[Areomyst]

'Ello friends. I've been noticing lots of smoke from the exhaust the past two days. There's no water in the oil, but the plug on cylinder 4 is caked up and I've just changed plugs 2 months back... the other 3 are fine. It's looking to me like my head gasket is leaking, and I'm ready for the job, but I had a curious find...

(1986 900 SPG, 192,000 miles)

Cylinder 1 - 150 lbs compression.
Cylinder 2 - 145 lbs compression.
Cylinder 3 - 145 lbs compression.
Cylinder 4 - 170 lbs compression.

Why would cylinder 4 be so much higher than the other 3? Is there perhaps some oil on the rings, or some excessive deposits causing cylinder 4 not to get that very minimal amount of blowby?

~Josh

 

[john-w]

I would expect to see all cylinders above 100psi.

Did you do a "wet test" also??

 

[Areomyst]

'Ello John,

I posted to late in the evening. All the readings were over 100, I was tired and forgetful. Sorry there. The plug wasn't too wet, but I had recently had the car running, if that'd make a difference.

~Josh

 

[Matthew]

'Allo Josh

Was the engine up to full operating temperature (fans cycling) and the throttle wide-open?

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[Areomyst]

Matthew


No, the engine had ben running only a few minutes.

I suppose I should check the plugs again with the engine being definately cold?

~Josh

 

[Matthew]

The compression test should be done with the engine at full operating temp (fans cycling), otherwise the figures will be a bit on the low side.

If you've just unplugged the HT leads, then fuel will still be squirted into the cylinders when you crank the engine for the compression test. That would wet the plugs. I disconnect the electrical connectors for the fuel injectors, as well as the HT leads. I seem to think that disconnecting the distributor's electrical connector would achieve the same result, but I canna remember

Always worthwhile checking the plugs when the car's been stood overnight, and peering inside the bores with a torch for signs of coolant leakage. The usual test for hydrocarbons in the expansion tank is a good test too.

If you suspect leaking piston rings, then try a squirt of oil down each bore. That will help achieve an effective ring seal and identify which ring is leaking.

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