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Loud Knock!


Well-known member
Apr 5, 2004
Jacksonville, Fl
'81 dark blue
Okay, I need some ideas. I have not driven my car much for the last month or so because it has developed an extreme knock. It is very loud and solid, like it's about to throw a rod, but it only happens at very low RPMs. When it first starts, it will make this banging racket for about a second, but then it goes away and the car runs like a scalded dog as usual, no problems. I can make it make this horrible noise by lowering the idle to the point where it is on the verge of dying, and then it is banging away. I only did this because it was the only way I could think of to find the problem. But I have still not been able to locate the sourse. It seems equally loud from above and below. It does not seem to be anything external, like the fan hitting something, and seems to be more from the rear vs front of engine. I took the torque converter cover off and could see nothing hitting the flywheel. I used a mechanics stethescope on the block, heads intake, frame, starter, transmission bell housing and the torque converter cover, and could not hear it at all, strange! (although I could hear it plainly enough not through the stethescope).

The transmission is a BTO Level II TH700r4 with about 30k miles on it, well serviced. The engine is a modified L81 that was completely rebuilt about 40k miles ago and has also been extremely well serviced.

Could it possibly be the torque converter? I do not know much about the inner workings of these things. Would there be something inside one that could knock like that at low RPM, then function fine otherwise?

God bless, Sensei
Sensei, I had a 66 Barracuda that had a loud knocking soung that turned out to be one of the flex plate attachment bolts hitting the bell housing. Pulled the bolt out, ran the motor to be sure that the knock was gone, then got a new bolt from "Mopar" and installed it. No more noise. You might want to double check those bolts with a wrench as mine was not backed out all that far.
That you hear the knock only for a fraction of a second after start up or when you idle the engine as slow as possible. In both those situations, you have low or no oil pressure. That the knock is only present during low/no oil pressure might be indicative of some kind of bearing problem.

What happens when, using the stethescope, you listen to the oil pan and the oil pan rails at the bottom of the engine block?
What happens when, using the stethescope, you listen to the oil pan and the oil pan rails at the bottom of the engine block?

I did not listen to the oil pan rails, but the oil pan and the engine block just above it were silent.

God bless, Sensei
You might want to double check those bolts with a wrench as mine was not backed out all that far.

I did look at them and they did not look like they were out, but I did not try a wrench on them, I should check that.

God bless, Sensei
Thinking back to 1978, I removed the oil pan of a 1976 in a chevy pickup because of the death knock that was coming from the engine. The engine had about 24000 miles on it. I didn't think there was any way at this mileage any problems could exist in the lower end. With the pan off and the oil tank at 40# hooked to the oil galley, the bottom end just dripped like a good bottom end did. After a can of CD2 down the carb slow enough to get most of the can down. And fast enough to kill it at a high idle. An over the night soak, the carbon on top of piston #6 that was disloged by the grandson's test drive of gramps pickup was dissolved. Don't pour it in too fast and hydo-lock the engine, it can be hard on top ring lands. Hope this is your problem. I have used Dextron with the same results. This mostly occured with engines driven by older owners and the younger crowd took them for a cruise. Again hope its this simple.
When it is idling as slow as it will go. What is your oil pressure at hot low idle? Has your oil pressure changed since the knock started. Is the knock a single or double knock? Is the engine built for the rpm its operated at?

Have you tried removing all of the converter bolts, pushing the converter all the way in the front pump (wire it to the trans) and start the engine to isolate the engine from the trans. Make sure you have clearance. Some aftermarket converters didn't have the same front pump engagement as the OEM ones did.

Additionally stock Flex-plate didn't survive well behind stall converters and high output engines. They cracked outside around the crankshaft bolts. Knocking would change or quit from neutral to drive. Aftermarket suppliers in the past sold 0.090 flex -plate for high performance converters and engines. I would verify the balance of anything aftermarket.

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