XJ XJ6 / XJR6 ( X300 ) 1995-1997

Thermostat Questions

Old 03-28-2019, 12:12 PM
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Default Thermostat Questions

I want to replace the thermostat in my '96 4.0 X300.
I ordered one and it did not come with a gasket or o-ring.
Shouldn't I use a new gasket for the housing?
Or, is there an O-ring.
I haven't swapped out a thermostat in years and that was a Subaru, not a Jaguar.
I've read, on the forum, that the o-ring should be either discarded and use Permatex instead , or "don't get the o-ring in the groove" , whatever that means.
I don't want a leak and I don't want to crack the housing in an attempt to not have a leak.
The temp gauge barely rises above C, but it does move , so I believe the gauge works.
I have heat when I need it and I don't usually worry about an engine that is far from over-heating but I'm concerned about not reaching optimum operating temp. and not running efficiently.
There may be plenty of reasons why I'm not getting much more than 15 mpg, but couldn't running at less than optimum operating temp. be one of them?
Old 03-28-2019, 06:58 PM
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Originally Posted by Jimi Shelter View Post
I want to replace the thermostat in my '96 4.0 X300.
I ordered one and it did not come with a gasket or o-ring.
Shouldn't I use a new gasket for the housing? [snip]
There may be plenty of reasons why I'm not getting much more than 15 mpg, but couldn't running at less than optimum operating temp. be one of them?
Hi Jimi,

It should have a gasket unless a previous owner used silicone sealant instead. If so, you can usually see the squeeze-out at the joint between the thermostat housing and cover . The gasket is part number NBB2217AC (Part 3 in the diagram below from jaguarclassicparts.com). Some thermostats come with an O-ring but don't use it on your car. The Workshop Manual torque spec for the thermostat cover screws is 21.5 - 28.5 Nm or 16 - 21 ft. lbs. Tighten each screw a little at a time, alternating sides to keep the pressure on the "ears" of the cover even, before bringing the screws to final torque.

A stuck-open thermostat can definitely cause poor fuel economy and is probably your problem since your temp gauge is reading Cold. Another common cause is the Engine Coolant Temperature Sensor. If the ECTS fails at a resistance the Engine Control Module interprets as "cold," the ECM will continue to apply cold-start fuel enrichment even after the engine has reached full operating temperature. If you have a scan tool that can read Live Data, check your ECTS signal for plausibility.



Last edited by Don B; 03-28-2019 at 10:44 PM.
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Old 03-28-2019, 07:43 PM
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You can verify the thermostat by reading the very accurate 2 wire coolent temp sensor that feeds the ECU

See page 78 and 86

you can double check your readings as a cold engine with the same sensor as the inlet air sensor , swapping option ?

The ECT code is P0140

If the ECU has not gone into closed mode the short term fuel trim will be rich at 8.0


More info on this thread including too large of O - ring cracking housing as you crank the bolts down


Last edited by Lady Penelope; 03-28-2019 at 08:08 PM.
Old 03-28-2019, 08:52 PM
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Exactly as Don said.


I apply a SMALL dob of RTV at 12 o'clock on the outer rim of the stat, sit it in its lip, and hold it there for a few minutes, usually 1 beer is enough, and that will hold it in place whilst you offer up the cover. If that stat slips out of of its mounting lip, the chances of you seeing it are zero, and when you tighten the cover, the ears will snap off, basically ruining your day.

Gasket only on AJ16, never seen an o/ring in that position.
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Don B (03-28-2019)
Old 03-28-2019, 08:59 PM
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AJ6 use 'O'ring!!!!!!!!!!!!!!!!!!!!!
AJ16 use Gasket!!!!!!!!!!!!!!!!

'O'ring on AJ16 will BREAK the ears on the thermostat cover!!!!!!!!!!!!!!

I keep the covers in stock for people that do not pay attention.

The ears can be welded back on if you break the rules.

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Old 03-28-2019, 10:05 PM
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If the new thermostat has a small hole near the flange then the thermostat should be installed with the hole at the 12 o'clock position. I understand the purpose of the hole is to bleed air in the system.
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