I posted a question recently regarding thermostat renewal because my temperature gauge rarely reached 'normal' only getting to the marker below 'N', but then it would move downwards towards cool only to move back up again and down again.
Anyway, thermostat renewal hasn't changed the situation, so my next thought turns to perhaps sensor(s) problems.
Could this be the cause? And if so, where is it (or are they if more than one) situated and is it just a case of disconnecting electrical connections, removing the old one and fitting the new one?
Thanks.

 

I had exactly the same problem but a new thermostat fixed it.
The temperature sender itself could be at fault or anything in the path from it up to and including the gauge itself. From memory there are two sensors located close together near the thermostat housing. The multi-pin plug is the ECU engine temperature sensor while the other with a single connection is the temperature gauge sensor. It's a relatively low cost item and is easily replaced. Disconnect the connector, unscrew the sensor, replace and reconnect.

 

What temp thermostat did you use? I believe 86*C is correct. I doubt the new thermostat is defective, but it does happen.

 

Thanks very much for the responses. I've no idea what type of thermostat I fitted. I just ordered it for my model and that was it!
As I'm getting the same results on the temperature gauge as I did previously, I'm a bit more reassured that at least the thermostat is probably opening & closing correctly and it may be the sensors. I'll replace them and see what happens then. If that doesn't work and the gauge is at fault, then I'll live with it I think!
Thanks again.

 

I f the thermostat isn't opening, then the radiator top hose stays cool, as opposed to getting almost too hot to touch. This is because the water in the engine/block heats up, but isn't allowed past the thermostat and into the radiator to be cooled down again, before recirculating. After the engine has been running a few minutes, feel the top hose, which should be just as warm as the hose running into the thermostat housing.

You can also do a physical check on the thermostat by immersing it in boiling water and you will see it "opening".

 

You only need to replace the gauge sensor - the one with the single connection. The multi-pin sensor is the engine temperature sensor for engine management.

 

Much obliged for that, saved me a job!
Thanks.

 

I'd recommend looking again into which thermostat you ordered up, before spending more money. Some shops have multiple temp stats (mainly "cooler" stats) that fit the same model. You want 86-88C which is about 185-190F. Thats the temp range the engine was designed to operate at for best efficiency and emissions.

The best way to check the actual coolant temp is with a scan tool, from the ECU, but this assumes the sensor is ok (which it very well could be)!

 

Thanks for that. I'm ignorant of such things so excuse the question if it sounds too simplistic, but surely whatever type of thermostat is fitted, they'll all designed to take the temperature to 'normal' or near to it when warmed up? Why would thermostats be designed which may give readings on the temperature gauge of near cool or near hot? Surely everyone want to see 'normal?
When my old thermostat was in place, the gauge went to near normal and stayed there. Only in the last few weeks did it start to move towards normal, then back towards cool, then back up and so on. Now I've fitted the new one it still does exactly the same.
Again, apologies if I seem to be question your knowledge, which is not intended, I've never before had to delve into the world of thermostats!
Thanks.

 

There's a lot more to thermostats than meets the eye! There is also a lot of misunderstanding out there, about the consequences of using different temp stats (or having a stat malfunction).

If you have an 88C thermostat that is operating normally, your coolant temp guage will read near Normal in the X300 XJ6. Any lower temperature rated thermostat (or a bad one) would keep the coolant cooler, and could cause the gauge to read less, not near the Normal mark. Another factor is that the temp gauge in our cars is not really an accurate measure of the coolant temperature, only an indication that it is more or less ok.

In the big picture, the thermostat is very important because the engine is designed to work best at a certain coolant temperature, namely 88C in our cars. That is when emissions are minimized and fueling is trimmed up properly.

If the computer thinks the engine is not warmed up yet because the coolant is too cool, it will add a little more fuel to get it warmed up. The proper fueling won't happen until the computer sees the engine is properly warmed up, which is highly dependent upon the thermostat.

Crystal clear now, eh? I think this was the short version too!!

 

Thanks for that and I thought a thermostat was just..........well a thermostat!
I'm still not pretending to understand the logic. To me a thermostat for a particular make/model of car should be such that when fitted and operating correctly, all other things being okay, the gauge should indicate 'normal'
If an 88 degree thermostat is the correct one, why would anyone fit another type which would make the car run cooler?
As I ordered the one for my model then surely it's the correct one and when fitted should read 'normal' at the gauge? Or are you saying that perhaps I may have been supplied with an incorrect one which was for another model?
Surely there can't be different ones for the X300 4.0 litre (my engine size)?
Thanks again by the way.

 

In theory, you are exactly right. But for some unexplainable reason, some folks think they should run their engines cooler than they are designed to run. That screws up the ecu ability to control the engine and ruins the engine's efficiency.

 

As I said in your other thread, you really want to read the numbers out of the ECU. It's usually best to perform diagnosis first and throw parts at it second!

If you already have a Android or iOS device for the user interface, good OBD-II readers are available from $50, plus a few bucks for the App to control it. If it saves you buying just one part in the future, it will probably pay for itself.

 

I am here because I am having this same issue. I live in East Texas, and it is never lower than 40 degrees on any given winter day that I would be getting out and about, it is normally around 50 most winter days. So my 1995 XJ6 when left to warm up a few minutes before driving, will reach normal, and the heater will blow hot. Driving just around town it will stay at Normal or just under, but if I get on a highway and go above 50mph the temp gauge will start to sink down to about 1/2 to 1/3 the way to Normal and the heater becomes ineffective. This is obviously due to the increased air flow through the engine bay/radiator. Surely this is not how the car was designed? To basically not have heat when driving faster than 50mph?? So my first thoughts on this were that the coolant concentration is off, in favor of coolant and not enough water? I was wondering if draining some of the 50/50 mix and replacing it with with more water to about a 40/60 ratio would solve this issue?
Any thoughts?

 

It's not how it should be but changing the coolant strength isn't appropriate.

See what people say could be the real fault.

 

Based on the symptoms and after talking to Paul Kobres, I'm almost certain it is just the thermostat not closing when the temperature drops too much during highway speeds with the increased airflow that occurs.

 

i had this exact problem and it was the thermostat. Bough a new one and problem fixed, it now sits just a bit to the left of the middle.

Hard was to take off the old thermostat as it seemed to be the original (it only said made in england) and was very stuck after 190.000km and 23 years.