Hi everyone, I've had this problem for a while now and I can't find the problem.

The overflow tank keeps getting full and squeezes the coolant out. After cooling down the car, opening the radiator cap, there is about a litre of coolant missing in the radiator. And this is within 1 hour of normal driving.

Coolant isn't leaking out of any hoses. Coolant has a one way trip to the overflow tank, it's not getting sucked back in. Temperature of the coolant is never more than 176 F.

I had the engine checked on compression, leak down test and an overflow tank coolant test for faulty head gasket. All tests were good, no issues. (it seems). Water pump was replaced a year ago so I don't think that would be the problem.

So my question for you guys, what could be the problem?

- air leak in the intake manifold gasket?
- head gasket must be leaking although tests say it's not
- leak somewhere else in the system?
- faulty radiator cap?

If there is a leak, why isn't coolant coming out of the leak and why is the system sucking in air instead?

 

I find it hard to believe your pushing coolant out, bringing air into the system, AND staying below 175 or whatever u said.

Maybe u have a faulty cap.
When engine is cold and u pull cap off n start engine can u see bubbles at hole, or is it pushing out?
it should only start pushing out after it really starts warming up.

 

are you running a stock block or a sleeved block?

 

Well it's happening. And it's 30 F here at the moment. At a certain moment the coolant level get's too low and the temperature will rise very fast. Then I have to stop and shut it down.
When it's fully bleeded and engine is cold I don't see any bubbles. When the engine get's warm it starts to push out the coolant.

It a sleeved Darton block. It's been sleeved in 2007 and fully built. The problem with this coolant has started in 2014 at which point I crashed the car at the Nurburgring.
The car has been rebuilt and this issue remains.

 

I don't care how cold ambient is. Air will not keep engine temps in check like ur explaining. When it shoots up, is that just the stat opening? It would be more helpful knowing how high it shoots up to. Let it idle and see if it goes over ...Maybe 203? That should be a somewhat solid high number that u would not usually see at idle with those ambients.

Also was not aware it's sleeved. Hopefully u get this sorted. GL!
Again - I think u might have a faulty radiator cap. U must hold pressure to increase boiling point, if not it will just keep expanding until it boils. Maybe that's why u think air is being sucked in. Air is not replacing the coolant, ur cap just can't hold it in. And of course their is a litre missing once u open it up because it's literally now in the overflow.
I have seen this b4 on my '11 Triumph Daytona with coolant boiling in my overflow :scratchhead: after catastrophizing, and flushing, and buying the "best" coolant. ended up being just a bad cap.
Once u have a known good cap or replacement check the hose for cracks and make sure there is the proper amount of coolant in the overflow.

 

I would say head gasket. Have you tried pressurizing the coolant system?

 

Did u try to do a full bleed of the system? The legit way ?

 

When I pushed coolant on my STi, my temps never ran up higher than normal, but I didn't drive it hard when I suspected I pushed coolant via exhaust gasses past the head gasket. That said, the traditional block tester you find at a parts store did not indicate exhaust gases in my overflow, however, my dad sent me one of his electronic CO testers and it sounded off like crazy. I also found small rubber bits in the reservoir as that MLS gasket also used rubber around coolant jackets.

At one point, I tried pressure testing the cooling system to see if it would leak back the other way, but it did not.

 

get a swirl pot or expansion tank.

 

perhaps there is a problem with your sleeves like on the picture below.
the sleeves on the picture are the dry sleeves. I have also seen the same problem with the wet sleeves.
or you have a faulty headgasket or there is a hairline crack somewhere.
definitely you are pushing exhaust gases into your cooling system.
that is the reason why you have air in the cooling system and the overflow tank keeps getting full and squeezes the coolant out.