I have a 1990 vet. After I had my air conditioner change over to the new stuff, my car started knocking badly especially after it warm up. Now its missing. Someone said it was my thorw out bearing, so I had my clutch replaced. Did not help. Seems to be getting worse. I have 75000 miles of the car and its been babied. Someone else said maybe it was a lower rod knocking. Others dont believe it that. Could it be the knock sensor doing this? How would I check this myself? I dont want to be taken advantage of again.
Thanks, Karen

The car's ECM runs a functional test on the Knock Sensor each time the engine is started. If the sensor fails the test the Service Engine Soon light is turned on. IIRC a code 43 is set.

Are you seeing the SES light on the dash?

Does the knocking sound change in frequency as the rpms go up and down?

How's the oil pressure?

Describe what you're hearing. Is it a sound such as when you rap the knuckles of your fist on the kitchen table or more like tapping a pencil on the same table?

Diagnosing noises can be challenging.

Jake

The former may well be a rod bearing, while that latter is more like something in the valve train.

knocking noise when warmed up
 

the knocking noise sounds like a knuckle knocking noise. I checked the timing it was ok. The spark plugs looked as if the car had been runing to hot. White ash in color. The plug wire was a bit corrosive. I had not change the fuel filter in a long while. The car is now missing along with the knocking noise when car it hot and mostly atidle. Running the car fast it does miss but the knocking is quite due to the high rpms. Any other ideas on what it can be. I am taking it to a diagnostic machine today. Maybe they can detect a problem. What do you think?

try sticking a vacuum gauge on it-Google for correct action of the needle throughout the rpm range.
You may also end up having to remove one plug wire at a time while it is idling (knocking) to isolate the cylinder that is the problem-

If I do have a cylinger problem, how much am I looking at to fix it. I took the car this morning to a shop to put it on a machine for the missing and to test the cylinders. It just seems odd that a car with only 70000 miles would have a major problem like that. Its also odd that as soon as I had the air condition change over the very day I got the car back it started making the knocking noise. Ill reply when I hear from the shop , what they find.

ok, i took my car to the dealership to see if they could find the knock. they took off the oil pan and said bottom end of engine was ok. then dropped the transmission. said my flywheel was bad due to the weak springs. said it was a 2 piece flywheel. Have you ever heard of that? It a 1990 model corvette. Said was for high speed and performance. said they could use a one piece heavy medal flywheel but did not know how much transmission noise and virbration would go on. The dealership cannot locate a 2 piece flywheel in stock any where they said. Said it would run 900 to 1300 dolloars just for the flywheel. That seems waY to high. Does anyone know where I can purchase a 3 piece flywheel for my car. Please help! I have already got 2000.00 in the work already just trying to find the knock. thanks to anyone that would please help.....

A dual mass flywheel if you can find it will be 1000 - 1500
A single mass should be considerably less - you will have some noise from the ZF 6spd.

Search is your friend. I think there was one vendor that was going to stock the dual mass.