Hello all, my Jag has started running cold after I replaced the thermostat, so I’m looking for solutions and I hope a simple write up might help someone in the future.

With the old thermo, the car warmed up quickly and the dash gauge would stay in the middle no matter what. All hoses would warm up and the expansion tank would warm too. The fan would sometimes come on, but only rarely.

I removed the thermostat by undoing the two 10mm bolts on the thermostat cover, and found the old thermo to have a ring of applied sealant around it which came out when I removed the thermostat.

I tested the old and new thermostats in the same pot of water, the new one opened at around 85-90 degrees, the old one appears to be stuck closed.

The new thermostat was installed with no O ring, but reusing a gasket which was on the housing already. The vent pin on the thermostat was at 12 o’ clock. The thermostat was positioned with the large side towards the front of the car, and the small side pointing rearward. I tightened the cover down bit by bit to ensure the cover didn’t crack, apparently they’re fragile.

I then filled the expansion tank with distilled water to the max mark, and squeezed all hoses I could reach to encourage air out. The ratio of water coolant is likely skewed towards water now, I will do a proper coolant change if I can resolve this first. I don’t think a majority water mixture would cause the temp to drop this drastically however.

Starting the car, I waited about 10 minutes to watch the temp come up and left the heater on full blast. The cooling fans did not come on at any point. The hoses to and from the rad were hot, but the hoses to and from the expansion tank were not. The tank did not warm up, I could put my finger in and it was cool. Satisfied that the level wasn’t dropping and the temp was in the centre, I took it for a spin. At speeds above about 30mph, the temperature would begin to drop. Eventually it went all the way down to zero. Stopping and idling allowed the temp to come up again.

When I got back, I was still able to put my finger in the expansion tank. It was warmer than before the drive, but not 80+ degrees. There was no great release of pressure when I took the cap off either.


These are tell tale symptoms of a stuck open thermostat, however it is a brand new item and has been tested in boiling water and appears to be working as it should. Are there any other reasons why my temperature would drop when moving?

 

Not sure if you saw my reply to your other post…..

but….

i had exact same thing.
2 year old thermo broken.

testing it in boiling water does not show anything other than that it does open.
i put mine in water of varying temperatures and found that it was starting to open when the water was as cold as 60 celcius.
replaced it and now all is good.

 

I put it in the water and brought it up to the boil, with a thermometer in. It seemed to open around where it should, about 85 - 90 degrees Celsius, but as the car is still cold it seems to be an issue still.

Could it be positioned incorrectly in the thermostat housing? When everything is tightened and reassembled, is the thermostat held in the right position? It was a bit fiddly getting it to stay in the thermostat cover seated properly while I was putting it back on the car. Can it slip out of the cover a bit, allowing water past?

 

G'day Toby,

You asked "Can it slip out of the cover a bit, allowing water past?"

Yes it can, my, car a V12, has a thermostat on each head and when I replaced them some time ago one of the original thermostats was indeed incorrectly placed in the housing and apart from the fact that it allowed coolant past it I am amazed that the whole thing didn't leak like a sieve so although it is a nuisance it would be worth checking.
Maybe you could start by removing the hose from the housing and having a look in there with a torch and a mirror.

Cheers,
Jeff.

 

Good idea to check with a mirror. I assume a bit of sealant on the new thermo would help it stay in place, I’ll have to buy some of that.

 

So I just went to check my thermostat position and stick it in with a bit of sealant, and one of my thermostat cover eyes has snapped off. Perhaps a leak was contributing to my cool runnings. Can this be welded back on, or is a replacement needed?

VERY annoyingly, I need the car tomorrow. So this won’t do. If anyone is in the east anglia area and has one of these available, I’d love to come and buy it!

 

This will have to do. Hopefully this lasts long enough for a new part to arrive.

 

Look real closely in the crack and look for dull oxidation color compared to the outside surface color to tell if crack started from previous owner / oxidation time

Your pictures were great for others to not have this

I see your oxidation metal bubbling up in your thermostat seat / groove area

Could be also caused by a shifting of the thermostat out of its seat / grove on bolts clamp down

Brands of thermostat for the record and if can micrometer the thickness of the on thermostat flange / rim

To seal the thermostat flange in the housing without using a O - ring used on the earlier X40 with the earlier inline 6 ( very close AJ6 vs. AJ16 ) .........................

