Help - temp cycling

[tvrbob]

Under normal running my temperature is cycling between 70 & 110. The fans are running OK but the over-temp is real because I had a puff of steam this morning. I've laid the car up now so that I don't overstress the heads. Any ideas? The only one I could come up with was sticking thermostat.

[Big Al.]

At what temp, do your fans cut in?

[GreenV8S]

Blocked bypass, or possibly a massive air lock?

[tvrbob]

Big Al. said:

At what temp, do your fans cut in?

Just before 90C so I'd don't think the problem lies there.

[tvrbob]

GreenV8S said:

Blocked bypass, or possibly a massive air lock?

Well I hadn't thought of this but the pipe from the top of the thermostat housing to the swirl tank is full of air. Is this normal?

[GreenV8S]

tvrbob said:

Is this normal?

Absolutely not! You need to fill and bleed the system.

[tvrbob]

So there was an air lock. 3ltrs of coolant required to prime the system through the swirl tank. What I can't understand is where it went. I've had no leaks and it's a good 2k since it's major service. Could that much air be trapped for that many miles before raising its head?

[ATG]

Perhaps that puff of steam accounts for a fair amount of the missing 3 litres? FWIW the two leaks I've suffered (one at the moment) were from the bottom left corner of the radiator (only just enough to leave a damp smudge on the ground) and from a failing water pump gasket (that only leaked when the cooling system was pressurised and therefore didn't leave much of a damp patch under the car). Given long enough a fair amount of coolant was lost by both leaks, but it was so gradual that I certainly wasn't aware of it until on the first occaision I noticed the level had dropped a lot in the swirl pot, and last week when the damn thing overheated going up a hill and I got a puff of steam and about 3 litres of missing coolant.