Low compression in all cylinders. What could it be?

Ser_Fed

Hello everyone,

Trying to get my Z22SE (US L61) back to life.

It came to us having major problems: there were a burned hole in third piston and oil/coolant cocktail in oil pan. This motor has very low mileage, about 20K. The problem was in a production defect, an air bubble remained when piston was casted, so it burned thru and the motor overheated.
We put a new piston and rings set from Kolbenschmidt and checked other three cylinders/pistons/rings condition - all like new. Next, we checked the head and found that it is not perfectly flat. We got it to specialized workshop where it was polished and checked for cracks. They took away from 0.05 to 0.7mm of head's surface.
After reassembly of the engine (of course with all new gaskets and bolts), we tried to start it. After a minute of struggle it started and ran quiet and smooth. We let the engine to wam up then switched it off. In a minute it wasn't started again. We found that compression in all four cylinders is low and almost even (only 6-7 bar each). The pistons and rings are ok because compression did not change considerably when we added some oil into cylinders when measured.
So there are problems above the block...
What could it be?

Thanks,
Serge

slindborg

did you test with the throttle fully open?

Ser_Fed

The throttle was opened, and even more - manifold have been taken off.

Today we took the head off again to check everything once more. We simply have poured some diesel in cylinders and leaved for four hours. When we have come back the level of liqud was the same. Valves have been checked in the same way. Everything is tight. Now the head is on again (fortunately, a gasket is cheap). The chain and sprockets not installed yet.

The only thing, that I can surmise is that because of polishing the height of the head has changed and cam phases have displaced.

vocky

could be a blocked cat, loosen the exhaust downpipe and try again 🙂

batcho

could be valve problem compression should of risen with oil added to the cylinders

Ser_Fed

Good idea. Will try.

And I have used the wrong term - the head surface was MILLED, not polished.

Ser_Fed

It becomes mystic...

There is no difference between trying to start the engine with exhaust pipes connected or disconnected. It does not start. And the pressure in cylinders is still low.

I think we have to start from the beginning. First, we intend to check all cylinders once again, this time with micrometer. We did not measure them before because there is no any traces of wear on surfaces.

I have few questions:
Should the nominal cylinder bore diameter be exactly 86mm? Is there any "maximum tolerance value" for cylinder's diameter? And what is the piston's exact diameter?

Thanks

batcho

you should measure the cylinder bore in three places top middle and bottom and at right angles to each other i dont know what the tolerance should be i am sure a engine recon shop would help you out if you can not find the information anywhere else

Ser_Fed

We use TIS info taken from here http://www.speedsterclub.nl/bibliotheek ... index.html
It's useful guide, all procedures are clearly described, but there are such references as
"Piston ring sizes and permissible piston ring gaps – see "Technical Data"
or
"The oversize identifications can be found in the "Technical Data"

Does anyone here have a link to that "Technical Data" for the Z22SE engine?