Failed head gasket?

[bigbeard]

Compression tested my 400SE last night. Cylinders 1,2,3,4,6 and 8 were all fine, showing pressures of 140 - 150 psi. Cylinders 5 and 7 (the back two on the right hand bank) were both showing exactly the same value of 107 psi.

This implies to me that there is a head gasket leak between these two cylinders. The only other sign of a possible gasket failure though is that the pressure in the cooling system always seems pretty high to me. There is no coolant in the oil (that I can tell anyway!) and the engine doesn't overheat.

Does my conclusion make sense to anybody else or am I barking up the wrong tree here?

[Boosted LS1]

Sounds like a reasonable diagnosis to me. There isn't a water passage there so you wouldn't expect a leak as such. It could pressurise the crank case. Any increase there? Has the valley gasket bowed upwards?

[adam quantrill]

If you put the spark plug back into #5 and measure #7, and the pressure rises, that would confirm it (and/or vice-versa) depending on the relative timing of each cylinder (which I don't know off by heart...)

[HeyAndy]

Boosted LS1 said:

Sounds like a reasonable diagnosis to me. There isn't a water passage there so you wouldn't expect a leak as such. It could pressurise the crank case. Any increase there? Has the valley gasket bowed upwards?

What are the symptoms to the valley gasket bowed upwards. Is it a big job to replace if it has gone?

[Boosted LS1]

HeyAndy said:

Boosted LS1 said: Sounds like a reasonable diagnosis to me. There isn't a water passage there so you wouldn't expect a leak as such. It could pressurise the crank case. Any increase there? Has the valley gasket bowed upwards? What are the symptoms to the valley gasket bowed upwards. Is it a big job to replace if it has gone?

If the valley gasket has bowed upwards that could be because of excess crankcase pressure caused by combustion pressure getting past the piston rings. If the engines fine it could be caused by blocked breather pipes. If the gasket has bowed upwards it doesn't mean that it has to be replaced. It may have been just the blocked breathers