Maybe get a spare Thermostat and take it with you in the boot/trunk, if the worst happens you can swop it out at pretty well any mechanics workshop in a short time.
Possibly existing one has failed or maybe it has a partial blockage preventing full shutoff. But it does not sound like a terminal problem, if the only effects are slightly variable temperature and interior heating not that strong those are relatively small inconveniences.
**
It plays havoc with the fuel consumption, DavidâŠ
Frank
xj6 85 Sov Europe (UK/NZ)
Maybe so on a fuel injected car without the right thermostat but is this also possible with the ASC? I thought not, so there should only be a small difference as hotter engines run slightly more efficiently. I could be wrong; I donât know the ASC.
Another possibility for your problem is tired radiator hoses - the bottom one in particular. This can collapse inwards when hot due to the action of the water pump if itâs tired. When hot, squeeze the hoses and if they are soft, change them.
Also, if you buy a new thermostat, try to get a good one, There is a lot of junk out there for sale. And donât forget the gasket, or youâll have to make one somehow if it needs changing away from home.
Itâs a bit complicated, DavidâŠ
Obviously the EFI use coolant temps as a parameter for fueling - implying that the engineâs mixture needs vary with engine temp. With carbs the same applies, but âsimpleâ carbsâs mixture is set the with the engine idling fully hot - and the mixtures is likely not changing with engine temps. Ie, engine fuel demands may not be fully met at lower (or higher) temps than normal - and the driver must compensate, unawares, with the gas pedal.
More sophisticated carbs may have engine temp compensators, which will alter mixture with coolant temps - generally running a fatter mixture with engine too cool by intent. Or, as Jaguar did, fit âsimpleâ carbs engines with auxiliary âcarbsâ to fatten mixture during warm-up - which of course will fatten mixture if the engine runs too coolâŠ
But my main concern is that while a faulty thermostat may keep you happy at say 10C ambient; it may not do so as the temp drops, overcooling the engine - which may be inadequate for cabin heating. And of course; at high ambient temps the engine may overheat. A crude test of thermostat function is that engine reaches thermostat rating within some 5 minutes of driving - if it takes much longer; the thermostat is likely faultyâŠ
As an aside; one of my Norwegian winter preparations was to cover 4/5th of the radiator on my cars. The heaters were so effective that even with a properly functioning thermostat the heaters cooled the engine to an extent that cabin heating was uncomfortably impaired - and reducing radiator air flow was an remedy. As a matter of fact, my fathers 38 BMW had slats, in front of the radiator, that could be opened or closed at will while drivingâŠ
Frank
xj6 85 Sov Europe (UK/NZ)
My C class is on its third thermostat and last time it failed I covered â
of the radiator so it could go over 60°, worked well. Winter, but not that cold⊠little traffic and not going that fast so no real cooling needs.
Consumption was up a little, otherwise I wouldnât have noticed for a while.
If the AED was still open way past 50° I would be surprised.
Of course I have the earlier system which runs a bit lean, then perfect, then a bit rich, switches off at 30° coolant temperature and must be a bit lean until it has fully warmed up. Itâs not sophisticated but it works and when it is off it is off.
The AED on the other hand may still add some fuel at warmer temperatures - but doesnât it measure exhaust manifold air temperature? Then it does not rely on any part of the cooling system and there would be no issue at all if the thermostat wasnât closing fully and overcooling sometimes.
I see no trouble taking the trip, wouldnât take a thermostat with me but I would replace it afterwards if the problem still appears. Cardboard for cabin heating helps, by taking away the capacity that was added for hot days in uphill traffic with the AC on.
Oh. And do look at the hoses before leaving, that is a good idea.
Thanks all. I have a manual choke by the way, not the AED.
I am pretty sure the thermostat is good. Warm-up is prompt, temp gauge reaches âoâ or so within maybe 6-7 mins and stays there. In a variety of traffic conditions (solid traffic jam idling for 20 mins, motorway at 70mph, and everything in between), movements of the gauge were hardly perceptible. As mentioned previously, temp on the water rail seems broadly consistent with the stat.
This time I got better and now reasonable interior heat at speed, but admittedly less hot than in town. But I am not sure this is anything to do with the thermostat.
Of course, I ordered several thermostats and a variety of gaskets so I am prepared for this anyway.
Head gasket may or may not be another story, but I am not suffering any coolant loss so will continue to monitor this closely.
Thanks
Nick
Sounds good, keep driving!
David
**
I 'think the ASC is controlled by coolant temps, David - but it somewhat academic. Apart from that, I cannot fault your logicâŠ
Frank
xj6 85 Sov Europe (UK/NZ)
**
Fair enough, Nick
The warm-up time is a bit longer than I would expect, but the proof of the pudding is what happens as the temp dropsâŠ
Frank
xj6 85 Sov Europe (UK/NZ)
I thought the AED is controlled by (warmed exhaust manifold) air temperature but what do I know - the ASC has an otter switch which matters coolant, and luckily thatâs what I have 
AED is the oblong device feeding behind the two carbs, ASC is the cylindrical device that hisses loudly and feeds all six inlets individually from below.
