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                                                                         model of the Jaguar engine.  


The webpage you have landed upon, deals with overheating of the Jaguar V12 of the most critical problems with the Jaguar V12 engine.  This page will attempt to explain, why your V12 engine seems to have the propensity to overheat, and what you can personally do, to avoid overheating,  a dropped valve seat, or other heat related damage to your V12 Jaguar engine.  Note...this information is primarily aimed at the '80s V12 engines.  Therefore, please refer to your owner's manuals for info specific to your vehicle.


Being a Jaguar owner (1984 XJS V12 HE),  I can tell you from first hand experience, that the potential for overheating of your engine, is not a fallacy.  It happens, and it has happened to me, on more than one occasion.  Back when I was a "newbie owner", and trusted a mechanic to work on my engine.  After two separate occasions of a dropped valve seat, I ditched the mechanic and learned about the cooling system.   So, the failures are due to ... LACK OF PROPER MAINTENANCE of the COOLING SYSTEM , and LACK OF UNDERSTANDING of the COOLING SYSTEM OPERATION.


The Jaguar V12 engine weighs in at approximately 700 pounds.  By modern day standards, it is a BIG engine.  It requires a large cooling system capacity to control running engine temperatures within specs.  The overall coolant capacity is approximately 5.55 gallons.  Compared to today's smaller engines, that's about 2 more gallons of coolant.  And again due the bulk of the engine, it is prone to after shut-down heat sink.  That's when the after shutdown engine temp's creep up from normal running temps, because coolant is no longer being circulated thru the engine, there is no longer a moving air flow thru the engine bay from vehicle movement, and the mechanical radiator fan blade is no longer spinning.  That's why the electric auxillary fan was incorporated into the original run after engine shut down, and dissipate the heat sink that occurs after shutdown.  

There has been much written on blaming the radiator design.  That is ... the OEM two bypass radiator flow design, and replacing/modifying the radiator, and replumbing of the hosing to accept a single bypass radiator flow.  I have read many, many postings on this issue.  I personally believe, and I believe the majority of the information I have read (from well experienced owners and automotive engine designers), would support a conclusion, that the original bypass radiator design is not flawed.  There is sufficient capacity within the cooling system, to keep engine temps within limits.  Therefore, if engine temps are above normal, the fault lies in a lack of maintenance of the cooling system. Certainly, this would not preclude an individual from making a system modification, if you have some advanced knowledge of  coolant system design.  However, this page does not attempt to explore the alternatives.  It attempts to give you assistance on maintaining your OEM system.


Therefore, given the above, lets see what you need to do to keep your engine cool.


Radiator or cooling system problems...


1)  Your OEM radiator fins have become externally clogged with foreign matter.  The XJS V12 body sits very low to the ground.   Maybe 6" of clearance.  As such, the radiator is prone to pick up more "road debris" than vehicles that sit higher off "road level".  Road debris can be anything that clogs the radiator fins...dirt, bugs, paper, leaves, plastic.  The result...the lower radiator fins and cooling tubes, get externally clogged with this "debris".... the ability of the radiator to dissipate heat diminishes, and running engine temps begin to creep up.


2)  The external fins on the transmission oil cooler gets clogged with the same "road debris".   Because the tranny oil cooler covers (approximately the lower 3 inches) of the engine coolant radiator,  (and sits in front of the engine coolant radiator),  the tranny oil cooler external fins clog with debris.  The tranny temps may increase,  and because the tranny oil cooler sits in front of the engine radiator, the engine radiator's effective coolant ability also becomes diminished...(ie.  the debris in front of the tranny oil cooler reduces air flow thru the tranny oil cooler)... which may increase tranny oil temps, BUT ALSO... since the tranny oil cooler sits in front of the engine radiator, it therefore decreases air flow thru the engine radiator, and engine temps go up.         

