'94 850 GLT Boiling coolant

fle · Post by **fle** » 13 Sep 2014, 06:24

Hi list,
I hope to get ideas on how to debug this boiling coolant problem.

Symptoms :
since some time ago, the car is consuming coolant. I noticed traces of coolant at the coolant tank cap. One day I noticed the coolant was boiling in the expansion tank, while the car had just been shut down. But this is usually not the case.
Also I noticed that the radiator fan was not starting as soon as the a/c is switched on. But it does after some time (few minutes).
And it is rarelly working, even when I let the car idling for 5 mins (the a/c is off)
The car is not overheating, my temp gauge goes to 3 o'clock and stays there while the car is in use (even during the summer w/ a/c on)
I don't see any white smoke at the exhaust.
I changed the thermostat something like 2 years ago (the engine temperature was staying too low)

What I have done so far :
I checked for codes, but there is nothing.
I used the diagnostic mode A6 to check the fan relay, and it is working on both speeds.
I checked the ETC and the resistance values seem normal (less than 3ko engine cold, and less than 200 ohms with the hot engine)
in june I performed a compression test, and it was ok, just at the lower spec limit (the car has more than 400.000 km)
I haven't found any coolant leak

At first I thought that the tank cap could be bad. I have cleaned the gasket, in case. But shouldn't it be leaking / boiling all the time
Then I thought the fan relay could be bad, but any time I do the diagnostic, it works.

Does someone have any idea on what to check next ?
thank you.

Post by **mikealder** » 13 Sep 2014, 06:40

Is the cap a grey coloured one or is it green, the grey ones were prone to cracking, the green ones can crack but not as frequently as the grey ones did.
If the coolant system is loosing pressure due to a bad cap/ expansion bottle then coolant will boil as the temperature it works at is over 100 Deg C, it needs the pressure to raise the boiling point above this value - I would also check the coolant pipe fitted to the bottom of the expansion tank as this can leak and go un noticed - Mike

fle · Post by **fle** » 13 Sep 2014, 07:49

thank you for your quick reply Mike

The cap is the green one, 20 years old. Do you suggest to change it ?
If there a pressure leak , then shouldn't it be boiling any time ? also when the car is running ?

Post by **rspi** » 13 Sep 2014, 07:51

There is a chance that you have a blown head gasket. You need to find out where the coolant is going. The are several places that it can be going and most people don't realize it but if the leak is small, it usually evaporates before reaching the ground.

Check under the carpet on the drivers side for a heater core leak and remove the reservoir cap when the system/motor is cold to see if the cap is bad.

How to test for a blown head gasket on a Volvo - …:

whoa · Post by **whoa** » 13 Sep 2014, 07:56

You could replace the cap with a $10 parts-store one to see if that fixes the issue (and then get a reliable Volvo one if so).

fle · Post by **fle** » 13 Sep 2014, 08:13

thank you for your replies, and for the video link. Robert, I am using your videos a lot, and find them very useful, like the pcv one.

So I will recheck the cap, and see how I can get a new one.
I'll also check the coolant level, as in the video.
I haven't noticed any leak at the heater core, but will check

I will do that tomorrow and let you know

Post by **erikv11** » 13 Sep 2014, 08:48

That's a good strategy. It really sounds like a bad cap, I would focus on that. A bad head gasket may pressurize the system but the only way for the system to actually boil is if pressure is escaping. And they almost never fail without the car overheating or some other abuse.

In addition the behavior of the cooling fan relay does not sound right, I thought the the fan should kick in right away when the AC goes on.

fle · Post by **fle** » 15 Sep 2014, 06:32

ok I did check :
the cap seems ok, I don't see any crack
I don't see any heater leak
I checked the coolant level as per the video. When the car is hot, the level increases by like 5mm. It seems ok.

I have taken the expansion tank out of its support bracket, and see cracks at the bottom (see the highlighted areas on the pics)
I will check this for leaks.

: cracks at expansion tank; flaborderie_2014_09_15_20140915_P9159800.jpg (43.22 KiB) Viewed 2728 times

whoa · Post by **whoa** » 15 Sep 2014, 08:19

My cap failed. There were no cracks; it just didn't hold pressure. In my case it held nearly zero pressure, so when I squeezed the upper radiator hose, air would come out of the cap. Anyway I wouldn't rule out the cap just because of how it looks.

fle · Post by **fle** » 15 Sep 2014, 13:33

I ordered a new cap.