No compression on Cylinder no. 5

[Patronus]

My '74 2.7 was running rough after it had been standing for a while (I had some work done on it, such as new tensioners, etc), so I decided to do a compression check. It turned out that cylinder no. 5 had zero compression. And I do mean zero. It did not smoke and there was no blow-by sound. So I thought it could be a broken ring.

I removed the head and found nothing wrong. No broken rings, in fact they look like new. The cylinder wall also looked like new. The valves were in perfect condition, and the cylinder head wasn't cracked or anything. I did, however, found the following:

1. One of the head nuts were loose. But I reckon it wasn't the problem, since I didn't hear any blow-by, either hot or cold. I assume I would have heared gas escaping between the cylinder and the head if that was the problem. The studs are in good condition, and helicoils have been inserted some time ago.

2. The exhaust valve wasn't seated correctly. In fact, there was a slightly irregular carbon deposit on the surface where the valve should have be seated. However, I believe that it would decrease the cylinder compression but not render it zero. Right? I tested it with gasoline, and it trickled through, indicating a leak around the exhaust valve. I have seated it properly now, but I fear that it may not be the problem.

My question is, could either of these be the reason for a total lack of compression, if everything else seems right?

Is it possible that the valve clearance could have been totally wrong, i.e. too tight - so that the valve never really closed?

Tell me what you think, before I put it all together. Please don't tell me to take it to a Porsche mechanic.

Thanks

[Bobboloo]

The carbon was your problem. Any time a motor sits for a long time moisture can loosen the carbon from the heads and pistons which due to the nature of a flat six gravity causes the carbon to fall landing in your exhaust valve seat. It's to bad you didn't get on the BBS before you broke down the motor because this is not an uncommon problem. You can search the archives for a previous post on this. Warren Hall has a method for cleaning out the crud with B12 Chemtool.

Bobby

PS the thread is opinions on my engine situation
requested.

[john walker's workshop]

don't you wish that you checked the valve clearance before tearing it down? as far as carbon caught under the valve goes, it usually blows out after running the engine for awhile.

[Patronus]

If that was the problem I am happy, even if it took me a day to open up the engine, and another to put it back together. All in the name of experience.

[john walker's workshop]

i would at least lap that exhaust valve in to check it's contact to the seat. as long as it's apart. check the guide slop too. maybe it's a good time to do the valves, for guide wear problems if nothing else.

[Patronus]

The reason that I didn't suspect the valve clearances was the fact that a Porsche shop had just adjusted it for me, before I put the engine back in the car.

I did re-seat the outlet valve, and it now makes proper contact with the seat.

Thanks for the advice.