Under what circumstances is your car getting hot? Most M50's used a water pump that had a plastic impeller on a steel shaft. This impeller can slip on the shaft, which will cause overheating while driving, but not when idling. In extreme cases the impeller can melt or break completely off of the shaft, causing overheat all the time. The new ones have a metal impeller on the steel shaft and should be replacing your plastic impeller if you are near 50,000 miles. This is very important to do this simple change. It could save you thousands down the road.
procedure is the same as the E36, listed below, with a few exceptions.
There is no plastic cover to remove from above the radiator, but you will need to remove the fan shroud, it is held in place by a pushpin on each side of the top. Use a small screwdriver to remove the center pin and remove the clip. Then you can lift the fan shroud up and out of the car.
replace the water pump, drain the radiator, the drain plug is located on the
bottom left of the radiator. You will need to remove the plastic cover above
the radiator. Remove the alternator air intake and the top radiator hose. To
remove the fan, you will need a narrow 32 mm wrench; the thread on the fan is
left hand, that is, opposite to normal threads. Make sure you turn it clock
wise to remove it. Do not use pliers on the pulley to hold it while removing
the fan, many of these engines used plastic pulleys and you will damage it.
There is a BMW special tool to hold the pulley in place while breaking loose
the fan, if you do not have access to this tool, you can use a hammer on the
wrench. A sharp hit is usually all that is needed to shock loose the fitting on
the fan. If using this method, and you have a black plastic thermostat housing,
make sure the top radiator hose is removed first. If you do not do this, there
is a strong chance of the wrench hitting the hose clamp and breaking the
plastic housing. Remove the lower radiator hose from the thermostat housing,
and move it out of the way. You do not need to remove the thermostat housing,
but may wish to do so for a better view of the pump. The pump is held on by 4
10 mm nuts, remove these nuts. In order to remove the pump from the block,
there are two threaded holes, check your new pump for their location. You will
need to screw a bolt into each of these holes; these bolts will bottom out
against the block, and extract the water pump.
The bolts need a 5mm thread and to be at least 30 mm long. Tighten each one a little at a time, alternating between the two to pull the pump evenly.
To install the new pump, lubricate the O-ring seal with engine oil or a similar lubricant, and push it straight into the hole. Put the nuts on the studs and tighten each one in stages to pull the new pump into the block. Replace all removed components, install new gaskets on the thermostat housing if you removed it. It would be a good time to change the thermostat also. This is essential if the car has been overheated, a thermostat is a temperature sensitive device, if it is subjected to an overheat situation, it will become unreliable.
Remove the bleeder screw from the expansion tank mounted to the radiator; fill the system until coolant is flowing from the bleed hole. When installing the bleed screw, be very careful not to over tighten it. Once you feel the screw bottom out, turn it less than 1/4 of a turn to secure it. This screw is sealed with a rubber O-ring, it will seal with very little pressure. Too much pressure will snap the hea