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What makes coolant circulate through the expansion tank?

morsing

Active member
Joined
Jul 20, 2007
Location
Aylesbury, UK
Good afternoon,

I bought a head gasket tester, and to use it, steam needs to be present in the expansion tank (no, this won't break the car).

I cannot get the coolant in the expansion tank to get above room temperature, circulate or anything else. This is a Volvo 240 (1989 B230E). I have just driven it hard for four miles on twisty roads, and it's just... cold?

So under what conditions would you expect hot coolant to enter the expansion tank?

NOTE: No, my thermostat isn't broken.

Regards,
Henrik Morsing
 
There isn't any forced circulation of coolant through the expansion tank. The only circulation path, aside from the heater, is down through the radiator from the top hose then back through the engine by the lower hose. The level in the expansion tank is just the level of the coolant in the system.

The temperature of the coolant in the expansion tank should be ruffly the same as the system as shown by the temperature gauge by simple heat conduction through the coolant as a whole. If not, then the coolant may not be circulating. Feel the temperature of the radiator near the center near where the overflow tank connects.
 
I bought a head gasket tester, and to use it, steam needs to be present in the expansion tank
So under what conditions would you expect hot coolant to enter the expansion tank?

Not steam, just combustion gasses. There might be a steady flow, or just the occasional bubble.

With the tank being the "high point" of the system, and bubbles always rising to the top of a liquid, any gasses that do make it through the head gasket will eventually make it to the top of the radiator, then to the expansion (actually, "degassing" is more accurate) tank via the small vent hose at the top of the right-side tank.

As stated, you won't see any massive flow through the tank, since both tank hoses connect to the same side of the radiator. There may be a slight trickle, or maybe just a calm rise-fall of the level as the coolant heats and cools.


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Ok, so what's the deal with the head gasket tester I bought? Do I just drive the car up to temperature and put the tested on the expansion tank and that should work?

Regards,
Henrik Morsing
 
Ok, so what's the deal with the head gasket tester I bought? Do I just drive the car up to temperature and put the tested on the expansion tank and that should work?
Yep. Like most of the instruction say, drain it down enough that you won't be sucking coolant into the tester, just any air (and gasses) that may be there. The engine only needs to be hot enough for the thermostat to be open, so the coolant will flow through the radiator, where any naughty bubbles will end up in the tank. If you're not using a constant vacuum source, you get to squeeze and release the rubber bulb until your hand falls off.
 
Ta,

The instructions actually says "steam in the tank", but I'll give your instructions a whirl later.

Regards,
Henrik Morsing
 
Good afternoon,

Well, I gave it my best shot and the test fluid remained blue. Assuming I did it right, I'm in the clear, which is both good and bad.

Regards,
Henrik Morsing
 
The test is actually very good for testing one of the many types a headgasket can blow, combustion chamber to coolant passage, one of the more common ways. It can blow in other ways though, combustion chamber to oil passageway, between 2 cylinders, combustion chamber to block wall, etc.

When they say steam in the tank, they just mean fully warmed up coolant.
 
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