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?

1 Answer 1


Here's the thing with the sniffer test, if not done right, you'll easily get a false positive, meaning the liquid will turn yellow or yellowish even though there's no hydrocarbons (my understanding is, the fluid is actually detecting hydrocarbons, but I could be wrong ... there should absolutely be no hydrocarbons in your coolant, but could be in your exhaust). If any of the liquid from the coolant gets into the test liquid, it will change the color, whether there are any hydrocarbons present or not.

If it were me, I'd keep an eye on it and see what happens. Top off the coolant, ensure the cap is on completely, then inspect the levels every day when cold and then again when hot. If you don't see any issues with coolant loss, then don't worry about it. If you have no other symptoms of a blown head gasket or leakage, I'd suggest you are worrying over nothing.

  • Thanks a lot! That could indeed explain my positive readings with the engine running at operating temperature. Naturally the Coolant was expanding/warming up when I loosened the cap to do the test, and then could possibly be evaporating into the test (since through the test would be the only outlet for the pressure/damp too leave). I never thought of that. Thank you so much!! Could also explain why it turned yellow just slightly.
    – Tim
    Dec 13, 2019 at 17:57

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