We've been chasing this problem for a bit now, I bought the car about 2 weeks ago and the coolant reservoir boils when I drive at slow speeds or stop, I can drive 50 miles at normal speeds but when I stop it doesn't take long for the coolant to boil in the reservoir.

Here's what my mechanic has done:

Block test a few times making sure it's not a head gasket.
Replaced the thermostat housing because it was leaking.
Installed a new radiator cap
Installed a new radiator.
New water pump.
Checked the temperature sensor and they check out fine
The temperature gauge doesn't indicate the car is overheating it doesn't go over half way he checked the temps they are consistent with the temperature gauge BUT it boils over. The fans are slow to respond because the car doesn't get too hot (as far as the gauges go), he did check the fans to make sure they are operation they work in low and high speeds and they will turn on with the ac on but they are slow to turn on without the ac because the cars temps aren't too high.
Today my mechanic said that it seems that the bottom radiator hose isn't hot enough until it's too hot, he will change out the thermostats even though they are new (changed out the thermostat housing).

What are we missing? He's a great mechanic with his own shop but I'm afraid this may have him stumped and the next stop might be to the dealer I don't want that.

 

does it sound like coffee percolating when you turn the engine off?

 

It does.

 

Just to be clear it does make that sound when I turn the engine off, it also does that when I stop moving or when I slow down.

I forgot to mention I also installed a new radiator cap.

 

Any suggestions?

The thermostats didn't make a difference he will install a new water-pump on Monday after that he is out of ideas. I feel somewhat confident that will fix the issue but I'm losing hope.

Have you guys heard of these cars having an issue with the water-pump?

 

I know you installed a new radiator cap, but it's a possibility that the new cap is bad. Try loosening the cap and run it briefly. If it behaves the same with the cap loose as it does with it tight, then maybe there's your problem. Doesn't hurt to try it.

When a water pump goes bad, it is usually because it leaks. Water pumps are pretty simple things, failing to pump water is usually not one of their failure modes.

 

I was somewhat thinking the same thing but 2 radiator caps? not likely but certainly possible. I don't have the car but I do have the old radiator cap I was thinking about installing it on my truck to test it that way.

My other thought was to buy a different brand of radiator cap.

 

I put the old radiator cap on my truck and it did fine....

 

Pressure test the cooling system!
After boil will occur if the system is not pressurized. You confirmed the cap but what about the reservoir and system???
Todays overflow tanks have 2 chambers. pressurized and atmosphere. If the is a leak between chambers it will do exactly what you describe.
How do I know?? Happened to me, slightest of cracks between chambers drove me crazy for weeks.......

 

I'll check with my mechanic to see if he's checked it. How do you check the 2 chambers?
Do you have a Sebring of this age and how did you figure it out?