3 - Cooling System ( ,  )

Overview

The core system is conventional, though of high thermal capacity and any movement of the gauge above normal should be investigated as a matter of concern. Likewise, warm-up should be quick with the gauge moving from cold to normal over a short period of time. Note though that the gauge is controlled by the ECU such that the centre points covers temperatures between about 70 and 98 degrees.

The main engine water pump is driven by the auxiliary belt. A second electrical pump is used for the heater circuit, mounted towards the rear right of the engine bay. XJR models also have an additional electrical pump for the intercooler circuit, mounted at the front right of the engine bay, together with an additional radiator mounted in front of the standard air conditioning and main radiators.

Two electrical fans are fitted, operating in two stages. They are controlled by a logic module which switches them on in slow ( serially connected ) mode when coolant temperature reaches 88.5C or air conditioning refrigerant pressure reaches 12bar. They are then switched to fast ( parallel connected ) mode when either the coolant reaches 100C or refrigerant reaches 22bar.

The control module itself is located at the front left side of the car, forward of the wheel arch near the EVAP system charcoal canister.

A problem was found with this system, in that the low-pressure refrigerant switch was unreliable, sometimes causing the fans to switch from standstill straight into fast mode with the subsequent start-up current causing the fuse to blow.

A dealer fitted workaround ( technical service bulletin ) was introduced which replaced the low pressure refrigerant switch with a wire link in the controlling circuit. This causes the fans to operate in slow mode whenever the air-conditioning is switched on.

3.1 - Thermostat Replacement ( ,  )

X300 thermostats appear to fail in the open mode after 4 - 5 years. This shows itself by the temperature gauge being slow to reach, or in some cases not achieving, the Normal mark. It is not unduly damaging to drive the car like this for a few days, but fuel economy will suffer and the throttle body is likely to get gummed up after a while, potentially causing it to stick partially open.

Thermostats can also fail in closed mode, causing rapid overheating. The car should not be driven whilst overheated otherwise severe damage will result.

To replace the thermostat, undo the two small bolts attaching the thermostat housing to the thermostat block. This can be found on the right hand side of the engine, attached to the large pipe leading from the top left of the radiator.

It may be necessary to pull quite firmly on the thermostat housing to break the seal. Note that a small amount of coolant will be spilt, though this can be avoided if you prefer by first draining some liquid by disconnecting the lower radiator pipe for a short time.

Remove the thermostat, noting which way it is fitted. Some vehicles do not have a gasket fitted, but if one is used then remove it. Carefully clean the faces of the thermostat housing and block.

It appears optional whether to use a gasket on replacement. It is not strictly necessary, but many feel more comfortable using a gasket or gasket compound.

Fit the thermostat into the block following the orientation of the old one; the actuating spring is normally on the engine side. Refit the housing, and tighten the fixing bolts. Whilst these need to be tight, excessive force will damage the threads.

Refill the cooling system to the maximum mark and run the engine up to operating temperature whilst checking for any leaks.

3.2 - Coolant Replacement ( ,  )

It is recommended that coolant be replacement every two years, as the anti-oxidants lose their effectiveness over time and leave the cylinder head at risk of corrosion. This in turn reduces cooling effectiveness and promotes head gasket failure.

Early models were also susceptible to a build up of sludge in the heater matrix, reducing heater effectiveness. The problem appeared to stem from the type of long life antifreeze used, and the best avoidance method is to use regular anti-freeze and change at two yearly intervals. Systems with the problem can be diagnosed as having a low heat output, assuming the engine is hot and the system set to defrost mode, and can generally be cured by repeated flushing.

Unless it is desired to completely flush through the system, it is not necessary to remove all engine drain plugs and remove the total amount of coolant.

The majority of the system can be drained by removing the header tank cap and disconnecting the lower radiator hose. The rush of fluid out of the system will also tend to pull any suspended particles with it.

Replace the hose and refill the system with a 50% mixture of quality anti-freeze and preferably distilled water. The exact percentage can be varied dependent on temperatures in your area, but it is safest to assume the residue of coolant in the engine is ineffective ad also bear in mind that as anti-freeze is less effective at cooling the engine than water, the mix should not exceed 65%.

Run the engine up to temperature with the header cap removed and heater switched to full to ensure any air pockets are released.

3.3 - Hose Replacement ( ,  )

It is recommended that hoses be replaced after 5 years service, to avoid risk of failure on the road which could leave you stranded.

