Help my 1995 V6 automatic 3.4 liter is having issues. Everytime I drive on the HWY my car stays cool no issues until I stop, then after shutting engine off my car gushes water from the over flow, after about 30sec to a min.
So far I have replaced water pump, thermostat, radiator cap, and hoses. Now one might think it could have a blown head gasket but there isn't water in my oil, nor oil in my water. I have shown it to several acreditable mechanics and the popular diagnoses is HUH! PLEASE if anyone has had similar issues and knows of anything or even if you have a suggestion let me know, before I give and retire my beloved car. :clonk: :clonk: :clonk:

Usually blown head gaskets will show up as "pumping" air into the system all the time...not just when at a stop? So, don't think that's going on per your description. But you can test for combustion products in the antifreeze easily enough. Does your car cool okay when idling and stopping and slow cruising?

When you stop and turn off your engine two things are happening. One is that the water in the engine continues to absorb heat but doesn't circulate and therefore can easily boil or gas out..... It's up to the cap to hold the pressure of the system. However, if you have introduced air to the system and can't get it out.... it's like a vicious circle. The entrapped air offers voids to the cooling jacket and allows veryhot surfaces to continue to nucleate boil-off more coolant as well as increases expansion in the system and pushing coolant out....again.

I would suggest that you have air trapped in the system and until you purge it all out, it's going to bug you this way.

First, you are sure you have the proper pressure two-way cap...right?? (I would get another new one just in case...I have seen new caps fail exactly like this) and get a bottle of WaterWetter or Purple Ice (surfactants help aleviate air)? Secondly, let the engine cool off and then open the cap to look at the level IN THE RADIATOR (not the overflow, but the overflow should be filled to at least the "cold" level if not fuller). The cold radiator should be absolutely full to the top. Third, lift the front of the car with a jack and run the car until you know for sure it's fully warmed up. This aids and abets air entrapment in the engine. Let cool to ambient temperature. Check to see if the cap is working and you should see the overflow level go up and down. If you expell any gas and you might not know it, the gas should be displaced by coolant in the overflow and thus the level should go down. Do not let your overflow become empty ever! If this happens and gas is still being expelled it just sucks air back into the system to do over again.

Hope this hits on something for ya!