Low compression 50psi

[Joes_meat]

I've just had a look at a mates car.

He was complaining about missfiring at low-revs (seems fine at 1200rpm.)
I had suspected the coil-pack or DFI module.

But I first did a compression test. An even 125psi on all cylinders except cylinder 3.

It showed 50psi dry and 80psi wet.

I'm just trying to work out what the most likley cause for this is?

Broken piston ring/bent valve/broken or stuck valve seat?

Thanks in advance.

[Morgasshk]

blown rings...

[maginoodle]

I've just had a look at a mates car.

He was complaining about missfiring at low-revs (seems fine at 1200rpm.)
I had suspected the coil-pack or DFI module.

But I first did a compression test. An even 125psi on all cylinders except cylinder 3.

It showed 50psi dry and 80psi wet.

I'm just trying to work out what the most likley cause for this is?

Broken piston ring/bent valve/broken or stuck valve seat?

Thanks in advance.

are the 3 cylinders on the same bank? if so it could be a bad head or head gasket, if they are on different banks i would say rings need replacing.

[FXST01]

Not 3 cylinders, cylinder number 3. Could be either your rings or a burnt or leaky valve. A vacuum gauge should point you in the right direction.

[mouce]

Cracked piston? I've heard of it happening.

[commy92]

could be so many things. In reality either way its going to have to be stripped ide say to solve the problem, I think that this is whats wrong with mine, a rooted ring or piston. Im to afraid to compression test it.

[Joes_meat]

I'll see if I can get a vaccum test. If it's a valve problem, I'll fix it, if it's a ring problem I got a quote for $880 for a low-km motor with warranty (full engine barring alternator.)

How would I go about a vaccum test and how would I understand the results?

[Tree cutter]

If it is a piston or ring problem it would blow a fair bit of smoke when you give it a good rev. More than likely a valve or valve seat problem.

[mouce]

http://users.bigpond.net.au/ergoff/vac1.htm

Posted on here a while ago, found it so useful I bookmarked it. It explains how to read a vac gauge.

[Joes_meat]

Thanks for the help guys, I'll let you know how it turns out.

[VT-565]

a bent conrod will also cause a loss in compression