Overheat / thermostat / temp guage

[BERGS2]

this old chestnut:

driving to work this morning - temperature seems unusually low after five mins or so (had barely picked up from the bottom of the guage)

Very suddenly it rapidly rises to over 90 so I pull over and flip open the bonnet with the engine still running to see if the fan kicks in

Unfortunately it doesn't and I hear the pressure cap blow on the catch tank - so I turn off the engine and let it cool for five mins or so.

Heading home to pick up the bike - all returns to normal fan cut in just below 90 degrees and cuts out when the needle is almost horizontal (about 80 i think)

Slightly preturbed by the erratic nature of this I decide to drive to work anyway (its only 5 miles) and all seems OK other than the fact the 'fan cycle' (time taken between the point where the fan cuts in and the time it cuts out) was incredibly quick - about 15 seconds)

The car had been unused for 4 days before this morning (but it often is)

The coolant levels were fine

can the thermostatic switch get stuck & wooudl that explain the rapid on/off of the rest of my drive in this morning?

Also where is the sensor for the temperature? Was a bit worrying that it was reading low for so long then suddenly went up like a rocket.

Sorry its a boring one, but any advice gladly received.

Tim

[greenv8s]

Sounds very much like an air lock in the engine. The sender for the temperature gauge is supposed to be immersed in water, when it is in fresh air it only reacts very slowly to changes in the metal temperature around it. The sudden temperature rise would correspond to the air lock suddenly shifting so the sender got immersed in water again. Suggest you bleed the engine and (more important) then bleed the radiator. Then take the car round the block giving it plenty of welly on the corners, come back and do the same again.