BMW Z3 M52TU Thermostat Replacement
Article reproduced with kind permission of Mike Fishwick

When the M52TU double-VANOS versions of the established M52 engine were introduced in September 1998, it featured a cooling system thermostat which was electrically controlled. These thermostats are now widely used, as they enable the engine management system to maintain coolant temperature within very close limits, thereby optimising operating conditions and so reducing exhaust emissions.

The thermostat operates against a return spring, compressed between the thermostat valve and its plastic housing, which also incorporates the cooling system inlet and outlet passages. The lugs retaining the thermostat to the housing are therefore under considerable pressure from the return spring.

Even though the prototype thermostats must have been subjected to a considerable number of test cycles, within a year owners began to experience failures on such a scale that BMW finally realised a widespread problem existed. Although – surprisingly – no general recall was issued, a Service Instruction dated May 2000 formalised the problem, which could manifest itself by the engine under or overheating. Overheating sometimes illuminated the ‘Check Engine’ warning lamp, with appropriate fault codes (168 or 222) being logged in the management system’s memory.

My Z3 has one of the early M52TU engines, having been built in October 1998, and after 63,000 miles it suddenly began to exhibit a suspiciously long warm-up period. I had already decided to replace the thermostat on a preventative basis before the summer holidays, and had therefore made the necessary preparations.

Although many owners may feel daunted by the prospect of tearing their BMW to pieces, it is not rocket science, and most BMWs are in fact easy to work on, coming apart easily and in large pieces.

A special tool was manufactured to hold the fan pulley while unfastening the nut securing the viscous fan coupling. This took the form of a piece of steel plate, shaped to fit around the coupling nut and two opposite bolts, with a long handle welded to it. The coupling nut is quite large, a suitably thin 32 mm (or 1¼” AF) spanner being required, which when in positon prevents the holding tool from slipping off.

Water pump pulley and holding tool

Another tool makes removal and replacement of the alternator belt much easier, by locking the tensioner pulley in its withdrawn position. This takes the form of a 6 mm pin (a bolt will do) about 60 mm long, but for added convenience I welded it to a 110 mm handle, made of 20 mm square tube.

Apart from the thermostat and its cover joint, the only parts required were a new radiator drain plug with ‘O’ ring, and the cylinder block drain plug sealing washer.

On cars over ten years old it would be good practice to also replace the ‘O’ ring seals inside the quick-release couplings which connect the radiator hoses to the thermostat housing. These are not listed by BMW, but are available from most hydraulic equipment specialists.

This is also a good opportunity to carefully examine all hoses in the cooling system – including the heater – and replace any swollen or damaged items. In particular, check the hose between the expansion tank and the block, as on some older cars this is known to be rather short, and can be affected by engine movement.

The first step is to completely drain the cooling system, by removal of the radiator and cylinder block drain plugs. Needless to say, this is an excellent opportunity to flush the cooling system and change the antifreeze! Remember to drain the heater by selecting its hottest setting and turning the ignition on, in order to open the heater discharge solenoid valve. The drain plugs can then be replaced, using new sealing rings, and tightened to 25 NM (block) and 8 NM (radiator).

While the front wheels are jacked up, the air conditioning drive belt – if fitted – and the alternator drive belt are removed, after compressing their tensioner pulleys using a T50 Torx key. With the tensioner for the alternator belt locked in the fully-compressed position, pass the belt over the fan by feeding it around each blade in turn. Mark both belts for replacement in their original direction of rotation.

Drive belt tensioner locking pin

These Poly-Vee belts normally have a life of at least 100,000 miles, but whenever removed they should always be carefully checked for cracks or damage.

Working from the top, withdraw the expanding rivets securing the top of the fan shroud, and remove the hose clips at the radiator connections. Withdraw the wire clips locking the radiator hose connectors to the thermostat housing, and the expansion pipe to the header tank. These connectors are a tight fit, and some gentle persuasion may be required to release them. It may be preferred to leave the bottom hose connected to the thermostat housing, and pushed out of the way until the fan has been removed.

The hoses can now be removed, then the cowl moved backwards against the fan, and secured with tape. The radiator securing clips can be released (unlock by inserting a small screwdriver into the clip, and lever forwards) and the radiator lifted clear. Remember to disconnect the air conditioning condenser fan auxiliary switch, if fitted. The cowl can now be removed.

