Temperature gauge behaviour

[streaky]

On occasions I have noticed the following behaviour of my water temperature gauge:

It normally runs at 80/85C but occasionally on a motorway blast (@ NSL of course) it will rise steadily to 95C (occasionally higher), then immediately fall to 75/80C and stay there. Then it might rise steadily to 90C and immediately fall again.

It looks for all the world like the thermostat opening and closing, but it isn't consistent. And it only happens on a long run at speed, not in town driving.

It does appear that it might be linked to the heater control, because I've noticed twice that shutting the control (ie. cold) stops it happening. Mind you, turning the fan on with the heater control open or shut gives hot air! And teh control is connected before someone suggests that .

The system is full and doesn't lose water (FLW!).

Any ideas?

Streaky

[GreenV8S]

This is just a guess, but one failure mode of ordinary top stats is they tend to be 'blown shut' by the water pressure when they are nearly closed. This means the temperature has to overshoot slightly before the stat opens, or a stat which is only open a crack will slam shut when you rev the engine and open again when the revs drop. All top stats show this effect to some extent, but I imagine a faulty stat could make it worse. Does this tie in with what you're seeing?

[njhucker]

mine does the same,(350)
but never goes above 95, and soon goes back down. no matter what the conditions.
So no worries.

[dickymint]

njhucker said:

mine does the same,(350) but never goes above 95, and soon goes back down. no matter what the conditions. So no worries.

Ditto.
I've Always put it down to an airlock where the sensor is.
as it clears the needle drops really quick.
I can never be absolutely sure that the system is full!
But I can nearly always squeeze more into it then problem gone.
Maybe when you turn the heater valve off theres more water available for the main system?

>> Edited by dickymint on Friday 10th October 10:07

[streaky]

dickymint said:

[snip] Maybe when you turn the heater valve off there's more water available for the main system?

Surely heater valve OFF = LESS water being circulated through the system. Whereas, heater valve ON = MORE water and MORE cooling - the heater matrix will do some cooling, enhanced by the fan. Or have I got that completely ar5e-about-face? - Streaky

[sevans]

Seem to remember a post about stats a while back. Someone recommended drilling a couple of small holes in the top. Maybe if it is slamming shut this would help.

[psb]

dickymint said:[snip] Maybe when you turn the heater valve off there's more water available for the main system?
[/quote]
Surely heater valve OFF = LESS water being circulated through the system. Whereas, heater valve ON = MORE water and MORE cooling - the heater matrix will do some cooling, enhanced by the fan. Or have I got that completely ar5e-about-face? - Streaky

Makes perfect sense to me,also was once advised by nice AA man to turn heater on when I had sticking thermostat problems, and it worked.

[dickymint]

streaky said:

dickymint said: [snip] Maybe when you turn the heater valve off there's more water available for the main system? Surely heater valve OFF = LESS water being circulated through the system. Whereas, heater valve ON = MORE water and MORE cooling - the heater matrix will do some cooling, enhanced by the fan. Or have I got that completely ar5e-about-face? - Streaky

I think what i am saying is: if your system is completely full then there is no difference if heater is on or off.but if there is air in the system and you turn heater off then there is a better chance that the main system is full.(well i know what i mean sort of)
Or does water circulate through the heater matrix regardless?