[Mk2] [Turbo] How can you lose cylinder compression?

[Rev2_mike]

As the title - how does this occur and why does this lead to an engine being 'blown'? Does it still run?

[cantfindausername]

My understanding is that you can lose compression in 4 directions.

1 - into the crank case via blown rings/piston skirt
2 - into the ports via cracked/damaged valves
3 - into the coolant system via the head gasket
4 - into the head via damaged combustion chamber

- I would say there are others, but it depends how anal you want the list to be.

As for the car still running, that depends how bad the compression loss is I guess. What ever the cause, its not a good thing.

[john_p_b]

the above is correct really, depending on how many cylinders you lose compression on will decide how well the engine will still run.

if you have a total compression loss on 1 cylinder then it'll still run but with bad missfire!

[Chr15py]

My car recently let go. It turns out that it was due to piston 3s ringland going awol. When I did a compression test it was 11.7ish across 1,2 + 4 but 6 then 4, then 2 in cylinder 3.
Drove fine, no hesitation at all. Only noticed something wrong when I pulled outside my house - sounded a bit like a scooby
Chris

[simmo490]

as above yeh i had that exactly on mine, tends to be number 3 due to running slightly hotter down to the inlet manifold design i am led to believe.

simmo

[mrdb]

Everyone says inlet manifold design but i feel its a combination of 2 things

1) mManifold runs more air into no 3 runner so leaner mixture occurs on no 3.
2) Fuel rail also provides poorer flow to no 3 and 4 injector due to pressure drop adding to the already lean no 3...

just my 2p