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Knock, ping or (!) rod knock Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 7 Mar 2005 06:06:59
In Reply to: Periodic knocking after cold start, Jimmy S., Sun, 6 Mar 2005 15:38:03 Members do not see ads below this line. - Help Keep This Site Online - Signup |
From your description, it seems that you hear the sound at idle. Highly unlikely to have "true" knock at idle without something seriously wrong. At idle, after a start, the cylinder temps are low, and cylinder pressure, especially with a turbo, is very low. Very unlikely to knock. But two things come to mind-
The first is that you have a stuck valve. This could be due to a lazy hydraulic lifter that is taking its time to pump up. The exhaust valve stays closed and pressure just keeps building up in the cylinder. It also exhausts into the intake manifold when the intake valve opens, and that sound can cause the intake manifold to 'ring', making a pinging sound.
I bought an Alfa GTV6 with a stuck valve. It made a sound just like somebody hitting a bell, as the pressure waves caused the intake plenum to ring. In that engine, the valve was stuck open and never moved, even as the engine warmed up.
The other one, which would be a real concern, would be rod knock. If the engine has had low oil pressure, the bearings on the bottom end will be wearing quickly. A typical failure progression would be to have a loud knocking sound when the engine is cold, that quickly goes away as the oil warms up. As time goes on, the knock stays around longer, and start to show up at speed. Then one day the engine just won't turn over, because it has siezed. This produces a knock which is a real mechanical knock, like somebody hitting the side of the block with a hammer.
If the sound is like true 'ping' or pre-ignition, I don't know how that could happen at idle and only when cold. It could happen, I just don't have a theory on that one. If the sound is from the bottom end of the engine, I'd have the oil pressure checked immediately. If it's from the top of the engine/intake plenum, then I'd try a few things. Some folks have had luck putting some very thin oil (Marvel Mystery Oil here in the US) in the engine and letting it idle for a few minutes, (Don't drive it!), and then doing an oil change. You may want to pull the valve cover and make sure that the oil lines to the lifters are OK, and none are plugged up.
posted by 192.249....
, Mon, 7 Mar 2005 06:06:59 <-- Viewing This Message, Mon, 7 Mar 2005 09:53:49
, Sun, 6 Mar 2005 20:50:25
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