V12 cooling and overheating

The system as built

The Series 3 uses a downflow single pass radiator, with coolant from both cylinder banks entering opposite ends of the top tank. Cooling airflow at rest depends entirely on twin electric fans, because there is no engine driven fan. There are two thermostats, one per bank, and the parts catalogue confirms the quantity as two for both the 82 degrees C and the 88 degrees C units. The 82 degree thermostat was the original specification.

It is worth stating plainly what a thermostat does, because owners routinely expect too much of it. A thermostat sets the lower limit of the normal operating temperature range. It has no control over the maximum temperature at all. Fitting a cooler thermostat does not cure overheating.

The thermostats themselves have two design features that matter. They carry a small wiggle pin to bleed air, which should be fitted uppermost, and a spring loaded disc on the bottom post that closes the bypass when the thermostat opens. Running the engine with the thermostats removed, which people do when chasing an overheating problem, makes it worse, because coolant then bypasses the radiator.

Why the V12 runs hotter than the six

Four mechanisms are documented, and none of them is a mystery.

• The radiator has little spare capacity. Roger Bywater of AJ6 Engineering, who worked in Jaguar engine development, records that the Series 3 downflow single pass radiator had significant pressure drop and did not really have adequate spare capacity for high demand conditions.
• There is more heat to reject. Five point three litres in the same nose that held four point two.
• Flow becomes unbalanced as the system silts up. A bank can increase its cooling at the expense of B bank, and because the gauge sender sits on A bank the gauge will show nothing wrong while B bank overheats.
• Heat soak after shut down. Damage occurs most often in the period after the engine is switched off, when circulation stops and heat rises out of the block with no airflow through the radiator.

The archive has not found any published measurement comparing heat rejection of the 5.3 V12 against the 4.2 six. The proposition that it runs hotter is well supported by mechanism and by specialist testimony, but no measured comparison has been located.

The gauge is not telling you the whole story

This is the single most useful thing on this page. The temperature sender is fitted to one bank only. If the other bank is running hot, the gauge does not know. Bywater recommends either moving the sender to the B bank thermostat housing or fitting a means of monitoring both sides. The needle should never go beyond the centre mark, and if it does, the correct response is to stop and deal with the cooling system before running the engine again, not to press on.

What actually goes wrong

• Fan circuit. The classic presentation is fans that run when fed directly from the battery but not through the car's own circuit. The fault is usually the fan relay unless the wiring itself has failed. This was a problem when the cars were new: the January 1972 road test recorded defective thermostatic fan action on a press car.
• Radiator condition. Burned on calcium deposits inside the tubes restrict flow. External blockage matters just as much, because the oil cooler and, on air conditioned cars, the condenser sit ahead of the radiator and pack with debris, so airflow is reduced before it reaches the core.
• Leak sealer. Both jag-lovers and American specialists are emphatic that radiator sealer plugs the lower half of the radiator and causes far more work than it saves. A radiator that has ever had sealer through it should be boiled out, rodded or replaced.
• Corrosion. The engine is aluminium and the system contains iron and steel parts. Documented failures include steel air purge tubing obstructed with calcium and rust, header tanks perforating, corrosion pitting in the water pump cavity behind the impeller, and erosion of the thermostat bypass seat.
• Water pump. Failure is characteristically at the seal, and often shows as coolant ejected past the seal after shut down, when circulation has stopped. Running plain water rather than inhibited coolant brings seal failure on early.

Coolant chemistry, which is not optional

Ethylene glycol is itself corrosive and is made tolerable only by the inhibitors added to it, and those inhibitors deplete over the life of the coolant. Water contributes free oxygen, raises conductivity and encourages galvanic action. Glycol oxidises in stages towards oxalic acid by way of corrosive intermediates including formic acid.

The practical consequences are three. Change the antifreeze annually, because the inhibitors wear out even if the coolant still looks fine. Never run plain water, which brings on water pump seal failure. And if a glycol coolant has boiled, replace it immediately, because boiling accelerates the oxidation.

Classic and Sports Car names the consequence of getting this wrong in its buying guide: a silted up cooling system, from inadequate corrosion inhibitor, leading to overheating and warped or cracked heads.

The failure that ends an engine

Overheating on a V12 leads to a dropped valve seat. The seats are pressed steel in aluminium heads, and thermal stress works them loose, most often in the period after shut down. One seat coming out destroys the piston, the head, both valves and the liner. The warning noise is a ticking. A Jaguar specialist put it to Hemmings in one sentence: you have got to keep the engine cool, or it is trash.

There is a nuance about which bank suffers, and the sources take different views. jag-lovers describes the XJ-S and XJ12 radiator arrangement as biasing the problem to the left bank, and says the Series 3 E-Type is less prone to that bias because of a better radiator design. Bywater, looking at the same two designs, judges the XJ crossflow the more efficient heat exchanger and the E-Type single pass unit short of spare capacity. Both can be true, because they are judging different things, balance between banks against total capacity. The archive records both rather than choosing.

What specialists agree on

• Use the correct inhibitor at the correct strength and change it on schedule. Every independent source consulted says the same, and jag-lovers and American specialists both say annually.
• Never use leak sealer.
• Deal with the radiator properly. Recoring, or replacing the original four row Marston core with a high efficiency unit, is standard practice.
• Consider an auxiliary electric water pump to improve flow at low engine speeds and to keep coolant moving for a period after shut down. Bywater's assessment is that an electric fan and an electric water pump can preserve an engine that would otherwise suffer badly.
• Monitor both banks, not just the gauge.
• Modernise the fan control. Replacing tired Lucas relays with modern equivalents, or rewiring the fans through a new relay, is common trade practice.

For reference, Bywater gives a normal temperature drop across the radiator of 5 to 10 degrees C, with a rise of the same order across the engine. An infra red thermometer is the easy way to check this, and 82 to 88 degrees C is quoted as the optimal range.

The reputation, fairly stated

The V12 does not overheat when it is right. The documented position is that the system was specified with little spare capacity and is intolerant of neglect, not that it cannot be made to work. The causes when it does overheat are a list, not a mystery: retarded ignition timing, a seized centrifugal advance, thermostats that only partly open, a blocked radiator, leak sealer residue, and missing shroud or airflow parts. A Series 3 with a properly rebuilt cooling system behaves quite differently from one without, and that difference is the single largest variable in Series 3 ownership.

Common questions