My cat seemed to have a slight coolant leak. So I pressurized the cooler and found a little leak in the lower hose of the coolant expansion tank, which I tried to fix.
Filled the coolant and let the engine run until the fan started.
Lots of coolant blew through the cap of the coolant expansion tank. There were big bubbles in the coolant tank.
I refilled on cold engine and started again: Same flow of coolant.


With the hot engine, I checked the coolant to be above max-level. This time I opened the little plastic screw on the coolant expansion tank for 30 seconds. ( does this make a difference ?)
I measured the CO2 in the coolant expansion tank. No bubbles any more, no CO2 found.

Questions:
- Can I be sure, that my head gasket is ok, as my CO2-measurement didn't find anything ?
- What else can cause so much pressure in the coolant expansion tank ? ( What about the auxiliary coolant pump ?)
- What else could I try to diagnose ? ( I own the SDD, too.)

 

With the engine hot the coolant reservoir is under quite a bit of pressure which is why there are warnings to not take of the pressure cap until engine has cooled down.

Your post implies you ran the engine to heat up with the cap off. Did you do that?

Also when refilling with coolant did you follow the recommended procedure?

 

Your post made me think about my actions and I must admit, that letting the engine heat up with the cap off was a bad idea.
For a reason, which I don't understand, the water flooding didn't stop, when I attached the cap. It just ran through the cap after a short time.
Now I cooled the engine down completely, refilled and heated up to 90°.. no water coming out anymore.
I don't think, I fixed my issue, as it only occured sometimes.

I will do some further tests to evaluate the conditions when flooding of coolant occurs.

 

Check the oil consistency on the oil dipstick. Does it look milky at all?

Also unscrew the oil filler cap and look for a milky residue on the bottom of it. Any milky oil residue means water/coolant in the oil.

Even if there is no milky residue it is not definitive.

If the coolant system is over pressurized when engine is running it could be a blown head gasket however there is one other possible cause.

I do not know your car. Does it have a transmission cooler inside the radiator?

IF it does that could be the cause. If the transmission cooler develops a leak it will cause increased pressure in the coolant system. when this happens it usually leaves considerable discoloring inside the coolant reservoir due to transmission fluid in the coolant.

You do not mention any discoloring so hopefully that is not the cause as the transmission will be shot.

To be sure on this you need to drain some coolant from the bottom of the radiator to check consistency of coolant.

 

I feel it's time to give an update on my trouble:
After swapping the O-ring on the lower hose of the coolant expansion tank, the pressure was pumped up to 1 bar and no air dissappeared over night.
I filled to max, let the engine heat up. Next day i filled again, then I drove about 40km.
Unfortunately, some days later I had to refill half a liter of coolant, again.
I will do one more drive, just in case there was still some air somewhere.
If I need to refill again, I'll have to search and think again.

 

The coolant expansion tank is not pressurized, The coolant engine/radiator is that's why the warning,

 

I've the same AJV8 engine and my coolant expansion tank is pressurised.

Not very much - you can run it in cool weather and non-stationary traffic without the cap and it's viable for testing purposes, but the level rises. Which is why it's under pressure with the cap on

 

Turn on the heater while you filling coolant. Heater/auxiliary pump will circulate more coolant into the system. Also, there is a small plug located on top of the coolant reservoir which you did. It possible that you still have other leaks i.e. water pump and thermostat housing. Tail tail sign is coolant smell after long drive. The coolant dried up and you won't be able to see it. Water pump, thermostat housing, and new belt are cheap. It's a a four hours or less job for DIY mechanic. I just replaced my last week. Water pump should be replace before 100K miles.

Be sure get water pump with metal gasket and not paper gasket

 

My bad. A seniors moment and too many years dealing with North American cars.

 

It's possible you have a failed cap.