High temp red sealastic ?

 

Very late update, due to parts taking forever to arrive. With a new thermostat cover costing £60, but a used entire thermostat housing only costing £20 (including coolant temp sensors!), I bought the used one and swapped my broken thermostat cover out, and fitted a new gasket thermostat housing gasket (no o-ring!).

This has cured the cold running issue. I can only assume that the thermostat was incorrectly positioned in the old cover, or it was allowing cool water past due to some cracks. Anyway, problem solved, and I now have some spare sensors and the thermostat housing just in case...

 

Red high temperature sealastic may not be best application as some ( by label and type ) release acidic acid which is not good for aluminum

But you may have gotten around to not using that

Good to see your engine regulation honed / tuned in

 

i used permatex copper rtv on mine.
last time used permatex thermostat and waterpump sealant.
but didn't have any around this time

 

A late update on this, as the weather cools down the temperature issue has reappeared. At the end of my 30 minute drive to work, the gauge was still low. It reaches the middle when idling at a standstill, but when coasting along the temp steadily drops. I was getting heat from the heater, although it seemed to go up and down in intensity as the temp gauge moved.

Could this be the thermostat problem again? Last time I out the brand new thermo in with a new gasket, so could it have shifted in place to allow cool water past? As winter approaches, I want it to warm up quickly, defrost, provide heat, and be running at optimum temperature so it's annoying that this issue has reared its head again.

Any suggestions for things to check would be greatly appreciated, thanks!

 

On the X300 the defrost and cabin heat out of the heater matrix / heater core operate separately than the core engine coolant circuit

So those 2 things should not be an issue ( ? )

Those 2 things together operate in a independent way of pulling a coolant amount from a pipe inside and along one of the rear cylinder walls , this is guaranteed to be hot and hot faster than normal cars using a coolant amount from the main / all engine block

But............................the coolant " base " temp can be started from a lower temp as the thermostat is too open or bypassing

You will see a coolant hose on the top engine block rear to cars left to do this , then the heater matrix coolant flow finds it's way back into the radiator

Another factor is if the radiator fans are running too fast at a cars slow speed , at highway speed the airflow across the radiator is independent of the radiator fans , base coolant temp returning to block related , the thermostat has a way of blocking the temp value inside the block to a operating range

There is a simple paper clip test if you need it

 

Does your 4.0 have the factory mod to keep the cooling fans on all the time ?

My 3.2 does not ( or the mod may only have been for the 4.0) and has quick warmup. I’m in Central Scotland so a similar,if a bit cooler, climate to sunny Suffolk.

Hope you solve the mystery.

Alan.

 

Hi Alan, I haven't got that mod, the fans only come on when the engine is hot (which isn't very often at the moment!).

It used to warm up quickly on the old thermostat, with the new one (when it was working properly) it took a bit longer, and now it seems it isn't warming up at all. After today's commute I arrived at the office 30 minutes later with the needle at the very bottom of the dial, although it had edged up slightly when I was in traffic, before coming down when moving.

I assume it must be that the thermostat has been dislodged in the housing. Are there any ways to get it to sit right? Obviously the thermostat gasket isn't holding it properly.

 

Very frustrating. Not been into the stat housing on mine.

i did have a new ECU temp sensor in mine when I bought it and it turned out to be faulty and one cause of poor consumption ( though engine ran very smoothly) so parts quality issues are sadly common.

 

I've got hints from other car makes that new thermostats are very iffy

I think you're chasing bad thermostats and your only option is another one

 

The TSB is also for the 3.2 liter as the end result is to not have the fans fuses blow and you have a overheated engine

You can just remove the 2 wire freon pressure switch connector and splice the 2 wires together to accomplish the TSB end result

 

Is the thermostat designed to be held in place when the cover is put back on, or is it possible to come loose and allow water past?

If it's held securely by the housing cover and the gasket, then it must be a dodgy thermostat. If it can move around a bit that might be the problem rather than a dodgy stat.

When I squeeze the upper rad hose and force some water around I can hear a slight metallic tinkling noise that sounds a bit like what I imagine a thermostat moving around and bumping into its surroundings sounds like. I don't know if it's the whole stat moving or something else loose in there.

 

i can only speak from my experience where a dodgy thermostat was the issue.
it was opening way too early preventing the car from warming up.
and knowing that stuck open thermos is a issue on these cars also