Car with expansion tank that overheats - what happens?

Joined
Mar 21, 2004
Messages
29,553
Location
Near the beach in Delaware
So a car with a radiator cap overheats, the pressure goes beyond 15 lbs and the radiator cap releases steam so you can look helpless on side of the road.

What happens with a car with no radiator cap but with an expansion tank and a tight cap on the expansion tank.

Where does all the steam go? No radiator cap to open at 15 lbs.

In this scenario. 15 lbs is an example. But it's around that pressure when most radiator caps open up.

This is just a question so I know. No particular car. Except it would have an expansion tank with cap.
 
No expert, but any I have seen have the radiator cap on the overflow / expansion / degas tank - and a high pressure cap with no spring on the radiator. My two Nissan's are this way - basically a no release cap on the rad, and a 17lb cap on the reservoir tank (Nissan's name). The pressure has to go somwhere.
 
As an example - both my Mercedes have a hose that goes from radiator to expansion tank. No cap in that connection. No restriction. Open hose.

But the expansion tank has a cap that is rated to a pressure. It has a valve and spring inside the cap.
 
That’s what the hole in the pressure cap fitting for the tank is for. Unfortunately it goes overboard and is not intended to be drawn back in, which I guess the overflow in a more traditional system is?

D4B994C5-EDEC-41B0-87A5-D4A0D8CCC5CC.jpeg
 
There are two kinds of systems in cars-- unpressurized reservoir and pressurized.
Unpressurized:
* The reservoir tank is not sealed. The cap fits loosely on purpose to let air in and out.
* System pressure is controlled by the radiator cap. The cap lets coolant flow to the tank every time the engine warms up to normal temperature. The cap has a check valve to let liquid return from the tank as the engine cools.

Pressurized:
* The tank is located at the highest point, so that a bubble of air exists in the top of the tank under the cap.
* The size of the air bubble reduces as the coolant expands when the engine warms up.
* The pressure valve in the cap is for safety relief. It doesn't open in normal operation.

In either system, liquid should never come out of the top of the tank during normal operation, so there is no need to return anything to the tank. Most cars don't even have a hose after the cap. It should vent only air when first filled.
 
Last edited:
The screw-on caps for the expansion tank don't look (or feel) like they have a spring to be pressed against by pressure and open up.

Just looks like a plastic cap. You can see something inside but does not look like it has a spring.

But I knew there had to be a way to relieve the pressure.
 
The screw-on caps for the expansion tank don't look (or feel) like they have a spring to be pressed against by pressure and open up.

Just looks like a plastic cap. You can see something inside but does not look like it has a spring.

But I knew there had to be a way to relieve the pressure.
Aaah, yes, the screw on ones. I believe the one in my old Saab that was a screw on still had a spring loaded inner section. Same for our vw.
 
The air bubble on the top of the pressurized surge tank is supposed to be there. Its spring-endowed cap may vent at its rated PSI, but it's supposed to vent air from the bubble. If it burps coolant the system was either overfilled or the car is overheating and boiling over.

In the old days, pre 1970s, the top of the radiator was the pressurized surge tank. It would burp out coolant the first time it got up to temp, replacing that volume with air, and that was it. The Chevy Vega needed all the surface area of its tiny radiator, so they invented the (non-pressurized) overflow tank (and vacuum/ siphon return system) around its inadequacies.

Then when cars got more aerodynamic, the top of the radiator was no longer the high point, and surge tanks became popular to help de-aerate and fill the system.

"Where does all the steam go?" Well there shouldn't be steam. If the cap lets a little air go at 15 psi there's still more pressurized air underneath it keeping the coolant from boiling. It doesn't reduce to zero psi. Aside from a little hysteresis it will keep on maintaining that pressure.

Your assumption seems to be that a cap venting (dramatically) is an emergency pressure relief, which it is, but it's also part of a feedback system that lets a little pressure out (due to normal thermal expansion) that you wouldn't even notice. If you look at the rubber flap on the bottom of the cap, it folds away from the metal. This is the check valve that keeps the cap from maintaining a vacuum, so as the car cools down, air finds its way back in.

To get "dramatic" steam, remove the cap on a very hot car. The pressure that was holding the coolant from boiling is suddenly released, the system flash-boils, and a lot of it comes out the rad cap scalding the hapless guy who removed it hot in defiance of all written instruction. A failure of a hose or anything else that holds pressure in can also cause this dramatic release.
 
Last edited:
Back
Top