get no clicks, no ticks, no knocks, nothing. It’s a VERY healthy, happy motor. I would think if it was a piston/bearing knock or a cam shaft bearing, I would hear it right away on a cold start. Anyway, any suggestions? She’s my baby!!!
Thanks Bill!
Peace,
John
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Hi John –
The knock that you describe sounds to me like detonation possibly caused by excess carbon deposits in the combustion chambers, engine overheating or the use of a poor grade of fuel. If so the knocking can be reduced by checking and adjusting your ignition timing or adjusting the vacuum advance unit ( If it is adjustable) . You can experiment with the above along with different octanes and brands of fuel to find out if the condition improves. An engine compression test will help to determine whether or not the engine is severely ” carboned up”. If you are concerned with the possibility of an engine bearing knock you will need to do an engine oil pressure test with the use of a master oil pressure gauge and the engine at FULL operating temperature. By the sound of your post though I think that you may find that the cause of the knock will likely be one of or a combination of the first suggestions above.
Sincerely,
Bill