prostreet505

Hi everybody, newbie to the forum here. I have a very strange problem going on here. Lets start by saying my car is a 99 GT Conv. with the 4.6 single overhead cams and a automatic trans. The car is bone stock, no mods at all. It has 145xxx miles on it. I just bought the car a week ago, knowing that there was a overheating problem, but I got a killer deal on it.

After I trailered the car home, I noticed that the coolant bottle was empty and very rust colored. I filled the bottle with straight water, ran the car for a while and the water was being pushed out the cap on the coolant bottle. I found the cap to be defective and replaced that. This time I removed t-stat housing and filled the block with water and topped off the coolant bottle with water. I ran the car for 1 hour at idle in the garage, cooling fan turns on at about 210 and turns off at around 185(had a scan tool pluged in the OBD to get temp reading from the PCM) and the gauge on the dash never breaks the halfway point, and don't overheat or push out coolant.

Now I drove the car 15 miles and pulled into the drive, opened the hood and the water was being pushed out from the cap again. The temp gauge was still about the halfway point. Let the car cool down, removed the t-stat and waterpump and tryed flushing as much rust out of the block as possible, replaced the waterpump with a new pump "not a reman",left the t-stat out this time, filled the block with 50/50 premixed coolant and topped off the bottle with the same coolant. Ran the car at a idle again for a hour with the cap off to check for any sign of bubbles in the tank and found none. The temp stayed around 165. Put the cap back on and drove another 15 miles, got back home popped the hood and the same thing. Coolant was coming out of the cap again. Let me add the hoses are all hot and the car has great heat, but the upper and lower hoses are rock hard. The temp never broke 180 while driving though.

I am replacing the radiator tomorrow just because of the rust condition and it only seems to act up while driving. I am hoping this will take care of the problem.

Has anyone ever seen a headgasket leak while driving and not at idle??

Any and all help will be appreciated.

Derf00

No smoke out the exhaust?

How's the oil look (milky)?

Let the car cool, take the cap off the radiator, run the car. See if you see any bubbles coming up through the neck. That would be compressed combustion chamber gases getting pushed into the cooling system through a blown head gasket.

Next step would be to run a compression test on all cylinders. They should be within 10% of each other.

Check the plugs as you pull them out for any rust or weird colors.

prostreet505

No smoke out of the exhaust and the oil looks normal, so milky signs of coolant getting mixed in.

I ran the car for an hour with the cap off and t-stat removed and no signs of any bubbles.

Replaced the plugs and the old plugs looked fine. No abnormal colors or build up of any kind.

I didn't do a compression check because the car runs great, no stumbles or missfires of any kind and this thing has great power yet. I can do the test, but wouldn't be able to until Tuesday when I go back into work and get my gauge set.