Is Cool Running Always Due to the Thermostat?

I’ve got problems with heating. All the system seems to be operating correctly and I’ve got “warm” pipes going into and out of the heater matrix, but the output from the vents is rarely better than luke-warm.

I noticed yesterday that even on a good run (40+ minutes at speeds between 20mph and 80mph) the temperature guage never indicated much over 60c.

This morning I started the car and let it idle. The upper radiator hose coming from the thermostat housing seemed to start warming slowly almost immediately so I jumped to the conclusion of a faulty thermostat and ordered a new one straight away.

A bit later, I got a break from my desk and so decided to test whether I was going to have problems removing the housing bolts and the housing itself. They came straight out (thank goodness) and so I took out the thermostat. It looks OK. It fully opened at about the right temperature and when cool appears to be closed. I can, however, see a very small gap and a glimmer of light through the top when the valve should be fully closed.

It really is the faintest of gaps, would this be enought to mean that the engine hardly ever gets up to temperature or could be some other reason for that? I used an IR thermometer to check it wasn’t the gauge itself under-reading but it seems about right.

I can’t see any topics on the forums which highlight other potential issues so interested in your feedback.

1982 XJ6 4.2 Series 3

I experienced a similar frustration with temperature reading seemingly low on my 1986 XJ6 literally from the day several years ago when I had the thermostat replaced with 82 degree (C) unit. Recent experience has convinced me that factors other than the thermostat may contribute to a ‘low’ gauge reading. Over the weekend, on a warm humid day we went for a 25 mile drive and it was only during the final 5 miles of the drive that the needle on the gauge moved to 90 and stayed there. It was interesting to note the moment went up that final ‘notch’ to 90 was when I made a sharp left turn. Then I remembered that over 10 years ago when navigating a sharp left interchange ramp between roads, gauge went a notch above 90. Ten days ago when driving on an extremely hot day, outside temperatures above 100 degrees F, 33C, as I backed the car into the garage space upon my return from the drive, I noted the gauge read close to but almost at 90 (just below), yet I heard the auxiliary fan come on, and it stayed on a minute or so after I shut off the engine. I know the fan is supposed to come on at termps above 90. Therefore, I’ve come to the conclusion that gauges and temperature sender units are not as accurate as we’d like for them to be.

You may be right, @MRCHB . I’ve actually swapped the temperature gauge and the readings still showed as low but it could be the sender. Having said that, it wouldn’t explain the other symptoms (cool heater temperature etc).

Overheating tends not to be an issue in the UK - I recall my auxiliary fan kicking in once since I’ve owned the car and in the recent heatwave here.

Thanks for your response.


Why not invest inan IFR temp guage. Not expensve here in USA at harbor freight.

measure at various locations on the system and get results.

bit, yes, A bad Tsdtst id likely the reason for low operating temeratures.

Been there in my 85 Fq50 ford 4x4.


Your heater running not hot enough suggests the possibility of the thermostat not opening and closing properly BUT keep in mind the possibility of a malfunctioning air conditioner amplifier. Remember,even when you turn on climate control to maximum warm settings a/c compressor comes on. If a/c amplifier is not working properly flaps of your a/c unit will blend the hot and cold air coming out of the matrix thereby diluting the hot air with the cold.

“Always” and “never” are words that I always avoid using. :wink:
Two additional possibilities, other than a thermostat problem, come immediately to mind for your lack of heat in the cabin. These include an inoperative heater valve, and a clogged heater core.
Observe your heater valve when you move your temperature knob from full heat to full cool. Does the valve go from full open to full close? You may need to remove it from the car and visually verify that the valve is opening fully with no vacuum and closing fully with vacuum applied.
Although the heater matrix pipes may feel warm it is possible that the internal matrix is clogged up with little or no fluid passing through it. One way to test that is by draining the coolant, disconnecting the hoses going into the heater matrix and running water from a hose through the matrix to see if it runs through easily.
Over the past 22 years of XJ6/XJ12 ownership (including 3 XJ6 parts cars) I have experienced first hand a variety of stuck heater valves and seen a badly clogged heater matrix that would only pass a small amount of fluid. Either of which could cause your problem.



If you confirmed your gauge is reading correctly, the only possible reason for overcooling is the thermostat.

