Gassing in Cooling System?

[Rus]

Hello everyone!

Yesterday, while giving my car her weekly inspection, I noticed a small coolant leak from the top radiator hose where it attached to the thermostat housing. I tightened the clamp and proceeded to clean up the leak. Since the engine was still hot, I couldn't get a good reading of the coolant level in the reservoir. This morning I checked the level and found the tank to be more than full. I've seen this before about the same time last year. When ambient temperature increases, my car begins to form gas in the cooling system which leads to a build up of pressure (even when stone cold). I've dealt with this issue last year by changing all hoses, thermostat, fan clutch and coolant. In October of last year the radiator was changed along with the coolant. Now, almost 6 months later of flawless operation, my problem is back. I have not yet had a chance to take any temperature readings around the engine to check if perhaps it is exceeding the boiling point of the coolant. My guess is that with our rising ambient temperatures, which get as high as 38 degrees C (100 degrees F), the coolant is on the verge of boiling and forms gas, which then accumulates and overpressures the system.

Has anyone ever dealt with this sort of issue? I can switch to a different coolant if anyone can suggest one that has a higher boiling point. I'm currently using the BMW blue coolant in a 50/50 mix with water. No other additives. Would something like plain water with Redline Water Wetter help? To answer some questions in advance: I've previously checked for headgasket leaks using a tester that detects combustion gases in the coolant. No leak detected. Thermostat is functioning to the best of my knowledge. The car isn't overheating. Fan clutch works...and its on a LOT in the heat. I've determined that gas is being formed based on the fact that when the car is warm and I open the bleed screw, gas escapes first and then I get coolant coming out. When engine is running, I get coolant only...thus the gas also travels around the engine.

Sorry for the long spiel, just trying to provide as much information as possible. Any input is much appreciated!

[healtoeit]

Well, Some things that come to mind for me are...
Do you properly bleed the system?
Do you use distilled or tap water? Distilled does not have anything that could turn into gas in it.

[Rus]

Well, Some things that come to mind for me are...
Do you properly bleed the system?
Do you use distilled or tap water? Distilled does not have anything that could turn into gas in it.

Yes, I have bled the system properly. As I mentioned, the cooling system has been flawless since October of last year. The problem spontaneously surfaced after we started experiencing very high ambient temperatures. I use distilled water in the coolant mixture. Water is capable of turning into a vapor. The vapor is condensible, however, so I doubt that would cause gas formation. If, on the other hand, the antifreeze additive is breaking down at high temperature, organic components may form gas instead of condensible vapors. I've done a bit of research, and it appears that higher ethylene glycol concentration may help increase the boiling point. Has anyone tried running higher EG concentrations?

P.S.: I will try to blow out my radiator vent line to the expansion tank today as I haven't done that yet.

[healtoeit]

Yes, I have bled the system properly. As I mentioned, the cooling system has been flawless since October of last year. The problem spontaneously surfaced after we started experiencing very high ambient temperatures. I use distilled water in the coolant mixture. Water is capable of turning into a vapor. The vapor is condensible, however, so I doubt that would cause gas formation. If, on the other hand, the antifreeze additive is breaking down at high temperature, organic components may form gas instead of condensible vapors. I've done a bit of research, and it appears that higher ethylene glycol concentration may help increase the boiling point. Has anyone tried running higher EG concentrations?

P.S.: I will try to blow out my radiator vent line to the expansion tank today as I haven't done that yet.
Could it be something with your thermostat?
Is the car overheating or is it just running hot? or is the expansion tank just getting full?

[Rus]

Could it be something with your thermostat?
Is the car overheating or is it just running hot? or is the expansion tank just getting full?

The car is not overheating. My temperature stays at or below the 1/2 way mark on the gauge. The thermostat works and is nearly new. I have a spare one as well, but would rather not mess with it if the current one works. the issue that I'm observing is that the system maintains pressure even after cooling off completely. This has happened before and I thought I had solved the problem with the repairs I listed in the original post. Unfortunately, with the summer heat, the problem has reappeared. In colder weather (temperatures below 30 degrees C), everything worked great with no abnormal behavior. I know the coolant is running hotter now because my cooling fan is on a whole lot more than it was before. I'm fairly sure that the pressure build up in the system is a result of some sort of gas being present in the system. I can actually vent the gas off using the bleeder screw. Another clue to the presence of this gas is the fact that the expansion tank level is very high even when cold, but decreases when gas is vented. The gas is not related to combustion byproducts according to the bubble tests I've performed before. I can't think of any cause for this gas other than the temperature of the coolant exceeding its operating limits. Hopefully I'm not the first to see this phenomenon.

[Rus]

What symptoms are expected to be observed when the line between the radiator and the expansion tank is plugged? This would cause a high pressure in the system due to the restricted relief path, but could it cause gas accumulation? If my radiator fails to vent via the pisser hose and pressure builds up, would coolant back up into the expansion tank and force the air in the tank up through the pisser (these have been known to occasionally become one way valve restrictions with dirt) and back in the radiator? Should the expansion tank cap prevent this? The cap is also nearly new and I believe its rated for 2.0 bar (purchased from dealer). This is 30psi...which is higher than any other cooling system pressure I've seen. AS always, input/experiences/opinions are appreciated :)

[whiskychaser]

When ambient temperature increases, my car begins to form gas in the cooling system which leads to a build up of pressure (even when stone cold).

Not got a solution but a couple of questions: if you depressurised the system then let the car sit for a couple of days, would it build up pressure on its own? If it does you may have some sort of chemical reaction going on there. Is there any chance the system is dragging air IN? Its a bit off the wall but central heating systems do it and this is a bit of an odd one:)

[Rus]

If I let the car sit (this is me recollecting my previous experience with the issue), there is no change in system pressure. It only appears to build up when the engine runs in extremely hot conditions. You may be right about the system dragging in air somehow. To be honest, I've messed with just about everything but the hard plastic line that runs along the driver side fender. I suspect if this line is partially clogged, it may force the coolant in the system to back up into the expansion tank and in turn force air from the expansion tank into the radiator. If this is happening, it could explain why the system builds pressure. Heat cycling makes the expansion tank act like an air pump. This, of course, is just me sitting at work and theorizing (instead of working :-p ). Maybe at colder temperatures the system can cope with the partially plugged line, but when hard times hit, it becomes incapable of relieving the radiator of coolant.

[Rus]

Nightly Update:

After getting home today I let the engine cool down a bit and opened her up. When I initially tried to blow through the vent line between the radiator and the expansion tank I felt a good bit of resistance. It cleared up when I blew compressed air through the line. I also cleaned out the port in the expansion tank with a pick. Upon restarting the car there was a steady stream of coolant from the vent port on the tank, so I think its pretty free flowing now. I went ahead and bled the system, but didn't find much air. I drove the car around and it seems to be doing alright. No signs of overheating. Now time will show if I resolved the issue. I'll update the thread with more info as my adventures progress. :)