No Compression

[sgeorgejr]

Need help diagnosing.    I just bored my 1.6 diesel and replaced it with a 1.0 mm oversized pistons.   The car ran but was smoking a lot after the rebuild.  After compression check, 1,2,3 had almost 400psi and 4 had 0.  The pistons are brand new.  I am suspecting a bent valve, but by eye, they look ok.  The old piston broke on the old engine, so that is why it needed a boring/rebuild. is there anything i am missing?   

[Vincent Waldon]

On a fresh engine some smoke is very normal... sometimes they smoke quite a bit.

A couple of questions:

- is the hole with the low compression the same hole that broke the piston the last time??
- was there any headwork performed while you had the engine apart?
- did you seat the rings during the first 20 minutes of run-time?


A valve that's not seating or some broken rings in #4 would account for low compression.. although 0 is really low!

[sgeorgejr]

Thanks for the reply,

The no compression was the same as the one which broke the piston, and i did not perform any head work  (although i just sent it out today to get checked).   I did not reset the piston rings.   The car did not run for over 20 minutes.

[maxfax]

I'd bet one of your valves on that cyl is tweaked a bit..  Just eyeballing them they can look fine, but not be perfectly sealed.. 

0 compression is impressively low..Did you notice an odd "hissing" sound from the exhaust or intake when cranking or running??   

[Quantum TD]

If you had done valve work, I would have guessed that your valve shims were too thick. But since you didn't, I'm guessing everyone else is on track with bent valve.

[R.O.R-2.0]

how on earth did you CRACK/BREAK a piston?!?!?! 

[sgeorgejr]

2 bent valves......thanks for the replies.   I didnt break the piston, the last owner did.    not sure what happened, but it was mine to fix for $400 so i took the chance. 

[maxfax]

Were they both on the dead cylinder???

[sgeorgejr]

yes they were.  valves must have wacked the broken cylinder