**
Indeed, DavidâŠ
As an aside; there is also the not negligible engine cooling effect of airflow at low temperatures even for the xk, air cooled engines swear by it - and controlling the airflow is one way of controlling the effectâŠ
Frank
xj6 85 Sov Europe (UK/NZ)
500 mile report: very smooth running. Temp gauge hardly budged. No coolant loss. However, still getting very lukewarm air into the cabin after driving for a while, hard to understand why as I get hot air at low speed so heater matrix must be getting coolant through? Perhaps therefore an issue with those crazy flaps and amplifier, cabin thermostat incorrectly set up�
More importantly, head gasket weeping is back. As noted above no coolant loss detectable but clearly not good. Probably the main casualty of the overheating so will inevitably need attention. Annoying given that I only just had a new head gasket âŠ.
By the way, is my fuel gauge experience typical ? After a fill, it drops to the 3/4 mark remarkably quickly, after like 10 miles. Makes you think there is a hole in the fuel tank and you wonât even make it to the next filling station, but then it calms down and sits for ages at the half way mark, so it is inaccurate when full and gets progressively accurate as it empties. I got a reasonable 21mpg on the motorway today.
Thanks
Nick
XJC 4.2 (1976), Canadian spec
Hello Nick!
The fuel gauge on my S2-74 does this as well.
Best regards,
Marc
i suspect the pragmatic is refering to quality of the product now available. Ie, new ainât always good!!!
And, aye my lump will run just a tad hotter at prolonged idle or slow traffic. i see it on the guage and hear the Efans engage,.
I see it as an airflow thing. And up hill, more throttle, more gas, more BTUâs to coolâŠ
**
During warm-up the system is in âfull heatâ and recirculating cabin air. As cabin temp is reached, the system goes into âmaintainâ - and vent temps will drop to a predetermined level. This may be perceived as a drop in vent temp - but is actually a characteristic of the systemâŠ
When evaluating the system; it is essential to measure temperatures with a thermometer - driverâs perceptions are unreliable. If all this does not fit your symptoms; read on for a âfuller montyââŠ
The heater core is fed from the water rail at coolant temp, Nick - then returned to the pump. Nominally, the heater core temp is independent of rpms and road speed, but three factors may interfere. A clogged core may not pass enough coolant to maintain core temp, lowering air vent temps - though increasing rpms/speed will increase water circulation. Increased speed may lower cabin temps, but should not interfere with air vent temps.
And as rpms increase; the compressor may increases evaporator cooling - and all air first passes the the evaporator for cooling. The vent temps is then regulated by heater core bypass, controlled by mechanically operated (by servo) flaps. But the AC amp is not aware of evaporator temp, and does not compensate for itâŠ
Crude evaluation, Nick; set the temp to âautoâ and â75â and drive along while cabin is heating up. System working as it should; as cabin temps reaches 75F, fans will go to low speed - and if the cabin temp drops; fans go to highâŠrepeated ad infâŠ
Measured at the in-car temp sensor, the the temp there should coincide with the set, 75F, temp - on the original AC amp this was adjustable.
**
You may have suffered the infamous tank âimplosionâ, reducing tank volume and shape - is it the same on both tanks? Detected by checking how much fuel goes into an empty tankâŠ
Or the tank sender unit(s) may have a glitch - usually on one side onlyâŠ
Frank
xj6 85 Sov Europe (UK/NZ)
Thanks all.
On the fuel tank gauge, itâs the same on both tanks and is more an amusing feature than anything else. It starts to get pretty accurate when around half full and I have tested accuracy at « E »âŠ) and the rest is merely a guide.
On the heating, thinking about it, I have noticed that when set to « auto », the fans rarely go into « hi » these days, as described to achieve the desired temperature, but rather cycles between low and mid. This suits me but its behaviour seems to have shifted over time.
Also, I should have said that most of the time I drive without the A/C running via the popular mod whereby the electric rear window isolation switch actually switches the compressor on and off. I havenât yet tried the system with A/C engaged, although my ignorance is such that I donât see why it would make a difference to max heat.
Thanks
Nick
**
To amplify, Nick, there are (originally) four fan speeds; âHIâ and âLoâ set manually by the switch - and different high and low operated by the AC amp/servo with switch to âAutoâ. System is designed to rapidly acquire set cabin temp by running fans at high speeds, then drop to low speed as set cabin temp is reached - repeating when/if cabin temps drift off.
The AC compressor is âonâ whenever the switch is out of âoffâ - compressor is only switched off by the Ranco thermostat when evaporator temp drops to +2C or below. The evaporator generally acts as a dehumidifier at all times - reducing window fogging. Turning off the compressor; the system, in âAutoâ will operate normally - but can only use ambient air for cooling. However, the AC amplifier is programmed to assume the AC compressor is on, so the flap settings may not be optimal at all times. Whether this is a factor in the fan speed operation, or temp variations you observe, is âuncertainââŠ
In any case, and as said before; it is essential to use a thermometer to measure temperatures when pursuing perceived problems - driverâs temp perceptions are unreliableâŠ
Frank
xj6 85 Sov Europe (UK/NZ)