3)  The air conditioning external condensor fins get clogged with this same "road debris".  If  you think placing the tranny oil cooler in front of the engine radiator is bad, then consider this.  The a/c condensor mounts above the tranny oil cooler, and covers THE REST of the engine radiator cooling fins !!  The effect...the tranny oil cooler and the a/c condensor effectively cover 100% of the engine radiator fins. Therefore, it is vital that both the tranny oil cooler, and the a/c condensor, are COMPLETELY FREE of any "road debris".  Any amount of this external debris, within the tranny oil cooler fins, or the external a/c condensor fins, will reduce air flow thru their fins, with the resultant effect of reducing air flow, and cooling capacity of the engine radiator.  Also important, is that the soft aluminum fins are not bent.  If you consider, there is nothing to prevent rocks, blown out tire pieces, bits of metal, or whatever "road debris", from bouncing off the road surface, striking, and bending the tranny oil cooler  fins, or the a/c condensor fins, then this is also a possibility.  In most cases, they can be returned to proper shape carefully, with a small bladed knife, etc.

4)  The "foam" insulation surrounding the engine coolant radiator  is degraded, or missing.  The presence of this insulation, "forces" air flow from all four sides of the engine coolant radiator (top, bottom, left, right), to be directed thru the radiator, (in addition to the tranny oil cooler, and the A/C condensor) enhancing air flow thru the radiator and increasing cooling effectiveness.  Without this insulation material, there is "some" loss of air flow thru the radiator, that escapes around the radiator, thus reducing the air flow "thru" the radiator.      

5)  "Road debris" gets trapped between the tranny oil cooler, the a/c condensor, and the engine coolant radiator.  You most likely won't be able to see the debris.  The debris will be more predominant on the bottom end of the radiator, and in front of the radiator fins and tubes.  The limited space between the tranny oil cooler, the a/c condensor,  and the engine rad, and the presence of the radiator top cover plate prevents visual observation.  Removal of this "debris", requires removal of the radiator cover plate, several electrical connections, the air purge tubing, and blasting with pressurized water or air, from every achievable angle and direction.  This is likewise, VERY IMPORTANT to remove, and well worth the time.  Remember, the engine needs every bit of cooling capacity available.  The debris needs to be eliminated from between all three sets, as well as between the fins of each unit.

6)  The engine coolant radiator core is internally clogged with "deposits".  Burned on calcium deposits within the rad tubes that diminish coolant flow.  This item, in and of itself, is a major problem in engine overheating.   You can't see it.  Your first indication may be a temp gauge that reads above normal...if you know where the temp gauge is suppose to be.  The pointer on the temperature gauge should never go above midline of the "N" on the gauge.  If it does, you need to shutdown, and start curing the problem.  Best to remove the radiator, and have it inspected, recored, rerodded, or replaced.  Remember, your radiator (which may be the original, and if so), may now be more than 20 years old.  Don't think that a DYI pull of the radiator, sticking a water hose in the inlet, and watching the water flow out, will tell you anything.  It won't.


7)  The radiator shroud has several rectangular cut-outs located below the electric fan.  The cutouts have rectangular rubber "flaps" that cover the cut-outs.  These flaps open when the vehicle is moving, allowing additional air to flow to the engine.  When the vehicle is stopped, they close and prevent air from the mechanical and electric fan from flowing out thru the cutouts.  They need to be in place.  

8)  The front air spoiler needs to be in place !   It's effective area of additional air flow thru the front of the vehicle is approximately 90 square inches.  That's  3 inches of vertical height, and 30 inches of horizontal width,  below the front bumper, and into the tranny oil cooler, the a/c condensor, and the engine radiator.  Obviously, it is effective only while the vehicle is in forward motion.  The spoiler "scoops" an additional air into the radiator.   If you think that may not be important...consider a possible reverse scenario.  What would happen if you cut out a piece of window glass in your home, in a 10" x 9" section, and think about the loss of air conditioning effectiveness, due to "escaped air".

9)  The auxillary electric fan must operate properly.  It should come on and shut off at the proper temperatures, and in accordance with whether the engine is fitted with (or without) air conditioning.  Again, check your manual for proper operation.  OEM fans are in the 630 cfm rate, so an upgrade of the electric fan (to 1200 cfm or so), is an effective upgrade, for controlling  after shutdown heatsink engine temps.  This electric fan, will usually operate after the engine is turned off, and may go thru several on/off cycles, until it's sensor determines it no longer needs to come on...that is, when the engine temperature is below the sensor cut-on temperature.

10)  The mechanical (belt driven) fan must work properly.  The viscous fan clutch is the heart of the mechanical fan.  The viscous fan clutch is internally filled with an special viscous oil, within the fan clutch assembly.  The oil expands at temperature and "fluidly" connects the clutch to the blade fan, and at idle (warmed engine), provides added air flow thru the radiator.  Testing operation of the clutch is subjective.  I have yet to see any relevant objective testing info.  The following may be useful in a subjective test of the fan clutch.

