Low compression: Piston-to-cylinder or head gasket?

First of all, this place is booming. I need to visit much more often.

I was looking at a start - stall on a high mileage, poorly maintained Honda 3.5L. I could keep it running with the throttle which produced a lot of white smoke out the tailpipe. It had very low compression (50psi) across one bank. Smoke in the spark plug hole entered the crank case. NO smoke made it into the coolant system. It was already destined for the scrapper, so even a head gasket was out of the question.

So, my question is, could a head gasket cause low compression and smoke enter the crank case through the oil passages? OR Is it for sure losing compression around the piston? Is there a way to make this determination?

Also, I didn't dig far enough to see if it had jumped time and damaged the pistons. The problem did't happen all at once. It worsened over a couple of weeks.

To determine where the compression is going, you need to roll it up on TDC of compression stroke and pressurize the cylinder. Now LISTEN to where the air is going.

Exhaust pipe = exhaust valves
Intake Manifold = intake valves
Dip-stick tube = rings
Radiator Bubbles / adjacent spark plug hole = head gasket

Here is Eric the Car Guy using a bleed-down tester. The tool helps determine how BAD the loss is, but it is not necessary. Adapt shop air to a spark plug hole, and listen.



This may, or may not, answer your question. But, it might be useful.

??? Radiator/bubbles that should be a head gasket



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I am willing to bet it is the rear bank of cylinders

Andy.MacFadyen wrote: I am willing to bet it is the rear bank of cylinders

That would be a good bet. What's the culprit on these?

Just so I'm clear... It's definitely losing compression into the crankcase. The whole bank makes me think it's not leaking around the pistons.

You could theoretically be getting leakage into the crankcase if the cylinder head and/or block is warped badly enough. Like, they ran it out of coolant, and had it glowing red hot before it shut off. :silly: Usually, when that happens, there's also external leakage of coolant/oil.

FWIW, I don't have confidence that smoke would make it's way back to the radiator. I've never tried it on a known bad car? :huh: But I wonder if a leak down tester would have produced bubbles in the radiator. Not being critical, just speculating.

Good point. If the head gasket was bad enough to leak into the oil passages, it may not be able to build enough pressure to push through the cooling system. This thing did have more leaks than a screen door so I can’t promise it wasn’t leaking oil out under the head.

Also, I use a homemade smoke machine that can supply more air than a leakdown. I’ve found it useful when there’s not enough resistance to do a proper leakdown

I believe this is the answer. Any type of low pressure gauge would work. Wish I had thought of that

PDM wrote: I believe this is the answer. Any type of low pressure gauge would work. Wish I had thought of that

That was a great video. :lol: I've done the same thing with my homebrew FirstLook sensor.

www.scannerdanner.com/forum/diagnostic-t....html?start=17#23481