I think I need a new thermostat because the engine isn't getting to operating temperature according to the dashboard gauge. I found some thermostats locally but they have slightly different dimensions. Which one should I get?

I have a 1995 3.2 sport x300 EU spec.

GATES TH14387G1
OR
BEHR/MAHLE - TX 18 87D

 

The X300 does not take a O - ring like the earlier X40

This can crack the thermostat housing on assembly

The gauge temperature reading is from the single wire sensor mounted on the thermostat housing

The ECU uses the 2 wire sensor next to it

The single wire sensor is dependent on the grounding of the mount hole and can be cleaned up

The gauge temperature reading can be wonky and is not truly accurate

Genuine Thermostat And Housing-3.2/4.0 Litre For Jaguar Xj 1995 - 1997 (from 720125 To 812255) Classic | Jaguar Land Rover Classic Parts

I think a problem accurse when a vendor gives you a X40 thermostat assuming the X300 is the same or so other stocking error

 

The AJ6/AJ16 thermostats most always fail OPEN.

I used to have to use coolant hose 'pinch-pliers' to stop coolant flow on the AJ16 engines when performing the RECALL T493 to calibrate TPS and perform O2 sensor orientation if the T-Mostat had failed and the customer declined repair. (coolant STAYED at 70 C)

The procedure would NOT begin until engine temp was (I think 88 C) so I deliberately stopped coolant flow until the engine reached operating temp with the 'hose pinch pliers'.

 

You can place the current thermostat in a pot of water and watch the gap open as you bring the pot of water up to boiling

Removed and in your hands you do not want to see it already opening

You can calibrate this against a meat thermometer however accurate that may be but it would give you a general picture

As Bob mentioned most thermostats fail in a fail safe mode protecting the engine with cooler fluid but your engine regulation will be off in the ECU open loop mode

Bob , 88 C was the number I was always looking for ( open / closed loop mode ) without the hands on experience you have

 

Well the gauge is showing normal operating temperature when pushing the engine uphill or driving spiritedly, but drops when driving normally. So I'm pretty sure it's stuck open. My question was which is the correct part number for my engine?
TH14387G1


OR
TX 18 87D



Both are listed for a 3.2 x300 but have slightly different dimensions.

 

EBC3621 for both the 3.2 and 4.0 but the price is steep

I see the bottom poppet dimension difference

The aftermarket manufactures seem to have a issue of accuracy

 

From what you are observing on your gauge seems to be normal on the X300 vs. the X40 from reading

There have been some to modify the instrument cluster to get it to respond correctly to what you would expect ( dummy gauge )

You can be observing this along with a thermostat not behaving in specification

The thermostat is not full open or full closed but modulates a fraction this way or that to keep it slightly above the ECU open / closed loop mode operating temp trip point

 

If you have an obd2 reader you should be able to confirm the coolant temperature at the temp sensor,
usually between 175f to 195f.
It’s possible the gauge could be having issues or there’s a problem with the connections.

 

Both cooling fans operation test ?

The 3 wire 2 temp switch is located aft left low on the radiator and you can place a paper clip in the connector

one of the two white wires to the black ground will give you one fan

remove the clip and place in the other white wire to the black ground will give you both fans

Unless the A / C lower pressure switch has ben modified with a jumper wire per Jaguar TSB in which case the single fan mode operates at all times

This is a low current section of the fans circuit so no arcing hazard'

the switches tripping points are 86 C and 100 C per wiring guide

Again you can place the switch in hot water if you remove it to watch it respond

 

I have had a Mahle TX18 87D fitted to my XJR for many years, no problems. It is the recommended fitment for Jaguar EBC3621 by Mahle and is of near identical dimensions. It is also the recommended fitment for many Mercedes models. Here in Australia it was sourced at a very reasonable price, despite being recommended for Jaguar & Mercedes.
Details on fitment attached.

John Herbert
(1996 XJR)

 

Engine temp is around 150F. Thank you everyone for your answers.

 

That being 150 F = 65 degree C which is low from my understanding and what Bob had to add about 88 degree C off the top of his head

Where you able to see if the ECU is in open or closed mode at this temperature point ?

This ECU tripping point has nothing to do with coolant temperature regulation

 

My opinion is a thermostat change

Remember no O - ring or you can crack the thermostat housing casting ears

With a second fan test to explain your higher temperature swing under heavy engine load

 

I'm fairly certain that the thermostat is stuck open. Open thermostat+lower ambient temperature+constantly running fans=low engine temp.

 

Yes 150f is low and that is confirmed by the dash gauge.

 

With the Jaguar TSB done you should only have one fan at all times until you reach a higher engine coolant demand or the A/ C high pressure switch ( higher A/ C freon 20 Bar pressure ) demand commands both fans

If the Fan control module is mounted upside down water can get inside shorting or making contacts to give more fans then needed

This module must be mounted wires pointed down and is located under the car fwd of the front left wheel

This white colored module is not interchangeable with the similar black one on the later model V8 X308

 

Coolant system TSB below

The modification is simply a jumper wire connector plug on the lower or single 2 wire A / C freon pressure 13 Bar switch wire connector

This lower pressure freon switch is not a too low freon switch which the X300 does not have

You can just splice the wires together and does the same thing

14-04 (jagrepair.com)

I'm going back to bed

 

Are you sure that only one fan is supposed to be spinning? I don't think it's even possible to have only one fan running when they are connected in series/parallel.

 

The wire routing inside the Fan Relay Control Module can be a headache to follow

but yes one fan only with a cold engine with A/ C off

you can remove the 4 wire higher freon pressure 20 Bar switch in case it has failed in the contact closed position , this would cause both fans to run if the higher pressure switch failed closed contacts . This failure mode can happen being observed in both fans running with the A / C off or on

The other halve of the 4 wire switch is a overpressure 27 Bar cut off the A / C compressor clutch

The V8 powered X308 has both fans running at a lower speed and then both fans running at a higher speed

Is your Fan Relay Control Module white or black ? they are not the same inside