I think I have a blown head gasket, but some things are just not making sense.

- When cold, idles rough for 10-20 seconds, then smooths out at 900-1000 RPM
- When warm, idles rough all the time (at correct revs), like a really lumpy cam, but smooths out above 1000RPM
- losing 300-500mls of coolant a day (about an hour's driving), but nothing visible under the car or the bonnet
- disconnecting cylinder 1 plug or injector makes no difference, but disconnecting the others does, so the problem must be in cylinder 1
- swapping plugs between cylinders 1 and 2 makes no difference
- replacing number 1 injector makes no difference
- compression is all over the place: cylinders 1 to 4 are 110, 110, 150 and 95 psi
- spark plugs show good colour
- new coilpack and new 8mm Champion steel-core wires
- No smell of exhaust or petrol in overflow tank
- No smell of coolant in exhaust
- Builds up high pressure in overflow tank, spraying coolant everywhere when opened, even when cold.
- No coolant in oil; no oil in coolant
- When switched off hot, can smell petrol fumes
- Changed from Autolite 985 plugs to the recommended NGK ZGR6B-11 (2 steps colder) a month or so ago

This can't all be caused by a blown head gasket. But for all this acting up, the engine is still producing excellent, smooth power from 1000-5000 RPM. The Engine is stock XR6, stock Tickford cam. Anyone have any ideas? What else can I check?

definite head gasket or cracked.

Replace the head gasket. You know you need to.

Thanks guys. At 253,000kms, I suppose I can't complain. It uses only a litre of oil every 10,000kms, so the bottom end and valve guides seem to be in decent shape.

With the head coming off anyway, maybe it's a good opportunity to up the compression, do a valve job, and do a little porting and polishing. Should be good for another 10kw or so.

Any recommendations or warnings?