As many of you know, this was a plug spitter I bought a few months back. After I did the timesert and a new coil, it still had a misfire on the same cylinder. A compression check revealed zero compression in #7. I did a borescope on the cylinder and there was liquid in the cylinder and signs that the previous owners helicoil had been chewed up and spit out, but no damage to cause zero compression.

I pulled the head and removed the cam to make sure the valves were sealing by putting liquid in the ports and looking for drainage. The intake valve weeps ever so slightly - just like the other cylinders. There is a chunk of the head missing where the spark plug threads in as pictured, but the plug seats and seals. I did a liquid test on it too.

The cylinder looks good to me and despite the obvious damage from the mastication of the helicoil, I can't see how this would cause zero compression.

The cam is good as well. I can only think it may be a blown head gasket. Here's a bunch of pics, what do you people think?

 

Valve seats?
Spark plug not fully seated in the head?
How did the head gasket look?

 

Can't be valve seats, they don't leak liquid when I filled the ports. I did the same thing for the spark plug hole. I didn't see anything obvious on the head gasket, but it wasn't stuck to the head or block.

 

What does the cam look like? Any lobes worn off? Are the valves opening and closing like they should? Just wild guessing?

 

Cam looks fine, no worn lobes.

 

The first pic looks like a crack in the head between the valve at the bottom at about 10 o'clock going toward the plug hole. Doesn't look like enough for zero compression though.

Did you do your liquid test in the cylinder to see if the piston and rings are holding?

When the damage happened the piston could have been hit hard enough to seize the ring and when it cooled down it may have contracted with the piston.

Just guessing though.

 

I agree there's a good chunk missing around the spark plug boss in the first pic. Might be enough that it's slipping past the sealing washer (when under pressure)

 

Worn cam wouldn't negatively effect compression. Valves don't open, or open less, but they still would be closed during compression stroke

 

Another idea, is it possible your compression tester didn't seal properly on the head due to the threads/repair?

 

That looks like a crack, but its really just dent. I had planned on doing the liquid test in the cylinder today.

If it were building enough pressure to slip past the plug seat, I would think that should at least register on the compression tester.

I think I'll TIG that part of the head and put it back together.

 

You straight edge check for warpage?

Which cylinder is down?

 

You should also consider cleaning up the top of the piston for a thorough inspection of it

 

Cylinder 7, and yes, I'll check for warpage.

I'm actually thinking of dropping the pan and popping that piston out.

 

I'd say that is a good idea. Your problem sounds like a piston ring issue to me.

- Jack

 

While I am not disagreeing with you...

1. I would think for a ring failure bad enough to cause zero compression, there would be scoring on the cylinder.

2. A liquid test should reveal a substantial failure.

3. I really don't want to drop the pan!