Cooling System Puzzle

[Ken's 57]

This one has me confused so it's over to the brains trust. Last couple of days I've been driving the FE to work: a trip of around 20 minutes each way. On two recent stinking hot days the temp gauge was reading around 185°F on the morning trip in - as it always has. Coming home in the arvo after the car has been parked out in the heat I noticed the gauge had barely moved up from the at-rest position. Reading around 120°F and I'm almost home. Pulled over fearing a loss of water and undid the cap. All fine.... slight hiss from under the 7lb cap and absolutely no fluid loss. Engine and radiator cool enough to touch briefly.

Back onto the road and within a minute or so it's climbing up towards the default position of 185°F. Huh
Engine, radiator and cooling system rebuilt 1000 miles ago. Heaps of power. No water in sump and no bubbles evident in the cooling fluid with the radiator cap off and engine revving. Original Smiths heater connected up but the tap is in the off position. New good quality temp gauge. Can anyone suggest what's going on?

Cheers, Ken

[Harv]

Some maybes, some more likely than others:

a) thermostat was stuck shut (for whatever reason). Radiator stays coolish as it has no flow. Temp gauge reads ambient temp as it is in the part of the cooling system with no flow when thermostat is shut. It would need the cooling system to have enough capacity to absorb the heat without boiling during the drive home... that's the unlikely bit. Releasing radiator pressure gave the thermostat a shunt, and it opened, letting system work like normal.
b) sticky temperature gauge needle. Turning the power off allowed the needle to swing back to zero, removing the stickyness from the gauge internals.
c) temperature sender unit covered in sludge/scale (unlikely) which was dislodged by the radiator cap pressure shunt.
d) crap connection on the temperature gauge increased/decreased the resistance in the sender unit wire, buggering around the gauge reading. Wire bent back to normal/crap dislodged from terminal when engine stopped, returning correct reading.
e) For a short period of time your car worked out it was an early Holden, and realised it was cooler than it thought  . It then realised it wasn't an FB/EK, and considered itself not as cool as it could be  . Sorry, couldn't resist that gag.

Cheers,
Harv

[ardiesse]

Ken,

Work's not uphill from home, by any chance?

What type of temperature gauge do you have - electric, or the classic style with (I forget) alcohol or ether as the working fluid?

Where is the temperature sender located - cylinder head or thermostat housing?  Shouldn't make much difference either way, actually.

I'm beginning to think it might be a bad electrical connection, if it's an electric temp. gauge.

Rob

[Ken's 57]

Thanks Harv and Rob. The gauge is a new "AutoMeter" mechanical gauge with a good connection Rob. To and from work is a fairly level run along a traffic-free country road. No sludge as engine was cleaned out thoroughly when rebuilt. Harv, thermostat is new as is the radiator cap. I tapped the gauge a few times but no change until the cap was removed while the engine was still running. It seems like some sort of air lock and I'm worried that a part of the engine isn't being cooled.....especially with what the rebuild finished up costing me. Does a closed thermostat actually block off circulation or just impede it until the engine reaches operating temperature? Is a 4lb cap worth trying? Oh and BTW..... I'm pretty sure the FE is happy in its own skin Harv. It's been turning heads since 13th August 1957 

[Harv]

Most thermostats block off all flow. Some have fancy holes drilled in them to allow some flow even when closed (sometimes called a bleed notch). Many also have a jiggler pin, which aims to let air pass through so the system can bleed.

Point taken about the FE's looks.... they are a cool machine despite my jives 

Cheers,
Harv

[Ken's 57]

Thanks Harv. I'll get another thermostat and see how it goes..
Cheers, Ken