crack in cyl. wall, need advice/opinions
 

Just a quick back story. This issue is with my '92 190E (170,000 miles) For the longest time the car has a had a rough/loping idle. Give it a little gas and it smooths out. Had a compression test done and #3 is low compared to the other cyl.

Now to present day, I bought a video borescope and just looked inside #3 cyl and it looked like there was a crack. Amazingly I can still see the factory bore marks, but I also saw what appears to be at best described as a hash mark/crack. It is brownish in color, which is easy to see against the gray cyl. walls.

If I had the ability to connect my borescope to the computer, I would upload a pic. I will see if I can take a pic of the video screen and see how that comes out.

Now, obviously a crack anywhere will cause low compression numbers. My question is, is it possible to have a crack in a cyl. wall, but not have any issues with coolant being burned out the exhaust? I don't have any coolant in the oil, or oil in the coolant.

Thanks

Ok, I managed to take pictures of the video screen. The hash mark I was speaking off can be seen on the right side of the screen. I know the pic isn't the best, but you can see this mark and it is horizontal. Also I took a pic. of the piston when it was further up the cylinder. Literally half of the piston has carbon on it and the other half is clean. None of the other pistons looked like this.

Any ideas?

This is not a problem to repair with the engine disassembled. A competent machinist can bore the cylinder and put in a dry sleeve then fit to the piston.

It would be advisable to check the other cylinders, bore and/or ring as required and do a valve job while it's apart. This is not much beyond a typical engine overhaul.

Hope this helps.

A couple of things.

How low compression?

Do a leak down test before taking the motor apart, see where the air leaks out.

Are you losing coolant from the rad?

Is this motor CIS or EFI ? If EFI, pull injector wires one at a time to isolate the weak cylinder. If CIS you will have to make a spark plug wire set that allows for shorting a cylinder at a time. You might be able to loosen a fuel injector line to drop a cylinder but it might take the pressure down for the other cylinders, have not tried it.

For a cylinder crack to drop compression much, you would be pushing lots of air out of the rad and have lots of antifreeze coming out the exhaust.

I had both my wife and neighbor look at the image while I had the borescope in the cylinder and they both agreed that there is a horizontal (ish) mark, that isn't from the factory. Again, the cylinder walls are nice and clean and the original boring marks are very visible.

Is it possible to have a crack in the cyl wall, yet have no issue of burning coolant (thick white smoke). I estimate that this mark, is about 1 - 1 1/2 inches down from the top of the piston. There is no evidence of oil in my coolant. Maybe I am chasing my tail here.....

The car has CIS (hard fuel lines) The car actually failed emissions for excessive HCs. Again, the garage figured this issue maybe a burnt exhaust valve and it's not only affecting compression, but letting unburnt gas get through. They were going to disconnect #3 cyl fuel injector, and re-run the emissions test, to get it to pass, but since it's CIS, no can do.....

The head gasket was replaced on this car 37,000 miles ago. The valve train was gone through while the head was off, and there were no issues. I always use 93 octane gas and every so often (no sooner than 3K miles) I put in a bottle of Techron fuel system cleaner. I am getting about 24 mpg in this car. So I don't see a burnt valve developing after only 37K miles. However, I could be wrong, hence my asking for opinions.