I have a 2011 R55 Clubman S, bought in 2017 with 70k miles and now has almost 110k miles.
I think it has a blown or leaking head gasket, but I want to make sure that's what it is before I start tearing things apart.

The symptom is every time I drive it, pressure builds up in the cooling system and starts bleeding off pressure out of the overflow tank cap.
Is there anywhere else the excess air/pressure could be coming from or is it probably coming from the cylinders via the leaking head gasket?

I don't have any white smoke coming from the tailpipes, and I don't see any signs of oil in the coolant or coolant in the oil.

Thoughts?

 

A few things I would check first:
Is the engine overheating and causing the excessive pressure (boiling over)?
Is the cap defective and not withholding the normal pressure that builds up when the engine warms?
You can buy test kits at the auto parts store that will detect combustion gases in the coolant. If it's positive then it's likely a blown head gasket.

 

I think I'll look for and try out one of those kits, thanks!

The temp gets high, but I think it's as a result of the excess pressure/gasses, not the cause.
When I've monitored the temps with my OBD reader it goes something like warms up to 215, drops to 185, warms to 220, drops to 185, warms to 225, drops to 185, warms to 230, drops to 185, warms to 235, drops to 185. (approximate numbers, I didn't write anything down). I can hear the cap bleeding pressure by the time it's up to ~220 though. Then after parking the car and letting it sit overnight, I can open the cap and more pressure comes out. Then open the bleed screw and even more pressure is released before any coolant comes out.

I did replace the cap a while back as well thinking that might be the issue.

I have also done a pressure test on the cooling system when I first noticed I was losing coolant, thinking it was a simple leak somewhere. But it held just above the prescribed pressure for longer than the recommended test time.

 

Normal for a cooling system to build pressure as the temperature increases. It is primarily the pressure that prevents the hot coolant from flashing to steam. If it does this cooling is severely compromised.

The up and down swings in temperature can be normal. I data logged a number of cars (engines) and coolant temperature goes up and down. I'll post a graph of my M-B cargo van's coolant temperature swings.





Doubt there is any pressure in the system after the car sitting overnight. What you are probably hearing is air rushing in because the cooling system is under lower pressure.

It is important the cap not release pressure at too low a pressure. I had a cap do that and this resulted in a loss of water vapor which over the span of several weeks had the coolant level down nearly a gallon. I found the cap leaking by running a hot pressure test and tenting the cap with aluminum foil. After a bit water had condensed under the foil. A new cap was the fix.

 

Good call, it could be air rushing in not out.

I know pressure and coolant is leaking out of the cap when it gets hot. After a 10 minute drive and hearing the psssst noise several times I can see some coolant splatter on surrounding parts.
The next day the coolant level will be a little above the proper level. When I open the cap and hear the rush of air, the level drops to around normal. Then when I open the bleed screw and hear more rush of air, the level drops to a bit below the 'low' line.
That is why I thought it was air/gasses being pumped into the cooling system, raising the coolant level and increasing the pressure beyond what the cap was designed to withstand.

I already replaced the cap once. Perhaps I should replace the entire expansion tank?

I think I will still try that combustion gasses test too just to make sure.

 

I'd bleed the system some more and get those air pockets out of the system.......

 

I'll see what I can do. I've topped off so many times following the proper bleed screw steps, and every time it gets over-pressurized again. That's what made me think pressure is entering the system from somewhere else.

 

Is that reservoir bottle an aftermarket one /oem

 

It's the same one that was in the car when I bought it at 6 yrs old.

 

Definitely....... change that bottle and top..... don't get a aftermarket.

 

[QUOTE=MiniBerg;4654561] Then after parking the car and letting it sit overnight, I can open the cap and more pressure comes out. Then open the bleed screw and even more pressure is released before any coolant comes out.

This is the best indicator of a blown head gasket, or other leak of combustion gases into the cooling system that you will ever get. If you have pressure in the cooling system after a cold soak, combustion gases are getting into the coolant. The great majority of the time it's a blown head gasket. The rest of the time it's worse, a cracked head or maybe even cracked block. You can feel the hose to verify it's pressure, not vacuum. It's pretty unlikely to be vacuum.

 

With the additional info I'm changing my mind and I believe what you are hearing is not air rushing in but out. If the hot cooling system is not able to maintain pressure steam pockets will form. If you shut off the engine steam will of course go away with the temperature drop. But this can leave air pockets in the cooling system. With the cooling system open this can release pressure and with this the air pockets will go away. This lowers the volume and the coolant level will drop.

The key is to find what the cooling system can't hold pressure. A head gasket leak is a possibility. You can test the coolant for the presence of combustion byproducts in the coolant. You can obtain a suitable sample of oil and have it analyzed. Anti-freeze compounds in the oil is sign coolant is getting into the oil and a head gasket is one possible explanation.

But I have had a bad coolant tank cap that when the engine was hot would release pressure and allow water vapor to escape. Over time this would have the coolant level low enough to trigger a low coolant level warning light. A tent of foil over the cap with the engine hot then shut off had water condensing under the foil and this confirmed my suspicion.

Yet another time suspecting a coolant leak I did a hot pressure test on my driveway and rather than a leaking radiator or even a water pump as I suspected the plastic coolant tank proved to be bad. It had developed a split along its bottom at the mold seam. Hot and under pressure this split opened up and hot coolant gushed to the ground. After letting the car sit overnight next morning I added about a gallon of distilled water to the tank and brought the level up. Started the engine and there was no signs of leaking. I drove the car several miles to the dealer with no signs of leaking and had the coolant tank replaced. With the tank out of the car it was clear the tank had been leaking for some time. But my usage seldom had me park the car with the engine *hot* that is hot enough the radiator fans were were running.

 

I ran the combustion gas test on the coolant reservoir and I saw no color change in the liquid.
Tested a second time, still no sign of combustion gasses. I think I'll try it again tomorrow or this weekend when I get a chance.

I put the cap back on the tank and let the car idle for a while. The temperature went up and down a few times without any pressure venting and I thought it somehow fixed itself, but eventually the tell-tale Psssst came back.