With typical bad fortune, the hose most likely to fail is the hardest to access. It runs backward from the water pump alongside the engine to the steel engine water rail pipe and is subject to the surges in pressure which accompany sudden increases in engine speed, for example on kickdown.

Replacement of this hose and the water pump inlet hose, which also links the thermostat block, inlet manifold heater, interior heater, header tank ( and intercooler circuit on XJR models ) requires removal of the inlet manifold, following the instructions above.

Hence it is recommended that both are replaced together, along with the inlet manifold heater hose which is the small hose running from the thermostat block to the throttle body.

When refitting, note the performed curve in the water rail hose and ensure it is correctly oriented such that the hose fits correctly onto the outlets.

3.4 - Water Pump ( ,  )

It is recommended that hoses be replaced after 5 years service, to avoid risk of failure on the road which could leave you stranded.

With typical bad fortune, the hose most likely to fail is the hardest to access. It runs backward from the water pump alongside the engine to the steel engine water rail pipe and is subject to the surges in pressure which accompany sudden increases in engine speed, for example on kickdown.

Replacement of this hose and the water pump inlet hose, which also links the thermostat block, inlet manifold heater, interior heater, header tank ( and intercooler circuit on XJR models ) requires removal of the inlet manifold, following the instructions above.

Hence it is recommended that both are replaced together, along with the inlet manifold heater hose which is the small hose running from the thermostat block to the throttle body.

When refitting, note the performed curve in the water rail hose and ensure it is correctly oriented such that the hose fits correctly onto the outlets.

3.5 - Air Conditioning ( ,  )

The system under the bonnet is conventional and appears reliable. The compressor is located on the lower front left side of the engine, viewed from the front, and is driven from the engine auxiliary belt via an electrically controlled magnetic clutch.

There is an inline fuse visible on the wiring to the compressor clutch, which is fed from a relay located behind the headlights.

The pressure switched for the system can be found on the pipes towards the rear of the engine bay on the left side. These control the operation of the electric cooling fans, as described under ‘Cooling System’, and also protect the compressor by preventing it’s operation if refrigerant pressure is too low, typically following a leak.

Diagnosis of this can be done from the climate control panel, as detailed in 11.3 below.

To keep the seals in good condition, and therefore keep the refrigerant inside the system, it is important to run the a/c frequently even though it may not be needed for cooling purposes. Five minutes use every couple of weeks is all it takes to keep them lubricated and healthy.

3.6 - Air Conditioning Condenser & Filter/Drier Repl. ( ,  )

Failure of the condenser is one of the most common reasons for loss of refrigerant, as it is quite exposed at the lower front of the car. The filter/drier is mounted alongside the condenser and needs the condenser to be removed for replacement.

To remove the condenser, first remove the radiator top panel which crosses the width of the car at the front of the bonnet area. It is secured by a number of large torx bolts which appear to frequently seize up. If necessary, they will need to be drilled out, or the heads ground off, but fortunately load on this panel is not high and loss of one or two bolts does not appear to be a problem.

Locate the pipes leading to the condenser as they pass to the left side of the main radiator. The union for one pipe is a few inches into the main engine bay whereas the second is on the filter/drier unit, mounted on top of the condenser and therefore harder to access.

Undo the unions slowly, listening for refrigerant pressure. If there is any pressure, then you don’t have a leak and the condenser doesn’t need replacement.

The top of the main engine radiator can be pushed gently backwards and the condenser & filter/drier then removed by pulling them upward. On XJR models, the upper hose for the intercooler radiator, mounted in front of the condenser, makes this more difficult but it is just possible without removing the hose.

The area around the mounting lugs for the condenser is a known rust spot and so you may consider treating it with a rust-proofing agent prior to refitting.

If you have to replace the condenser, replace the filter/drier at the same time as these do wear out and otherwise need replacement on their own.

When refitting the condenser, carefully lower it into the space and from underneath ensure it is settled correctly onto the locating posts. Reattach the pipework and refit the top panel.

The system works at high pressure so ensure O-rings are in good condition and ensure unions are tight during refitting. Always use two spanners on the union to avoid stresses other parts of the pipework when loosening or tightening the unions.

3.7 - Coolant Cap ( ,  )

The cap maintains pressure in the system, with venting controlled by a spring under the brass centre piece of the cap, which lets excess pressure escape via channels in the threads.

Failure of the cap is indicated by traces of coolant on the top of the header tank, accompanied by coolant loss from the system. Failure can occur if the spring becomes weak or if the rubber sealing washer becomes worn or deformed.