327/300 deceleration knock

[Ed Driscoll]

Hi,
When I decelerate I get a knock. Specifically, if I rev (with/without load) up past 3800, the engine sounds fine, until I lift off the throttle, then I get a rattle/knock until I get below 3800. Then it's fine. The sound is localized in the front of the engine. Someone has suggested that this is a timing gear problem. I replaced the gear, chain and cover as well as the water pump. The cam had no noticable play, and the dist cam rotated immediately when I rotated the engine from the crank. I've been told that it's possible that the timing cover (aftermarket) is incorrect. I've also been told it's possible that the water pump, even though said to be correct, is slightly too small, compressing the timing cover and causing the lack of clearence that results in the chain hitting it on decel. That sounds crazy to me, but when you ask questions, you get all kinds of answers. Hey Joe L, ever heard of this?
Thanks for the help,
Ed

[Chuck R.]

Re: 327/300 deceleration knock

How about the fuel pump/push rod assembly?

Harmonic balancer?

Thermostatic exhaust flap

Alternator?

Just thinking what else is up front that could be making noises.

Chuck

[Ed Driscoll]

Re: 327/300 deceleration knock

Fuel pump, pushrod are new, harm. balancer seems normal, installed correctly and tight. i don't know what the thermostatic exhaust flap is? and the alternator is new as well. Crazy...

[Joe L.]

Re: 327/300 deceleration knock

Ed-----

Well, I've heard of noises coming from the front of the engine, but it's very hard to say what's the exact cause in your case.

The information that you've been given regarding the timing cover/water pump interference is not so far-fetched. This CAN happen. To check for it, you need to ascertain if the water pump legs will contact the block WITHOUT gaskets installed and WITHOUT the rear cover plate on the waterpump contacting the timing cover. If the water pump legs are "held off" the block because the rear cover of the waterpump contacts the timing cover, then it's a problem that needs to be corrected. If the timing cover has significant space between it and the waterpump back plate, that might be an indication that the timing cover is not right. Basically, the waterpump back plate should not contact the timing cover BUT there should be VERY little space between them.

If you wish, you can obtain a new GM cover under GM #10243967.

A more ominous problem with symptoms roughly as you describe could be a rod bearing problem. If the rod affected were one of those near the front of the engine, the noise might sound like it was coming from the front of the engine

[Ed Driscoll]

Re: 327/300 deceleration knock

The rod bearings have all been checked and are AOK. I'll check the timing cover/water pump issue. I've heard that the water pumps were cast @ 2 different plants, and one seems better then the other in terms of fit. The aftermarket cover seems suspect as well. Thanks guys.