Temperature Gauge keeps moving up and down?

[Justin]

For some reason my dash temperature gauge stays at cold for a short while when my cars first started, then the red light will flicker and it will start moving towards the hot symbol, the dial then stays at about quarter the way up, then occasionally the light will start flickering again taking the needle up to the thinner line before the hot symbol, then once again will return back to the quarter position?

Could it be that my fan isnt working and could it be as simple as replacing a fuse?

[MCHUDD]

Hi mate.
you could try changing the thermostat.It might do that if it wasn't opening and closing properly.
Cheers Mark.

[Justin]

Thanks mark, today it's staying right up on hot for ages then occasionally dropping back down, my fan is working and the water in the filler tub is warm but not really hot, theres also plenty of water in the system.

Is it easy to change the thermostat, where does it live and where can I get another one?

[MCHUDD]

Hi Justen.
If you follow the top rad hose from the rad back to the engine,where hose meets engine thats where the thermostat will be.
They can still be got at a motor factors.Just tell them witch modol you have.
Cheers Mark.

[Justin]

Thanks mark, it was much cooler and not as sunny this morning and driving around town for about an hour didnt cause the gauge to go towards the hot, infact it stayed constant at the quarter way mark.

A mechanic friend of mine said its unlikely that its the thermostat as the cars not overheating or using loads of water, he seems to think its the heat sensor/water temperature sender?

If I do have to replace the thermostat does it come with a gasket as my Haynes manual strongly suggests that this be replaced and also will I have to drain the entire cooling system or can I just get away with replacing the small amount of water that will be lost when I disconnect the top pipe where the thermostat lives?

Jus

[MCHUDD]

Hi Justen.
You do not have to drain the system. Just disconect the pipe[engine end],pull out old thermastat and bung a new
one in.You will lose some water but just fill it up again.
Cheers Mark.

[Justin]

Is it worth replacing the heat sensor first or shall I just replace the thermostat anyway?

the water in the header tank feels very hot today, maybe I should put antifreeze/coolant in?

J

[pettaw]

I doubt very much its the temperature sender. I've not known any of those go bad, but I've seen plenty of thermostats wear out. They go soft with age, and start opening, causing cool running, or if you've not been using anti-freeze in the engine, they can rust and seize shut.

What type of engine is it? If its a 1.4, it sits in the top radiator hose above the water pump, there's a double jubilee clip there. With the warm weather now's the perfect time to do a proper flush on the system and fit a new thermostat. MCHudd is quite right you don't need to drain the system down to change the thermostat, but I bet you haven't replaced the coolant in the car for a few years? Should be done every two years to keep the anti-rust additives topped up.

Best way to drain the system, warm the car up first, run until the engine is nice and hot and the radiator cooling fan has come on. Then switch off and allow to cool for about 30 minutes so you don't scald yourself. This is to get the hoses nice and soft so they'll come off the pipes without breaking anything. Then loosen the bottom hose on the radiator to drain the system. You might need to raise the car up to do this.

Then you can change the thermostat, button the system back up and refill with good quality coolant, mixed 50:50 with water. I find the Volvo own brand coolant works best for me, nice and thick because its got lots of anti-rusting additives and also wetting agents to improve the conductivity for heat transfer. Its also not too expensive at about 12 quid for a 4 litre big bottle. I believe you can also buy 1litre small bottles. For a 1.4 you'll probably only need about 4-4.5 litres mixed, so probably get away with 2 litres of concentrate.

Hope that helps.

[Justin]

Thanks Pettaw thats a great help. its behaving itself now but I will replace the thermostat if I can still get one easy enough, if not I might just repalce the coolant anyway, I've had the car a year so dont know how long the previous coolants been in there?

My haynes manual says its best to drain the coolant when the cars really cold? and am I correct in thinking that the interior heat slider should be on hot?

J