A)  From a cold start, crank and run the engine (at idle).  Listen for the wind noise (air turbulence) produced by the mechanical fan. As the engine warms to operating temp, listen for an incease in wind noise, as the fan clutch engages the fan.  No difference indicated a non-operating (or weak) clutch.
B)  After performing step A, (and on the now warm engine), turn the engine off and immediately try to spin the fan blade. It should only spin about 1/4 to 1/2 revolution, indicating the viscous coupling has engaged the fan blade.  If the blade spins more, the clutch is suspect.
C)  Check the spin of the fan blade at cold engine and warm idle engine. (Spin the fan with a hard finger push on a fan blade).  On a cold engine, the fan may spin one turn. On a warm engine, maybe 1/4 to 1/2 turn. The important point is to measure any difference in the amount of rotation of the fan blade.  No difference, the clutch is suspect.  If the blade won't spin at all, the clutch is seized, and needs replacement.
D)  Check the fins on the clutch for oil, and or dust/dirt.  If the clutch is weak/bad, the oil within the clutch will have thinned, and weaped out, causing oil/ and or dirt on the fins.
E)  Any one or more of the above, would cause the clutch to be suspect.  


11)  The "air purge" valve is internally clogged with calcium deposits., or rust tubers ( bubbling of the internal wall due to tube corrosion).  The air purge valving is the tubing that lays on the top radiator cover plate, on the B bank side of the radiator cover plate.  This tubing (OEM steel) will most likely be partially or completely obstruced,  if there has been no maintenance of the tubing, or the engine coolant system.  It will reduce or prevent purging of air from the cooling system, to the atmospheric catchment tank.

12)  There are two thermostats.  One on the A bank. and one on the B bank.  Either one or both are non-operative, or the thermostats are not correct.  There is some discussion on what temperature rating these thermostats should be...  180 degrees or 190 degrees.  I don't think it really matters, as long as they work!  The thermostats must have the little air hole within the thermostat, and it must be oriented (installed) at the 12 o'clock position. The holes allow trapped air to flow  out and be expelled from the cooling system.  The Jag V12 thermostats also have an extra disk at the operational end of the thermostat.  The extra disk (when the thermostat is open) block coolant flow thru the radiator bypass piping, such that all coolant flows thru the radiator. Most likely, you can find an aftermarket replacement...but if there is any doubt, spend the extra few dollars, and get the right ones.

13) There are two radiator caps.  One on the engine coolant filler pipe, (at the front of the V), and one on the expansion tank, in the engine bay, and mounted on the front of the B bank fender well.  Both should be 15 lbs, (16 lbs caps are also acceptable).   Forget what you read in in the Jag manuals, about a 20 lb cap on the coolant filler pipe cap located above the V.  That cap acts only to seal the V (the valley) filler pipe.   It does not act as a "true" radiator cap, allowing a two way coolant flow, as engine coolant expands, and contracts. The upper sealing surface on the engine filler pipe, and the expansion filler pipe, must be honed smooth for a proper seal of the cap.

14)  The atmospheric catchment tank must be fully operational.  It is located on the B bank side, behind the B bank wheel well, and within the car wheel well body.  It's rear of the B bank front tire.  Covered with a plate.  Another prime problem area, because the tank is completely invisible, and you won't know it's there, unless you read about it, or see coolant leaking from that location.  The tubing to this tank may be cracked (and sucking air, or blowing coolant), the plastic atmospheric catchment tank may be cracked, the tank lid seal may be sucking air, the metal tank overflow drain may be clogged, the metal drain pipe seal between the tube and the plastic tank may be shot.  Remove the cover plate (which may be sealed with some black sealant), inspect all components, and repair as necessary.  Repair any rust while you are there.  When reinstalling, fill the atmo tank 1/2 full....(on a cold engine).

15)  Generally, the proper mix of water/anti-freeze is a 50/50 mix.  However, for your particular location, you should mix according to instructions on the container, based upon high and low ambient air temps in your location.  Because anti-freeze degrades over time, the cooling system should be drained, flushed, and refilled with a new mixture annually.  Very important.

