He said GT so it has a 4.6. A '99 would indeed have the bad manifold. He says its venting through the pressure cap, though. Not around the thermostat which is the usual failure point on the plastic coolant crossover, though I have seen/heard of other places. The temp sensor area is another one. The extended warranty on the intake manifold was 7 years/...I forget the mileage, but it does not matter as it is long out of the year range officially as the last vehicles covered were MY01 which would have been up until 2008. Interestingly, I have heard of two or three MY02 vehicles which had the bad manifold and were denied coverage because it was not within the extended warranty year range.
But this is getting off track.
I would have the new cap tested just on the off chance you got a faulty one. The suggestion to check the surge tank to make sure you are actually sealing is a good one.
If you know someone with a scangauge or similar that can read raw data from the computer, that would be great. As it would solve any question of it actually running hot or not.
Heck, even if you have to shell out for one, they are extremely useful for any troubleshooting like this and it is way worth the money.
Headgasket is possible, but unlikely; they rarely fail on these engines.
But it will build pressure like this.
Question: If you take it out cold and hammer on it, will it vent out the cap, before the gauge even enters normal?
I have found that coolant from a engine with a blown HG will smell like raw fuel in addition to the usual sweetness. A better test would be to use one of those kits that sucks the air coming out of the coolant system through a liquid that reacts to hydrocarbons.
A leak down test would would work too, but a bit more complicated, since you have to ensure both valves are closed on the cylinder you are attempting to test.
Ive found straight compression tests unreliable for a bad head gasket, interestingly.
