Overheating-weird problem

[Mac626]

Hi everyone. I am having a problem with my '98 626 (6-cyl) overheating and expelling fluid from the overflow. Here's what I have done:

1. Replaced Thermostat (a PITA, BTW - no effect)
2. Flushed system (4 times, 4 different products - no effect)
3. Replaced sensor (to PCM, not the gauge - no effect)
4. Replaced radiator cap (and verified with pressure tester - no effect)
5. Checked fans (both are functioning as they should, but see below)
6. Pulled hair out (no effect)

What happens is that the car will run and the temperature will stay normal during most driving conditions. When I stop and turn the engine off, the fluid will boil out of the overflow <1 minute later and I end up losing about a pint+ of coolant/water. If I let it idle, open the hood an watch it, the following occurs:

1. Car heats up
2. Eventually begins to boil over
3. 2 to 3 seconds later fan kicks on
4. Stops boiling over (fluid loss is about a cup or so)
5. Process repeats until out of fluid (if I allowed it to continue)

It seems as though the fan is kicking on a little late or something. I wonder if maybe using a higher pressure cap might be the answer (to raise the boiling point), but that's a wild guess. Obviously something is not right, either the PCM is goofy, or perhaps the stars are misaligned.

If anyone has any ideas, please let me know - I'm getting tired of having to put fluid into the darn thing every day.

[Mark E M]

Yea, sounds like the head ahs a minor crack or the head gasket is leaking into the coolant system. need to have it checked at auto shop,they have a tool for checking compression gasses in the coolant.Hope this helps. Later, Mark.

[Mac626]

Originally Posted by Mark E M

Yea, sounds like the head ahs a minor crack or the head gasket is leaking into the coolant system. need to have it checked at auto shop,they have a tool for checking compression gasses in the coolant.Hope this helps. Later, Mark.

Thanks Mark. I only checked the oil for cross contamination. I'll see if my local shop can do that compression gasses test (not that I could afford it if the heads need to be replaced).

[Mac626]

I just thought I'd post the solution to this. I finally became frustrated enough to take the car to a mechanic. He pressure tested the cooling system and discovered a leak in the overflow tank. The leak was at the seam and I would have never seen it. Anyway, this allowed coolant to steam away while I was driving and also caused air to get into the system. I bought a new tank ($85 and it comes with a new filler cap) and had the mechanic install it. He charged $40 to install the tank, refill the system and suck all of the air out of it with a special pump. It has run nice and cool ever since and the leak is gone.