Recently I came to noitice that my car lost almost everything of the coolant from it's expansion tank (not all the coolant from the system, just the tank) in a short period of time.
The expansion-tank showed a level of just below minimum, while there were clear signs the coolant left through the expansion-tank-cap in a burst. It could be a possibility I didn't close the cap properly the last time (two days before that), but I'm not sure.
Anyway, I pressure-tested the old cap and it seemed to be a bit faulty. Just to be sure I replaced it for a new one. With the new cap installed I don't have any overflow issues anymore. But, while I was busy I did some additional checks just to be sure it wasn't anything worse what caused the coolant to leave the expansion tank.
My results are as follows:
- Absolutely no signs of coolant in oil
- no coolant loss (always steady at max. except this one time when it seemingly poured from the cap)
- Water pump works properly.. as in that the coolant does flow.
- Car holds temperature perfectly steady in all conditions
- some minor traces of 'stuff' in the coolant, but nothing exceedingly (I guess). Does not look like oil since the coolant itself looks clear.
- As part of some regular maintenance, related and thus worth mentioning, the car recently had: a new water-pump (with timing belt), a simple coolant flush (with cleaner) and fresh coolant.
- barely any oil-consumption
- Heater works properly to the point of sweaty buttocks :-D
- Coolant system holds pressure (below blowoff threshold for the cap @ 1.4bar/20 pounds)
- No co2 in coolant system at cold start (did sniff-test)
- I tested compression for all cylinders: everything OK
- Spark-plugs look perfectly well
- Checked inside cylinders with boroscope: no sign of cracks, as far as I am able to tell.
- externally the engine, other hardware and specifically the oil cooler/heat exchanger looks clean: no visible leaks.
But here's the catch:
I also did the sniffer test on the coolant system with the engine at operating temperature (and running). Was somewhat of a hassle to test this since I had to act quickly before the coolant level would rise and create a mess, but I managed to do so and the sniffer-fluid turned slightly yellow-ish. My control-test (directly taken from exhaust fumes) turned out way more yellow. But still, it turned slightly yellow... which indicates some co2 was present.
This worries me, but do I have to? Taken into account that everything else looks fine... Could it have something to do with expanding/contracting? Because I didn't have these results (co2 in coolant) on a cold engine.
Hope someone could help me out.
/edit: forgot to mention: petrol 4 cyl. turbo Multiair Alfa Romeo engine.
/edit: Does co2 in the coolant system always point to a faulty head gasket? Isn't there any other part that could be responsible for exhaust fumes entering the coolant on a petrol car?