Coolant problem

[oldmercfan]

Dear experts, any thoughts on a coolant problem with my M110 engine would be appreciated. Intermittently, I will head off for a drive and within a minute or so it wildly overheats. The radiator is cold, so its clear that water is not circulating. I used to be able to fix this by loosening the radiator cap, which would trigger a rush of water from the radiator into the block. Recently, this 'fix' is no longer working. I then discovered I could get water circulating by taking off the water inflow hose to the heater taps on the bulkhead, topping it up with coolant (at least a litre) and reconnecting. But even when I do that, a few days later while driving, water will come flooding out of the radiator overflow pipe. I have replaced the thermostat (installing it correctly with the arrow pointing rearwards). I have also had leaks fixed in the radiator. Any suggestions really appreciated!

[Lance]

Me too, I had the same problem then it cleared by itself so is OK now, I changed the coolant but created an airlock and don't know how to clear it or prevent it happening.

[Mercmad]

Is the thermostat seized shut?

[markb]

Yes it sounds like an airlock.
The best way I have found to avoid this is by filling with coolant into the top hose rather than into just the radiator

[Lance]

Is the thermostat seized shut?

On mine the thermostat was new, but it behaved as if it was not working. After the airlock cleared it worked fine.

The best way I have found to avoid this is by filling with coolant into the top hose rather than into just the radiator

I will try this on my W123 when I change the coolant (on the list of jobs to do).

[oldmercfan]

Thanks, all. The thermostat is not seized shut as the cooling system will work fine for a few days (after filling up the heater hose) before spewing its guts out the overflow pipe. I will just have to keep doing things this way and hope it gets its act together eventually.

[Mercmad]

Before you cook your engine, go to a radiator shop and get them to test the coolant for carbon monoxide . It's a simple test and takes seconds to do . Temperature gauges running up rapidly then dropping is what happens when overheated coolant goes past the temperature gauge sensor, followed by a belch of coolant which makes it drop rapidly . Two things cause this, stuck or incorrectly fitted thermostats and cracked or corroded cylinder heads .A constant air lock is actually a leaking head gasket, because it shows coolant is being drained off somewhere. Is it a genuine behr thermostat or a cheap ebay version ? .

[oldmercfan]

Thanks, all. The problem has fixed itself. The rush of coolant from the overflow pipe slowed to trickle over a few days and then ceased altogether. Have been driving it now for two weeks with coolant system working perfectly. I can only conclude it was an air lock as my trust mechanic in Adelaide, Barrie Green, had suggested. He says old BMWs had a similar issue, so now I know.