It's been a while since I updated on my overheating problems with the C8, I am just back from 6 weeks in Canada so here goes.
Original problem being the coolant being pushed out of the radiator so after lots of investigation/repair and finally having the radiator stripped and cleaned, today I checked for combustion gas in the coolant and whilst doing this with the cap off I watched the coolant rising and pushing out of the radiator filler, once it was finished rising the level settled down just below the filler tube. The engine was running while this was going on so I carried on with the combustion gas check of which all was clear, I replaced the radiator cap and had a run up and down the garden with no ill effects. I will take the truck for a run out on the road tomorrow and hopefully all will be well.
Paul
