Hello all,

I have an overflow leaking issue that develop recently. My stock overflow tank started leaking at the seams. This is the 2nd one that has leaked. To get me by I sealed the tank completely around the seam with some sealer. This didnt work so well, as the pressure in the tank apparently was to great for the sealer & started to bubble through the sealer. The good thing about that was I could see now that the entire seam was leaking the whole way around. I did pressure test the stock unit with my compressor and it took about 25psi before it started to leak. Thats pretty high, but I figured with the heat of the coolant that the tank probably expanded some so it didnt take that much pressure to push through. I was always under the impression the coolant system of a car was only running around 7psi or so.

So onto my resolve. I found a forge tank (the nice aluminum one) to replace the stock one figurering that would resolve the issue. Well to my dismay, now the bloody thing is leaking out of the cap! I thought maybe the seal inside of the cap wasnt making contact when in place, but I removed it and sure enough it went on almost a full turn further without it. So with being unable to see if the gasket is seating completely I will have to assume it isn't although it appears to be.

My questions are as follows:
1. Has there been issues with overflow caps in the past?
2. I noticed in the cap there appears to be a pressure valve of sorts, could this have failed, or is my cooling system having issues creating higher than normal pressures. (i have tried reducing the amount in the overflow, but it still seams to well, overflow!)

Thanks for any insight you may provide!

 

The stock cap has a pressure relief valve built in. As you have found, the stock tank is prone to leaking in earlier models Sealing the tank from the outside rarely works due to the nylon construction of the tank not allowing good adhesion of sealers and the pressure coming up from under any sealing attempts. Since you got a new aluminum tank and are still having issues, it's beyond the original tank being the sole problem.

A few other causes and things to check:

Overfilling. The stock tank should never be more than about half full when hot to allow space for coolant expansion. The same is true for any aftermarket tank. You should not have more than about half a tankful of coolant when hot, filling to to this level when cold may cause an overfilling condition that causes the cap to vent the excess pressure and with it, the excess coolant.

Air in the system. Occasionally air will work itself into the system and cause pockets and cavitation and with it localized overheating. Purge any air from the system by using the vent screw located in the upper radiator hose.

Overheating. I can only wonder how many MINIs are driving around with failed cooling system fans due to the infamous power steering fan failure. In most earlier models the failure of this fan takes out the fuse responsible for the control circuit that runs the main radiator fan. Check the 5 amp fuse in the underhood fusebox labeled F05. If it it blown, the radiator fan will not come on and cause the car to run hot when not in motion. Many people will not be aware of this happening, as the stock gauge does not read 'hot' until the temp has gotten well over 230 degrees. This will cause excessive pressure and venting.

Head gasket failure. This is rare, but can crop up, especially if your car has overheated repeatedly. Combustion chamber pressure will leak into the water jackets and cause a rise in pressure that the cap will take care of by venting. A pressure test can be done to see if there is indeed an internal leak. Not likely, but still a possibility.

 

Thanks for your reply Greatbear! I did check my f05 and it is good, as well as my fan does turn on a few times after I shut down the car. This lead me to put the car into test mode so I can read the current running temps. I was reading between 90°-96°C which I believe is around 194° to 204°F.

Its funny it doesnt always leak, but at times it does. I keep the res well under half in both the old and new tank, so I am pretty sure its not from having too much coolant in the system.

I will try bleeding the system, as I didnt realize there was a bleeder valve accessable. I dont know if the engine ever overheat before I owned it, but it hasnt since I have. I did check the oil fill cap as this sometimes will collect coolant condensating if the headgasket is blown. I dont think it is that, but if it is, I have another head sitting in my shop incase the orginal is warped.

Thanks again!

 

What color expansion tank do you have? The original white one's leaked like crazy. I went through two of them before they switched to the new and improved yellowish one. That one has been working pretty good for me although I've seen posts about that one leaking as well.

About the bleader valve...yes it is very easy to bleed the MINI's cooling system. Bleed it with the car on and heater blowing. This will purge any air in the heater core as well. If you get any air the 1st time, bleed it and drive around the block. Do this until solid coolant flows out.

 

Kapps,

I replaced the leaking yellow tank with a forge aluminum one. It is leaking out of the cap now even though my temps are fine.

I will try the purging of the coolant system soon.

Thanks.

 

Ok i was running the car at idle & opened the bleeder valve & nothing came out. I left it idle for 10 min or so & eventually it started spewing coolant. It was at 110c. I shut the valve & let it cool down. I ran it again at idle and it staryed around 93c then it started to go up to over 100c. I shut it down. The fan never came on the 2nd time, but it was on the 1st time. At what temp does the fan kick on?

Also back to the cap. It still is opening the valve even around 95c or so. Is this normal? Should I maybe replace this cap?

Thanks