14)  If you suspect engine heating problems, buy an infrared heat gun, and check engine temps.  Locations to check are the top of the thermostat housings, the water rails, radiator inlet and outlet hosing, front and rear of the heads (on both banks).  180 to 190 degrees is good.  If you get reading above 190-195 degrees, start looking for problems in any of the above.

15)  Check the radiator banjo bolt....located on the A bank side of the radiator, on top of the radiator.  The holes, in the banjo bolt, must be unobstructed, the spacer washers must be installed, and not deformed.  The banjo bolt allows air within the coolant system to bleed out to the air purge valve, and ultimately... to atmosphere.  Air pockets in the engine, cause hot spots, and diminished cooling.  It may be possible to remove "crud" from the holes, with appropriately sized  drill bits, run at a low rpm.  

16)  The coolant system must be filled properly.  There is an "air venting port" on the "B" bank side of the radiator.  At the top.  Remove the port.  Remove both radiator caps.  Fill the expansion tank to the bottom of the neck.  Slowly fill the V filler coolant neck.  Coolant (and air) will purge from the air vent.  Continue to fill until no more air bubbles escape from the venting port.  Replace the port bolt.  Start the engine and warm up.  Shut down and reconduct this procedure until no more air bubbles escpe from the air vent port.  With a cold (and properly filled) engine, the coolant level should be about 3 inches below the top of the filler neck.  Be sure to follow your manual, on setting heater controls.

17)  Radiator leak (coolant system) sealers.  I won't mention brand names.  DO NOT use these products.  They are a temporary "band-aid" repair.  They do not fix the problem.  They (may or may not) temporarily resolve a leak.  More than likely, these products will clog the radiator,  the heater control valve, or other heater components.

18)  Other engine mechanical devices which may cause excess engine temperatures are...incorrect timing, non or weak operation of the distributor centrifical advance mechanism, weak or non operation of the distributor vacuum advance mechanism.

19)  DROPPED VALVE SEAT....So...after all that...why does a valve seat drop?  It's really very simple.  The V12 heads are aluminum.  The valve seats are steel.  Aluminum and steel have different expansion/contraction temperature ranges.  At higher than normal engine operating temperatures, this variation in expansion/contraction may cause the steel valve seat to separate from the aluminum head.  With the valve seat separated from the head, the opening and closing valve, cannot seat properly, it beats on the valve seat, and you now heard....the dreaded "ticking" noise.  It sounds very much like taking a screwdriver, and tapping on the aluminum head. The ticking noise, increasing and decreasing, as engine rpms go up and down.  And if you continue to run the engine, the seat may even begin to break apart into pieces, and possibly damage the head, the valve, and the piston.  I can assure you, it will cause your Jag lovin' euphoria, to take a roller coaster dive.  

18)  WHEN WILL THE VALVE SEAT DROP?...If your engine overheats, and you continue to drive, it could happen at any second.  Another 30 seconds of driving, could cost you several thousands of dollars.  Even if you stop and shut down an overheated engine, the aforementioned "after shutdown heat sink", may cause the valve seat to drop. And only become noticeable, the next time you start your engine.    

19)  Buy a repair/operation manual for your specific vehicle !!  Read it, and understand how the coolant system on your vehicle operates.  Then, overhaul your cooling system before you go showing off your ride, and before you get burned !!

19)  Become a member of one of the many Jaguar web sites.  I like  Lots of friendly, and knowledgeable members.

20) NOTE...There are other conditions which may cause overheating....lean fueling, engine misfires, clogged cat converters, etc.  So, learn about your vehicle, and be sure to maintain your own servicing records.

21)  Here's a handy conversion factor for C to F, and F to C.


       F =    9/5 C + 32.                         example.....212F =  9/5 x 100C + 32

       C =    212F - 32 X 5/9                 example.....212F - 32  x  5/9


Don't forget to view my information on the importance of balanced fuel delivery flow of your vehicle's fuel injectors.  Get your cooling system "up to snuff", and then get your injectors cleaned and serviced, for maximum engine HP, better fuel economy, and for passing emission tests.


To learn about, and to prevent V12 engine fires (the other cause of death to the V12 engine) need to   CLICK HERE


THIS PAGE WAS LAST UPDATED ON Thursday, February 2, 2017.





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