[patrickt]

I think I would just loosen my plugs and let the pressure out of the cylinders and pull a cable off of the battery, and then when the engine has cooled off put the cap back on it and see if it continues to build pressure while it's just sitting there in the garage. If it does then it has to be a chemical reaction. You don't have any other things under the hood that are under pressure, do you? Anything like an AccuSump, Fuel Injection, a bottle of NOS, something else that's under pressure?