interesting compression problem

[diesellieber]

I've got a 240D with about 300k miles on it. Number one cylinder has low compression.... something like a hundred pounds. But, when I run a leak down test on it, it tests identically to the other three cylinders. I've been trying to think of what could cause this. I thought maybe the intake lobe was worn off, but that wasn't the case. Yesterday a friend suggested that the problem could be a worn valve guide on the intake. He thinks the valve is not seating properly during cranking, but manages to seal when the leak down test is applied.

Has anyone seen this problem?

[pdxman]

Dieselleiber, whats your car doing that you are doing the compression tests. Reason I ask is I'm working on mine right now and thinking I may have same problems. My engine shakes, smokes somewhat, worse than normal bad accelaration but runs great on highway. Dies on me once in awhile putting it into drive unless I let her warm up well first.

Thanks.

Could be a worn valve spring.

When the metal heats up it expands and with
that this is a spring it would increase the tention
of the spring,thus closing the valve.
And when cold metal shrinks thus reducing the
tention on the valve and not closing it all the way.


I could be wrong so feel free to correct me if I am.

Hope this helps.

Louis.

[psfred]

More likely worn out valve guides and a bad rotocap on the #1 exhaust. This allows the valve to "wear in" on the seat asymetrically, so that it seals at one point in rotation but doesn't at others. Common on high milage engines.

If you don't have oil consumption problems, I'd bet a good valve job will restore the engine to like new running condition.

A tight exhaust valve will act similar -- it's OK on a cold engine, but won't seal all the time warm. Crud built up on the stem will do the same thing -- it moves completely closes slowly.

Cure for that is to set the valves about 0.010 inch loose for a while a see if they close up.

Peter

[diesellieber]

pdxman: sorry, I just recently bought the car and haven't run it on the street yet.... can't give you any driving characteristics.

I think a valve job is in order..

thanks