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Thanks Everyone. New Thermostat going in today and to @Paul_M_Novak 's reply, next stage is the heater matrix. The heater valve is new (old one was leaking) and working fine.

On other points:

  • I’ve got and IR thermometer and the results of using that still seems consistent with a faulty stat

  • No air conditioning on my car. This should make it simpler but it still seems bloody complicated

New stat going in today. Will report back.



The sole purpose of the thermostat is to keep the engine hot, Mark - it’s the radiator’s sole task to maximize coolant cooling…

If indeed the IR shows water rail temps (at the temp sensor) at less than thermostat temp, the only cause can only be the thermostat. It is circulation that allows engine cooling - and the thermostat is the controlling agent. There may be many cause for hot running, but only one for too cold - so changing the thermostat is an obvious first step…

As for cabin heating; low coolant temps may be a factor, of course, but if you do not have air condition - cabin cooling should be the only problem.

Question; did the car originally have AC, now not functioning - or was it fitted with heater only? In either case, flaps are used to control cabin temps. Either guiding ambient air through the heater matrix, for heating, or bypassing the matrix, using ambient air for cooling. A cabin temp sensor is used to acquire and maintain set cabin temp - giving the necessary information for flap settings…

Vacuum plays a vital part in operating flaps - so verifying vacuum at various places is necessary. The water valve, vacuum operated is either fully open, heating, or fully closed - cooling. With AC fitted the valve is closed when vacuum is applied, but even if the valve is open there will be no cabin heating with flaps in the ‘cool’ position…

The IR can be used to measure difference in inlet and outlet hoses, indicating the amount of heat transferred to the cabin. But that depends on both valve and flap positions - and interpretation may be difficult; with no circulation, for whatever reason, there is no coolant flowing. However, it is also important to measure temp at the cabin vents temperature (with the IR) - if they read around ambient air temps it of course implies that air is not being heated in the core. Nominally; in- and outlet temps are equal…

xj6 85 Sov Europe (UK/NZ)

xj6 85 Sov Europe (UK/NZ)

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Hi Frank, thanks for your response - as articulate as ever.

This car never had air-con so it’s the basic heater system.

The water valve appears to open and close perfectly, moving from one extreme to the other as soon as I move the cabin temperature control from the “Vent” position. Can I presume this indicates that the Vacuum control is working well?

Once I’ve changed the stat, if there’s no improvement in the heating I’ll then start looking at the flaps and the matrix. I think @Aristides described this as akin to adjusting a swiss watch. In which case, I’m in trouble :slight_smile: Presumably, patience will see me through.

Thermostat arrived. All office work stops so I can get it in the car. New bolts for the thermostat cover, nicely coated in coppaslip go back in and hoses are connected.

Coolant replenished, car started and the first success is that there are no leaks. Second success is that the temperature gauge needle is off its stop much quicker than usual. Took it around the block and within a mile or so it’s up to an operating temperature over 80c - rarely seen previously.

And, yes, I have heat. The throughput is still slow - slower than you’d expect from the noisy fan - but there’s no doubt about the hot air coming from the front floor vents and rear console vent. Good enough for now but when the opportunity arises, I’ll do a full check of the system. I suspect the defrost isn’t going to be impressive.

I could only get a genuine 82c thermostat yesterday but I’ll pick up an 88c version in the coming weeks - that seems to make sense.

Thanks for your help.


as to the first question, is cool running always due to the thermostat: back in the old days…available were “summer” and “winter” thermostats, tho few actually changed them out. Summer and most often used year round were in the 68 to 78C range, while winter stats were 84 to 88C. The job of thermostat is to regulate coolant temperature constantly to keep in in the desired optimal engine operation range, for older cars around 190 to 210 F, and newer cars higher but with higher psi radiator caps. So the stat has the job of raising coolant temp by closing the passage to the radiator while at the same time opening the passage to the engine, OR…lowering coolant temp by opening the passage to the radiator while at the same time closing the passage to the engine. In most cases the closing is not 100% as there are slight clearances, and some by design. Part of design is the volume and rate of flow–some think that by removing the stat there will be more flow and thus run cooler…: not necessarily so: too rapid flow thru the radiator may not allow sufficient time for the coolant to be cooled. Some think a lower temp thermostat will result in solving overheating: not so. The cause of overheating will remain, and the stat will open coolant to the radiator at a lower temp but the coolant will from that point heat up due to whatever the problem is–clogged radiator or passages or faulty water pump impellor etc. In a good cooling system though, a lower temp spec thermostat WILL keep the coolant at a lower temp–about that of the spec of the stat, (usually coolant will be about 10 D C higher) as that is the whole purpose of the opening-closing temperature of the stat.` A 70C stat will normally have engine coolant at about 75 to 85C on warm summer days…but will be at 70C on cold days…too cold…thus the winter stats of 86 to 88C. As to the heater, 70C is about 158F…warm enough for nice heat in the cabin. With desired coolant temp of 75 to 85C even more nice heat. If there is not heat, then the issue is within the heater system.

