Sorry for the crazy title. But that's what my thermostat does 15-20minutes into driving. I cannot think of what could be the problem. My fans are working fine. I let the car reach the brink of overheating, then when I popped the hood, the coolant reservoir was almost full to the cap bubbling and boiling. I recently got my engine bay sprayed and clean. Is there any chance that my car is nor overheating as a result of what they used or maybe what ever they used may have gotten on to something?

 

try replacing the cap

 

Is it possible that the cap could be the problem? That seems kinda crazy

 

if it is not sealing properly... you would be surprised.....

 

well?

 

I would definitely stop driving the car and don't let it hit near overheat again.

Try burping the radiator, I'm thinking there might be air in the cooling system.
https://www.google.com/search?q=burp...rping+radiator

You could also try
Open the radiator cap (Not the reservoir) - Start the car COLD - and squeeze the radiator hose.

Other thoughts would be that the thermostat might be broken and not opening.

 

Reservoir cap or radiator cap?

I have a mishimoto radiator fan shroud, replaced the thermostat and flushed cooling system about 10k miles ago and I also have a Z1 radiator cap. I burped the system using the Lisle funnel until my heater started scorching consistently.
During last week's Socal heatwave with temps hitting up to 110 on my car's thermometer, I had this similar problem. Jumped on the freeway then looked down to see my water temp rising above halfway. Got off the freeway to park, the needle lingered a little over half way then it would drop to normal temp. After a minute, give or take, it'll jump back up, and that's what it'll do continuously. I drove home locally so I can shut off the car in case it got too highy.
I tried burping the system again the next day then took it for a drive. The problem only occured once during this drive, but I'm assuming it was because the ambient temperature started dying down. After this single occurrence, it never happened again.
My reservoir cap is cracked, though, could that have caused the problem?

 

radiator, overflow isn't a sealed system like the rest of it

 

Liquid under pressure has a higher boiling point. If your system isn't holding pressure your coolant will have less thermal capacity before it reaches saturation temperature.


To the OP was ANYTHING else done to the car between when you weren't having this problem and when you got your engine bay cleaned?