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MG Midget and Sprite Technical - What is going on with my Tempeature Gauge

I have run the car up today its the Easter ritual,
But the temperature gauge started to read got to about 40 then dropped to nothing.
The car was running for approx 2 hours so I am sure it would be at operating temp. I did a test later on removed the bulb from the radiator and dunked it into a mug of hot water it went straight upto 60 so why would it not register the coolant temp?
Thanks Ian
I Pickering

do you have an air lock in the cooling system?

the gauge can ONLY register temperature if immersed in a hot liquid

I don't know why (though there are stacks of cleverer peeps here who can) but bloody hot air doesnt do it...

what car

what type of rad

does it have one of they overflow bottles with coolant in

often the overflow bottle can be full and the head can still have a huge "hole" in the water due to an airlock

the bottle stays full because the feeder tube blocks up, usually with rusty particles out of the cooling system, 'specially if the system is often/usually run with plain water

Antifreeze / summer coolant is designed to stop rusting happening and the rad cap should have a relief valve built in to allow equalisation
bill sdgpm

Is this on a 1500?
Once, when I blew the HG on my 1500, the first thing I knew about it was a drop in the temperature. As Bill says, when the water level in the head drops and leaves the sensor high and dry, it stops registering.

Guy
Guy Weller

Its a 68 midget 1275 engine vertical flow rad with the temp probe going into the top of the rad. No overflow bottle is fitted. The airlock does sound like it could be the problem as there was a fair amount of (gurgling), liquid moving about when I shut it down.
Next question what is the best way to get rid of the airlock?
Thanks Ian
I Pickering

fill the rad while squeezing the bottom hose...
David Smith

Ian

I think you missed the vital point in Guy's post

Air in his system came from Head Gasket Failure

and sad to say it is a strong contender for your basic problem too

if you fill the rad and run the engiine with the cap off you will soon find out if compresssion gases are replacing coolant inside the water jacket

dont stand with your face anywhere near the filling hole but listen for bubbling inside

If you are a member of the AA they have a chemical detection kit to test for exhaust gases in the coolant, if not you can still look for the bubbles

If theres air in there it didnt get in through porous hoses I'm afraid
bill sdgpm

Thankyou for the info Bill I am hoping it is not the head gasket, I have checked the oil and there does not appear to be any water contamination. Also there is no yougurt on the oil filler cap so I am hoping the head gasket is ok I will report back when I next annoy the neighbours with running the car up.
All I need now is 4 tyres and a door sprayed but the budget is tight !!!!!!!!!!!!!!
Thanks Ian
I Pickering

Hi Bill

The bulb at the end of the capillary has alcohol in it. It needs a great deal of heat energy to heat it up and cause expansion that will cause the arm in the gauge to move against a spring to regster temp rise.

Water is a good conduction and can supply this heat; air is a poor conductor and so doesn't transfer enough energy.

A
Anthony Cutler

I knowed there'd be someone!




ta Anthony
bill sdgpm

This thread was discussed between 03/04/2010 and 05/04/2010

MG Midget and Sprite Technical index

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