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Since I live in a warm weather location and never use the heater, I think my mechanic was correct in installing the 82 degree C thermostat. Though it takes a bit longer for the temperature gauge to reach 90, once it does it remains there, even in outside driving temperatures rising above 33 degrees C with the air conditioner on. So, like “back in the old days” as Nick references, my car is fitted with a year round summer thermostat.

It sounds as though your thermostat was faulty.

It is supposed to be completely closed below the indicated temperature stamp on the thermostat. Expect it to just start opening within a couple of degrees of the spec’ temperature and then to keep opening slowly for the next 4-5’c rise. It then ought to open a lot very quickly over the next ~4’c and then reach maximum opening ~12’c above its spec temperature.

A thermostat which is always a little open will have bled water to the radiator even below the minimum design temperature when you started the car from cold. This means it can take an age for the engine to warm up, which are the exact symptoms you have reported.

Everything you might want to know about thermostats is on the website.

kind regards


Just record the vent temps versus outside air temp with the IR, Mark - then you sort of have a baseline when/if you start ‘doing something’ to the system…:slight_smile:

You should also verify temps the same way when the system is turned to ‘cold’ for the same reason. Which will also indicate whether the flaps work as they should - and indeed, with the combination of results, that the water valve works as it should in heating/cooling. Actual data seldom come amiss…:slight_smile:

As for the defrost; the same, but essentials are that flaps are open only with ‘Def’ selected, and when selected, blowing hot air at high fan speed. However, without AC the air will not be dried out through the AC’s evaporator - so demisting the screen may take long time, also influenced by ambient air humidity…

Good result with replacing the thermostat - can expect good cabin heating with low coolant temp! The icing of the cake would have been to record the vent temp increase with the thermostat replacement - but hardly vital…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

That temp rise above thermostat spec sort of implies that your cooling system is less than perfect - which is not that unusual…:slight_smile:

xj6 85 Sov Europe (

To add that it ensures vital circulation through the block while the thermostat is closed, Nick - and as less water is being heated; warm-up takes shorter time. Which of course is also stretched if the thermostat stays ajar and let water through the radiator - adding both to the extra volume to be heated and the radiators cooling effect…

As an aside, as the coolant reaches thermostat temp and start opening one may experience a slight temporary drop in temp as the cold radiator coolant enters the circulation…

xj6 85 Sov Europe (UK/NZ)

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I’ve always suspected that my car runs a bit rich - there’s no visual indication but it smells rich.

Could that be because it’s been usually operating at too low a temperature?

That’s what I tell my team all the time but it’s a case of “do as I say” etc…I’m going to collect some this weekend and then check all the flaps. If I know the system is working as well as it can, then I’m satisfied. I have to say that I was happy to find an car without aircon when I bought it - avoiding complication, I thought. With hindsight and the experience of a small heatwave in the UK, I am of the opinion that I was wrong.

That is correct.
With a 82°C thermostat your engine should be running at 82°-85°C in all but extreme conditions, i.e. extended idle or extended climbs on hot days.
For the time being it’s ok, but keep an eye.


Enrichment for cold running tapers off until about 70ºC or so. If you were running 60ºC then you’d still be getting a small amount of enrichment…akin to driving with oldie car with the manual choke partially applied.

If you still smell a rich mixture with the higher coolant temps there are various other possibilities to look into.



At 40C, a ‘small heatwave in the UK’ is masterly British understatement, Mark…:slight_smile:

And cold running, especially on EFI and more sophisticated carb set-ups, may/will certainly cause fat running…

xj6 85 Sov Europe (UK/NZ)