low compression diagnosis
 

The car is an 81 300SD. The engine has 278k miles on it and worked fine until it was overheated. Now it won't start even with ether, and the compression is about 120 in all cylinders. Anyone care to suggest a diagnosis? much appreciated if you do.

Richard

I would guess a blown head gasket or maybe even a cracked head just from reading other posts. Any water in the oil or oil in the water? I'm sure someone else that has a better idea will chime in.

low compression after overheating
 

Yeah, I thought it might be a cracked head or a gasket, too. But, the thing that bothers me is that all the cylinders are equally low. You would think that a blown gasket or a cracked head would cause one or two cylinders to be quite a bit lower than the others.

I just overhauled my head at 300K . In a leakdown test, it showed a bad cyd. but when tore down the head needed everything. But it was still smoking along when I torn it down. 120 psi a cyd is not enough pressure to burn fuel, so it does lean towards a head gasket, but I would have my head checked & overhauled (with 278K)....only after a leakdown test shows me that my cyd. are not leaking real bad. Replace the head gasket and you are back on the road.

Is there a possibility the timing could be off, stretched timing chain causing the valves to open and close at the wrong times? All of them being real low and close to the same does sound a bit odd.

We pulled the head off last night. The head looks ok. The cylinder walls have some discoloration that looks unusual. It's not scoring like you would get with pieces of metal floating around the cylinder. It looks like there has definitely been some kind of unusual wear between the rings and the cylinder wall.

Diesellieber,

You probably destroyed the rings. Extreme overheating will take the "spring" out of the rings and they will leak compression because they are not tight enough against the cylinder walls.

Lucky you didn't have an aluminum head.

P E H

That's what we have be speculating here. Have you seen this happen before? I guess the next question is whether or not a simple ring job will fix it, or whether we have to bore and do a complete rebuild.

Yes, this happened to me on a 220D. It ran real good and would start when hot, but it wouldn't start cold unless the block heater was on for hours. I could roll start it cold because it was a manual transmission. If I was where an electrical outlet was not available, I had to park on a hill or parking ramp. I had another engine so I installed that instead of overhauling the original engine.

I don't know how much the cylinder walls of you engine are worn. You will have to measure them to determine if they are within spec. I think you have a sleeved engine so you can replace the sleeves if necessary and used the same pistons which are very expensive.. You should check the ring groves in the pistons. Usually the top ring groove is worn. The pistons can be rebuilt by a competent auto machine shop by installing a steel spacer in the groves.

If you don't replace the sleeves, remove the ridge and rough up the walls with a glaze buster or cylinder hone.

P E H