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View Full Version : What would cause a hollow knocking noise that goes away as the engine warms?



phils89sc
12-05-2010, 11:45 PM
This started sometime after I replaced the HG's. The engine will do it when it is cold, or after it has been sitting for a couple hours after being shut off. What it sounds like is like a hollow tap / knock, as if you were tapping a hammer on a piston. After the engine runs for a couple of minutes, not even to full warm up, the noise stops. It goes with engine RPM as well. I am kind of hoping it may be a lifter and nothing deeper.

Any opinions?

90turbo1
12-05-2010, 11:52 PM
does it sound half engine speed or with engine speed.

take a section of vacuum hose and place in dipstick tube and listen with ear. if its a rod bearing you will hear it very loud through the dipstick tube.

phils89sc
12-06-2010, 12:34 AM
does it sound half engine speed or with engine speed.

take a section of vacuum hose and place in dipstick tube and listen with ear. if its a rod bearing you will hear it very loud through the dipstick tube.

It sounds more like it is coming from the top engine, left side, but cannot be sure. It does increase with engine RPM. You cannot feel it, just hear it. I will try the vacuum hose to the dipstick and see what happens.

Can the SC itself make a noise like this?
Thanks

RalphP
12-06-2010, 01:11 AM
Did you have anybody helping you?

Did they disappear while you were putting it back together?

*KNOCK KNOCK* LEMME OUTTA HERE!






Seriously, it's going to depend on exactly where it is.

It could be something as simple as a pushrod that's not quite set right.

RwP

phils89sc
12-06-2010, 08:48 AM
Did you have anybody helping you?

Did they disappear while you were putting it back together?

*KNOCK KNOCK* LEMME OUTTA HERE!






Seriously, it's going to depend on exactly where it is.

It could be something as simple as a pushrod that's not quite set right.

RwP

That's funny!!

I probably should have, but didn't. When I put her back together, I should have coughed up the extra money to replace the lifters and push rods, but did not. I am kind of hoping that maybe by having the heads shaved and not using new lifters / push rods, and I did not shim them that maybe a lifter is just not pumping up all the way on startup, and taking it's time.

Is there an engine treatment / cleaner out there that I can pour in on my next oil change that can clean off all my internals and possibly free up any possibly sticking lifters?

the-big-e
12-06-2010, 09:54 AM
Marvel Mystery Oil has worked wonders on my cars for many years.....:D

phils89sc
12-06-2010, 06:21 PM
Sorry, had to ask. Of course the preferred method would be to pull the engine, but that is going to have to wait. Is it possible to get the oil pan off with the engine in the car by removing the SC, plenum, and mounts?

David Neibert
12-06-2010, 06:31 PM
Sorry, had to ask. Of course the preferred method would be to pull the engine, but that is going to have to wait. Is it possible to get the oil pan off with the engine in the car by removing the SC, plenum, and mounts?

To do that you would have to support the motor from the top unbolt the mounts and a few other things, then lower the K-Member and front suspension to gain access to the oil pan.

IMO, it's easier to pull the motor and make the repairs on an engine stand.

David

XR7 Dave
12-07-2010, 09:47 AM
Rod bearings are not going to get quieter as the engine warms up. It's more likely a stuck piston pin.

phils89sc
12-07-2010, 11:14 AM
Rod bearings are not going to get quieter as the engine warms up. It's more likely a stuck piston pin.


Is there anything I can do short of full disassembly to alleviate the problem? How could a piston pin (is the wrist pin the same thing?) get stuck?

90turbo1
12-07-2010, 11:28 AM
Rod bearings are not going to get quieter as the engine warms up. It's more likely a stuck piston pin.

well mine kinda did actually when the problem first started. but all it took was about a week and it was doing it all the time.

when I tore my motor down all I found was number 3 rod had no bearing left, just a rod on the crank. the bearing shell was as thin as paper down in the pan. and some stuck to the side of the rod.. but like I said it would go away for the first week after I heard it and then progressivly got tot he point where it was knocking all the time.


Another check you can do is take the super charger belt off and run motor without supercharger (it will idle just fine) and see if noise goes away or gets worse. Sometimes the supercharger coupling can make a noise (but would be all the time) and it can really mask some other noises.

XR7 Dave
12-07-2010, 11:32 AM
Is there anything I can do short of full disassembly to alleviate the problem? How could a piston pin (is the wrist pin the same thing?) get stuck?

If it is a stuck pin, not much you can do other than let it warm up until the pin quiets down before driving the car. It's caused by varnish or carbon buildup on the pin. When the piston is cold it shrinks and sticks to the pin. As soon as some heat gets into the piston (long before the coolant gets hot), the piston expands and frees the pin. This condition is not repairable and will require new pistons/rebuild to fix properly. If you are cautious with it when cold, it may run a long time like that.

Worst case scenario is that you could have a bent connecting rod causing the piston to bind, but that is less likely than a sticking wrist pin (very common).

phils89sc
12-07-2010, 11:50 AM
If it is a stuck pin, not much you can do other than let it warm up until the pin quiets down before driving the car. It's caused by varnish or carbon buildup on the pin. When the piston is cold it shrinks and sticks to the pin. As soon as some heat gets into the piston (long before the coolant gets hot), the piston expands and frees the pin. This condition is not repairable and will require new pistons/rebuild to fix properly. If you are cautious with it when cold, it may run a long time like that.

Worst case scenario is that you could have a bent connecting rod causing the piston to bind, but that is less likely than a sticking wrist pin (very common).

Thanks Dave, for the explanation. I always wondered about wrist pin noise versus rod knocks. I often wondered if a rod knock noise would go away as the engine heated up. I baby it anyway, but especially until it warms up (=no boost till warm).

Anyone out there got a rebuildable shortblock (or long block) for sale?