After posting this thread we finally got round to buying a compression tester and checking the cylinder compression. All cylinders are within tolerance (190 - 195 psi) but I still suspect exhaust gases are getting into the coolant through a head gasket failure but I just can't quite get my head around how it's happening, especially as the compression on all cylinders looks to be ok? Before pulling the engine out, stripping it down, and going to the expense of buying new gaskets, it would be nice to be absolutely certain it's a HGF. Is there any other way that the exhaust gases could possibly be getting into the coolant?

Before I buy an engine block tester kit to confirm there are exhaust gases in there, I don't suppose anybody has one lying around?

Phil

A compression test won't show a head gasket failure unless there was a huge lump out the fire ring, in which case you would get a massive misfire.

Get a friendly garage to sniff the gases at the top of the rad header tank with their Exhaust Emission tester.

Kit you can buy are chemical test kits with a limited number of shots.

Only other test that might show up something is do a pressure test on the cooling system, pressure the cooling system to just below 1.4 bar or whatever the pressure cap is rated at, shut off the valve on the tester and see how long it takes to leak down.



[Edited on 7/10/13 by britishtrident]

Aye there's dozens of ways you could be pressurising the coolant through minor cracks and leaks. On a number of cars I've seen cracks that only open up when the head is hot and let a tiny dribble of gas into the coolant, no misfire, no obvious compression loss.

TBH on most engines it's easier and cheaper just to strip it and see, but it's bloody hard to find some cracks.