Viscous coupling nut being unscrewed

With the radiator removed it can be thoroughly cleaned, and the cables which pass around it checked for abrasion damage etc. Removal also enables the front of the engine to be properly cleaned, and after an hour or so spent rubbing with steel wool it will positively gleam. It will not make it go any better, but it looks nice!

While holding the viscous fan coupling nut (32 mm or 1¼” AF spanner) remove three of the M6 bolts securing the fan pulley, and – using longer bolts – secure the holding tool in position, using two opposite bolt holes. Now unfasten the coupling nut, remembering that it has a left-hand thread.

Thermostat housing exposed – note water pump with pulley removed

With the fan out of the way, depress the wire securing clip of the thermostat electrical connector, and unplug it. Remove the bottom radiator hose, if it was not previously uncoupled from the thermostat. Now slack off the front nut on the stud at the engine lifting eye (which Heather uses to secure the washing line when we are camping!) and remove the bolts around the thermostat housing – three M6 and one M8. After tilting the lifting eye as far as it will go, the thermostat housing can be swivelled and removed.

Defective thermostat – note broken lugs

In my case, both thermostat mounting lugs had fractured, allowing it to expand under the influence of the return spring. I was lucky, as it had not completely closed the return from the radiator, so permitting a reduced flow into the engine at all times. Although in summer this would have caused a slow warm-up followed by overheating, during winter conditions it simply extended the warm-up period.

New thermostat – improved type

Replacement was, as they say, the reverse of the removal procedure, with no unpleasant surprises. Fit the radiator bottom hose to the thermostat housing before fitting the fan – after inserting the locking clip, firmly push the connector over the thermostat housing until it clicks into position.

The water pump pulley is again held by the special tool in order that the viscous fan coupling nut can be tightened to 40 NM (30 lb-ft) which is about the threshold of pain for one hand at radius of 12 inches.

The radiator can now be carefully replaced, then its securing clips engaged and fully tightened with a small screwdriver by gently levering backwards. Fit the bottom hose and (if fitted) the air conditioning condenser fan switch connector. Remember to place the fan cowl over the fan before replacing the radiator!

With the tensioner pulley locked in its retracted position, the alternator drive belt can be positioned, after which the tensioner can be slowly released. The air conditioning compressor belt does not have a locking device, but is easy to replace.

Although the drive belts can be fitted before replacing the cowl and radiator, fitting them afterwards provides good practice for any future roadside emergency. Do not wait for it to happen – practice in your garage! Needless to say, a spare belt, with the Torx key and locking pin are worth carrying, but preventative replacement is the best option.

When refilling the coolant system switch the ignition on, with the heater set to its hottest position, in order to purge the heater system. For every litre of antifreeze used, fill the container with water (preferably de-ionised) to give the ideal 50% mixture. This will maximise the anti-corrosion content to last for the next four years or so.

Although in M52TU form the engine does not have any bleed screws, there is one on the expansion tank – a large circular plug, marked with a deep cross. Remove this while filling the cooling system, and replace it before starting. With the filler cap removed and engine idling, remove the bleed screw, and replace it when water begins to flow. Briefly run the engine three or four times to about 4,500 rpm, then switch off, check the level, and replace the cap.

Run the engine for a few more minutes, then check the coolant level. Repeat this check a couple of times until normal operating temperature is reached, then stop the engine and allow it to cool down before finally checking the level. Remember to also check the hose clips, drain plugs, and thermostat bolts for tightness, and examine all connections and joint faces for any sign of coolant leakage before road testing.

The engine was now back to its usual half mile warm-up period, instead of about five miles – a great improvement, which was followed by an oil and filter change to remove the inevitable build-up of moisture and unburnt fuel caused by prolonged cool running.

This is an interesting task, which is within the capabilities of most practical owners. It is always satisfying to find the definite cause of a problem, and for those who have the interest such work helps to improve the man-machine relationship beyond that attained by simply driving the car. Is it really the Ultimate Driving Machine? Maybe not, but it is certainly the ultimate toy for those who enjoy playing in the garage!