Almost boiling over when parked

[blk95lumina]

Hello.

I have a '95 3.1L w/ ~150K mi.

Whenever I park the car after an hour trip or so, I notice that after sitting with the engine off for only a few minutes, the coolant in the reservoir begins to rise (already at the "Hot" mark before turning car off) past the "Hot" mark to just a couple inches short of spilling out.

With the coolant at this level in the reservoir, I have then started the car and the temperature measures "normal"....usually right in the middle.

Also, after the car was idling for less than a minute, the coolant level goes back down to the "Hot" mark.

The coolant level will also subside once the engine cools off from sitting.

I have checked the coolant temperature sensor near the thermostat, and it is working properly. I have also recently replaced the upper and lower intake manifold gaskets. I do not suspect a head gasket leak b/c there is no coolant in the crank and no oil found in the coolant...after I replaced the intake gaskets. The coolant also passed the test for not having exhaust gasses...this was tested before I replaced the intake gaskets.

Any ideas on what could be the problem? Faulty rad. cap?

Why does the coolant rise only when the engine is off and not while running?

Thanks.

[maxwedge]

I would change the cap for sure and make sure there is no air trapped in the cooling system.

[blk95lumina]

Thanks. I'll try a new cap.

How do I confirm whether there is air trapped in the cooling system?

Thanks.

[merit4242]

follow the top hose to the thermostat with the car hot there is like a gold/brass fitting there looks like a flat head could fit there open that little valve if fluid comes out and no air then close it up

[blk95lumina]

You mentioned doing this while the car is Hot, but shall I do this w/ the car running or engine off?

Thanks.

[maxwedge]

T'stat should open to do this.

[Bearwulf]

Engine running.