Your symptoms seem to be closely in line with those for a blown head gasket. Essentially, after warm-up your engine is effectively running on one cylinder.
The engine you have is notorious for blowing head gaskets. They always seem to blow between the combustion chamber and the oil drain hole or pushrod chamber, sometimes passing through the cavity around a head-attaching stud. There have been suggestions that a common cause of the problem, is overfilling the sump with oil.
I suggest you check for signs of heavy blow-by: remove the dipstick or oil filler cap, run the engine and see if gas is blowing out of the hole where you removed it. If you have heavy blow-by after 150 hours, and the engine has not been seized up, it is very likely one of your cylinders has blown its head gasket between the combustion chamber and the pushrod chamber.
In its early stages, a Vanguard with the characteristic blown gasket tends to run satisfactorily from cold then leak worse and worse as the engine gets warmer. Then, as the gasket damage progresses, it leaks right away from a cold start.
It is quite easy to fix, but it is not a good idea to run it under load in the meantime. It will erode the top of the cylinder and/or the underside of the head, and make a small job into a big one.
Last edited by grumpy; 06/12/11 03:05 AM. Reason: Add detail, second para
