When the engine is cold, the coolant level sits about a quarter of the way between the low and high marks on the coolant expansion tank. But lately after driving long enough for the engine to get up to temp, the coolant level is just a couple inches from the very top of the expansion tank.

The car is getting up to its normal temps, 215-220, thermostat is working properly, plenty of hot air from the HVAC system, and after the engine cools down, the level drops to its normal range in the tank. Coolant itself has a proper mix ratio.

What could be causing it to expand so much? No signs of a bad head gasket (had it replaced less than a year ago), no bubbles in the tank, no coolant/oil mixing, no odd-colored smoke from the exhaust, no pressure released from the tank once the car has cooled down.

Ideas?

 

I have had this problem twice in the last 8 months, it's because you're losing pressure in your cooling system. If air is allowed to escape the cooling system the coolant will fill up where the air is supposed to be. I noticed my problems started with my coolant tank boiling over and my caps pressure relief valve opening up like it's supposed to otherwise the coolant system would explode.

You're losing air pressure somehow, first things first, replace your coolant reservoir cap as this is the most common thing that fails. It's like $40 bucks, it very well might solve your problem. I believe it's on its about ready to completely fail. If it's not the cap (I strongly believe it is) then you need to have you cooling system pressurized to see where your losing air pressure.

 

+1 on what Systemlord writes

Motor on!

 

+1 check the cap first and replace it. Then check the tank for leaks itself. 17107515499

Coolant Expansion Tank #1 in the diagram Part number 17137823626


http://www.ecstuning.com/Search/SiteSearch/17137823626/ES2166017/



And the cap for the tank, #2 in the diagram part number 17132754264


http://www.ecstuning.com/Search/SiteSearch/17132754264/ES38481/




We also stock the MINI coolant right here, mixed it 50/50 with distilled water. Coolant if factory MINI will be a blue color and you will see the residue if its leaking on the transmission or in other locations.

MINI Anti-Freeze / Coolant - 1 Gallon Part number 82140031133


http://www.ecstuning.com/Search/SiteSearch/82140031133/ES196253/

 

Got a new cap today, that seemed to fix the problem, thanks guys!

Only down side is I needed it now, couldnt wait for one to be shipped as my other car is in parts as I'm fixing it (hit a freakin dear!). Nobody had one in stock, except the dealer...THIRTY FIVE DOLLARS!

I must say, the quality of parts on these cars is utterly ridiculous. My other car is a 2001 VW GTi VR6, I've had for over 10 years, got it when it only had 16k miles, now at 150k, and other than a couple wheel bearings and a couple very minor electrical fixes, its had no problems. Pretty sad when a 14 year old car with 50k more miles than my mini is so far superior in reliability...still using the original timing chain/tensioner, valve cover, coolant expansion tank and cap. Other than the Minis handling, I'd have to say its a piece of junk, and I dont know if its just poor engineering or the worst case of planned obsolesces ever. What a sad statement.

 

it's unbelievable that MINI can't even make a reliable, non moving, reservoir coolant cap

 

I'm going to buy a second cap just to have a spare ready just in-case. I have never had to replace a reservoir/radiator cap in my life, they weren't plastic either. Mini/BMW outsourced much of what make up a Mini Cooper, AC, transmission (Japan), TSW calipers to name a few.

 

Great,

Checking the link above and it went to the ones not in stock, error, its fixed now and we have them in stock, ready to go.

Thanks