COOLANT LOSS - IS IT HEAD GASKET??

[V8JOHN71]

I have a Discovery 1 on carbs which has been losing coolant water and I can’t work it out! There are no obvious leaks anywhere and as it is not a regular use vehicle it’s hard to determine how much coolant is going though I am guessing around quarter to half a litre on a 50 to 60 mile trip.
There are no bubbles to be seen in the coolant reservoir when the engine is running and I have done three block tests with no indication of exhaust gases getting into the coolant at all
.
There are some slight misfires on idling and some wispy traces of smoke or steam on idle when hot, but very little. Video link shows idling when warm, not sure if water drips are condensation or coolant!



I have had all the spark plugs out and as expected most of them are quite black and sooty as I know it is running rich, indeed I had a problem getting it through the MOT because of this, so at some point the carbs need attention.
The only exception to the appearance of the plugs were cylinders four and six (adjacent to each other) which were both very clean and light brown colour, no trace of black sooty appearance.

Cold compression test shows regular pressure on all cylinders at around 150 psi with minor variations. Using a borescope I looked at the internals of each cylinder and number six had what looked like a wet appearance on part of the piston top, can’t believe this is oil as in that case I would expect the sparkplug to be dirty – it was one of the clean ones.

So I am thinking, is the most likely thing a blown head gasket around the area of cylinders 4/6 with coolant getting into the bore??
I had the heater matrix out some time ago to fix the fan and it looked almost new with no evidence of water leak in the heater box and there are no drips anywhere in the coolant system or radiator. Any thoughts will be much appreciated

[scudderfish]

When I was rebuilding my engine I had coolant getting into the sump due to a dodgy valley gasket.

[Ian Anderson]

I had a leaking core plug….always was dry but “salts” deposit gave it away.

Ian

[ChrisJC]

Is it still a 3.5?,or is it a 3.9?

Chris.