I have been stumping every mechanic in our area. I have a strange knock in the my 350 that ONLY knocks after it has warmed up. The engine only has 26,000 miles on it and it has NOT been driven hard and I know all but the first owner who only had it for one year. The problem is NOT bearings as it only is knocking when I stop...come to a stop sign... or just let it sit idling. NOT when it's cold... only after I have driven it for awhile. I have literally driven 30 minutes without a sound at times, and then it starts... and it doesn't go away until the engine has completely cooled down. I have one mechanic who wanted to change out the block, another who says he's nuts and that it's something in front of the clutch, and another who says "hey, if it's been that way for so long, leave it alone and live with it. By the way, it has been going on for a couple of years and does not get worse.... same old thing, but I want to fix it. Without your help, they're going to start tearing it apart very soon. I WOULD REAAAAAAAAAAAALLLY APPRECIATE any help out there if anyone has anything to offer.
Thanks........... I am at jbernander@yahoo.com

 

whats your oil pressure at when it starts knocking?

 

down around 13 -15.... told by one mechanic that that is fine and another who says it's way too low for a vette. You can see the drop in oil pressure as the engine warms up, which was why my one friend said it was bearings. thanks.

 

13-15 is low....My old stock motor had like 25 and it had 127,000 on it when I pulled it out.

 

Put it on jack stands and check the bell housing,
I bet it is the DM
I had the exact same problem when I tried to resurface my DM
It would only knock when it got hot and at low RPM
Get the thing hot hold it at the RPM you can here it best at and have someone under the car listen with a mechanic stethiscope at the bellhousing close to the FW and I bet you find the knock.

 

The oil pressure is a bit low, but still meets the 10 psi/ 1000 rpm rule of thumb. I don't think that's your problem.

I had exactly the same problem back in '93 when my 89 6 spd had about the same number of miles on it. The noise you hear is the dual mass flywheel. The 89's were the first year for the DM, and they had a problem with the springs on the inner part of the flywheel taking a set. This greatly reduced the preload between the two flywheel halves and allowed the inner hub to rattle. The noise only occurs at hot idle and sounds identical to a rod bearing knock. The springs were changed for '90 to make this noise issue much less likely to occur. In any event, I was assured by the GM engineer responsible for the flywheel at the time that there is no danger of flywheel failure due to this noise -- it's just really annoying.

I was certain that I was going to have to replace the flywheel. I even went so far as to pull the trans and flywheel and test the flywheel only to find that it was fine.

It turned out that the problem was a rough idle due to 3 bad injectors. My GM source told me that even a minor variance in the idle could cause the noise in an '89. I would check all the basics that could cause a rough idle, like plug wires, cap and rotor, vacuum leaks, etc. If this doesn't fix it, then I would check the injectors.

 

i agree with above. it isn't time to tear motor apart yet. get a mechanic stethiscope or use a wooded broom stick handle and find location of noise, then go from there

 

also....your mechanics answers are just guesses and that will and can get expensive

 

hahaha! the oil pressure is supposed to drop when the engine gets hot.

the oil pressure gauge reads BACKpressure at the pump... when the oil is very cold and thick, theres lots of backpressure. As it warms up it thins a lot. Dont equate pressure with flow always.

 

Sounds like the Dualmass flywheel took a dump...

 

Thanks.... I am going with your suggestions of the DM. I will let you know how it turns out. AND YES I agree that if they just start going to 'get 'er done' it WILL get expensive. The one mechanic that I want to work on it was the one who also advised me to check out sites like this.... and he was also the one who said that the level of the oil pressure when hot was not a concern. Thanks again... it's the first time I have been on the site, but surely will not be the last.