Mechanical Temp Gauge Flucuations

[pamitchell]

Hi all;

My temp gauge normally reads steady but recently I fitted a new 180 thermostat and experienced brief moments where the needle flucuates +/- 10 deg. I'm not unduly concerned, just curious. Any ideas or links to literature on mech temp gauge operation?
TIA
Phil Mitchell
San Jose CA.

[type26owner]

At what frequency? Do you have a overflow/recovery tank on the cooling system? Is the radiator cap holding the correct pressure?

[nebogipfel]

Hi Phil,

I wonder if it could be an air lock? Hard to imagine, but I suppose it is possible.

[pamitchell]

Car has a recovery/overflow bottle and 10 lb. cap. Never boiled over.
How do you check for/get rid of airlock?

[type26owner]

Phil,
The cooling system will self purge any air out via the overflow tank. Just check after everytime it's stone cold and top up the water in the tank if it needs it by sucking it dry. Oh, you'll need to have fitted a two-way rad cap also. There is a lightly spring loaded brass valve exposed in the center which is the vacuum break valve. The cooling system will not work correctly without the proper cap. By that I mean it will not recover the overflow from the hot coolant when it cools back down.

What is the estimated time does it take for to it swing from indicating the +/- 10 degree hot and cold points? This an important clue as to the realtime effect you're observing?

[pamitchell]

Keith. It's more of a flicker than a swing. My rad cap does not have the brass valve. Can you recommend a part no. Are rad caps universal fit?

[ElliottN]

Thermostat - when you fit a new thermostat it may exhibit an initial 'flicker' first time you run the car. The temp will climb above it's normal opening temp (say 10-15 degrees) before it opens with a rush and the temp usually does a quick drop below ambient temp (cooler uncirculated coolant coming through) before settling. It's a bit like 'breaking the seal' and in my experience only does it the once. If it's persisting you may have a faulty/sticking thermostat - especially if nothing else has changed but the installation of the new thermostat.

Rad caps - are either 'Recovery System' caps or standard caps and all are available in a range of pressures. Recovery System caps should labeled as such and have the small valve as described by Keith (note - it's NOT on the outside of the cap). Without a recovery cap your overflow bottle is a useless appendage - except for catching some of the coolant if you overheat and the rad cap blows.

Have part numbers for after-market caps made here in Oz but that won't help you so the best thing to do is take your existing cap to a good auto parts store and get a 10 psi recovery cap that physically matches your current cap. For the most part there are only 2 sizes of caps (OK - there are some exceptions) and standard type Elan rads use the larger cap.

[type26owner]

Adding to Elliot's description of the rad cap a little, what you want to look for is two rubber seals on the cap. Only the two-way caps will have dual seals.

Suspect there is still some trapped air still in your cooling system. The Robert Shaw thermostat already has a small indentation to allow the air to bleed past it.

One of the downside issues with the Smiths mechanical bourdon tube type water temperature gauge is the very fast response time is exhibits. Because this gauge moves so quickly and shows the thermal drift caused by the wasted heat of the engine in realtime I suspect it spooks most folks. Running a higher temperature thermostat causes this normal process to happen even faster. What you're seeing is the engine obeying the Laws of Thermodynamics. It can be quite unnerving if you don't understand those laws. Hence to avoid being bothered by an effect they don't understand most folks are inclined to install a lower temperature thermostat to lessen the speed of the drift. Ya kinda shoot youself in the foot if you do this for that very reason.

And of course the gauges' clockworks mechanism is prone to dither some from vibrations and cornering forces. Mine reads low by 10 degrees when ever I corner one particular direction. I can't recall which direction that is at the moment though.

[pamitchell]

Thanks to all for the valuable information.
BTW, what is inside the Bourdon tube? fluid, gas,?

[type26owner]

Best guess is ether is inside the bulb of the Smiths Water temp gauge.

[pamitchell]

Pleased to report that fitting the 180Deg thermostat cured the needle flucuation as Keith predicted. Car runs with near constant 85 deg C. indicated. Using an infrared tempature gun to set the electric fan cut in switch.
Thanks to all for your advice. Still curious what;s inside the Bourdon tube.
Phil Mitchell-San Jose CA. 69 S4 FHC

[type26owner]

Almost certainly it's diethyl ether. If you ever run across an old bottle of this stuff don't touch it, call the bomb squad instead. The distillate peroxides are extremely unstable.