B6 1.8t Quattro no compression issue

[JB5]

Hello everyone,

Need your thoughts on an issue i'm recently experiencing with a B6 1.8t I purchased with a blown head gasket about 5 months ago. After replacing all necessary components including a rebuilt head with new valves and seals, I ran the car for a good 1K miles with no problems. one day of all days i drive the car around again, warm it up and shut it off. not even 5 minutes later, i try turning the car on and it begins to missfire on Cyl 1. so i tried the following to begin my troubleshooting:

1- replace spark plugs- same cylinder wet spark plugs and engine sounds awful when running.
2- changed coils- same result, even removed coil while engine was running and there is spark at the coil and engine sound is the same
3- changed injectors- i thought perhaps injector was opened causing the wet spark plug.
4- made sure engine is on time- it is perfectly timed
5- finally found the problem to be no compression on Cyl 1. I mean, the needle on the compression tool doesn't even move when cranking the car.
other cylinders proved to be good 185 average psi on Cyl- 2-4


so I put beroscope into the cylinder to see if somehow the engine jump timing or if there's valve/ piston contact, piston is clean and shows no marks whatsoever. all valve springs are good, none are broken and when cranking with Valve cover off, everything moves good nothing out of the norm.


what's weird is the car never gave any hints of anything bad or noises or anything while driving. literally it was running perfect when i shut it off and now it wont turn back on, when it does Cyl 1 has misfire.

I put a lot of money on this motor to be running perfect and having this happen really sucks.
complete rebuilt head, replaced all gaskets new oil pump replace every maintenance item, just wanted a good engine that could be driven without worry for a while. only gave me 1K miles of use.

any thoughts?
next thing will be leak down test.

[aslusher]

Man, that blows. You may have already come across or done this, but here is a quick method for gathering further information: put a small amount of oil in the offending cylinder and run the compression test immediately after. If you have compression then, then you've got a pretty good clue pointing to piston rings not sealing properly. Did you happen to replace those in the course of the head gasket replacement? EDIT: I am pretty ignorant on this. I just did some reading, and it looks like piston rings require a whole lot more disassembly and work beyond what you're doing for a head gasket. Going to assume the rings were not done.

This may sound stupid, but are the threads for the spark plug port on cylinder 1 intact? Just trying to think of all possible routes for air to escape from the cylinder.

Also, did you check compression right after doing the head gasket? I ask because if not, then its possible that cylinder 1 was already on its way out and that the failure was not in fact all that sudden.

[JB5]

great point aslusher,

I actually did try putting some oil in the cylinder, forgot to include it on the list of troubleshooting steps, but still, there was no compression. when i had it all apart i only checked the rod bearings which looked very good but didn't change the piston rings.
originally it was cyl 2 where head gasket blew, so i thought all other cylinders were good.
Right after doing all the work, i didn't check compression. kind of wish i did. Hopefully when I do leakdown test this weekend the problem would be identified.