Question about 1995 Jaguar XJ12

1 Answer

1995 Jaguar XJ12

Car is overheating when going over 55 MPH with AC off.

Car is overheating with AC on.

Car is overheating with when stopping at red light, with AC off. Need to put car in Nuetral at light.

Seems like the thermastats are closed.

Waterpump is not leaking, nor the hoses, nor the radiator, etc. No leaks.

Hoses going into radiator are not extremely hard.

Car cools down in 30 min or so.

I then can drive car under 55 MPH and temp guage is at N or one notch to the left. If I go over 60 MPH, the temp gauge goes to H, red zone.

The aux fans seem to work. I turn off car and the aux fans keep running.

I turn AC off, put car in nuetral and reve the engine to 20 to 25 rpms, it seems to lower the temp guage back to 2/3 to right.

Any suggestions welcome.

I plan to change the thermastats and related relays and switchs.

Also, I will buy new relays and switches for related cooling system system components.

I will also take the car to get a computer diagnostics.

Just purchased the Jag. The prev owner had the XJ12 sitting in the garage for 3 years. He changed the antifreeze before I purchased, one week ago.

Any suggestions welcome. Thank you.


Posted by on

  • Victor Hernandez Apr 17, 2009

    I will check the things you state, next couple of days.

    Perform a cylinder leak-down test on the engine which will show if you have a blown head-gasket and/or cracked cylinder head.

    1. How does one do a cylinder leak-down test?

    2. Would the above problems, cause the cooling antifreeze not to circulate? blown head-gasket and/or cracked cylinder head.

    3. Would exess air or air pockets in the cooling system cause this problem?

    The previous owner flushed the raditor and refilled it with antifreeze to specs.

    4. The engine is super clean, it has been well maintained, it seems.

    See pics at

    5. Do you have a resume of your experience, etc.?

    THANK YOU for the advice. I will do as you say.




1 Answer

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  • Master
  • 3,614 Answers

Try to to DO the ff

  • When the engine is running hot and the radiator fan should be on, check for 12 volts to the fan. If there is no voltage, there is a relay or sensor that has failed. If there is voltage, then the fan motor needs replacing.
  • Perform a cylinder leak-down test on the engine which will show if you have a blown head-gasket and/or cracked cylinder head.
  • Check for blockage in the radiator. Flushing the cooling system will not fix a partially plugged radiator. Replace radiator with new to repair problem.
  • Check for a partially plugged radiator. When the engine overheated sludge or debris may have been dislodged and plugged the radiator. Replace the radiator with new to repair problem.
  • there is a fuse that controls the fans. There is also a power relay in the circuit. Check the cooling fans themselves by jumping the circuit with 12-volts to see if they operate. If not replace with new to repair problem.
  • You may have a faulty thermo-switch. In addition, if you have a blown head-gasket it would allow exhaust to get into the cooling system, which in turn would fool the thermo-switch.

Posted on Apr 17, 2009

  • micromaster
    micromaster Apr 17, 2009

    good to hear that let me know again soon.

    kindly acccept my solution here.



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