MTFCA Forum

I have an engine knock during light deceleration that only occurs when there is no "load" on the engine. When the engine is slowing down the car the knock goes away and is never there during acceleration.

I tried removing shims from the rod caps and although one cap "seemed" loose apparently it was not the culprit.

What else could it be? Crank?

Centre main bearing.

Is it a "knock" or a "rap"??? Some would say that there is no difference but I think that a knock is a deeper tone and could be the crank. I think a rap is a higher pitch sound and in many cases related to the cam/timing gear. I have had several engines with rapping noise and acted as you describe. Usually by moving into a retard position the noise goes away. Also when the engine is under power the noise goes away. I'm sure you know this but just in case!!

Piston slap.

Generally a "higher pitched" knock under deceleration is a rod knock, that or maybe a piston pin knock... Main knocks are deeper, you can not only hear them but feel them. Piston slap is a different sound (takka takka takka)...

Do you know the condition of your u-joint? Excessive play in your u-joint may show up under "no load" conditions.

A big end bearing knock is most prominent under acceleration, if it gets to the stage of hearing it as well on deceleration, then it's ready to throw a leg out of bed.

Jerry VanOoteghem wrote: ↑

Sun Oct 25, 2020 6:28 pm

Do you know the condition of your u-joint? Excessive play in your u-joint may show up under "no load" conditions.

No I don't.

Susanne wrote: ↑

Sun Oct 25, 2020 6:10 pm

Generally a "higher pitched" knock under deceleration is a rod knock, that or maybe a piston pin knock... Main knocks are deeper, you can not only hear them but feel them. Piston slap is a different sound (takka takka takka)...

Probably not piston slap then.

A rod knock usually occurs when you are lightly decelerating such as slowing down from 35 to 30 gradually on level surface. If you decelerate quickly it will usually not knock nor when you accelerate. At a gradual slowdown when the engine is neither pulling nor slowing down on full compression will leave the rods slack and it is at that point where the knock happens. If it hasn't been knocking for a long time, you can usually take out a shim or two between the cap and the rod and reach the .0015 clearance and it will stop the knock. You need to mic the crank when you have the cap off to be sure it is not out of round and also check the babbit in both the rod and the cap for cracks or wear through. It might need to be re-poured.
The book on "Engine" gives good do it yourself instructions on how to do this.
Norm

.0015" is what you look for to have a happy bearing clearance.

You''e right. I stand corrected.
Norm

I had an engine with similar symptoms.
Pulled the pan and things seemed tight.
Turned out it was the front main.
A blocked oil hole was the problem. (Low mileage "rebuild".)
I seem to remember a remetalled cap and rebluing the crank was the solution.
Hope this helps.
Bruce