QuestionI hav ea 95' 325i. i recently replaced the water pump, thermostat, and thermostat housing. When I crank the car, it has a high pitch hiss, sounding like it is coming from the front, top of the motor. It isn't constant but does increase in pitch when the engine speeds up. It seems to disappear when the engine reaches the normal operating temperature. the engine doesn't overheat at this time. It is my wife's car so I don't know if it was making the noise prior to my maintenance. I did have trouble bleeding the system but I think I got all the air out. When I crank the car, the system checks ok, but I get a low coolant warning when the switch is turned off.
AnswerBobby,
To be honest, I am more familiar with the previous body style (E30) although cooling systems are generally simialar. I do own a '99 E36 328i, but I'm still learning the in's and out's of these cars.
Most of this will probably seem redundant, but I'll say it anyways. When you remove coolant for a water pump change, or any other time you open the coolant system you introduce air in the coolant's place. This air is sometimes tough to get rid of. If you have any air in the system, it will heat faster than the coolant, allowing for additional pressure to build. On the older BMW's it is recommended that you turn your heat on 100% leave the doors open and start the car. This will allow coolant to flow through the heater core and remove any air inside the cabin as well as under the hood. There should be a bleed screw or some apparatus for these engines to be bled. On the older M20 engines (You have an M50) there was a screw on top of the thermostat housing. If you loosened this screw while the engine was running, coolant and air bubbles would flow out, removing any air in there.
I found this at a website for the E36:
"With all of the coolant removed from the system, use a funnel and place it into plastic containers. One gallon spring water or milk containers make excellent storage containers for used coolant (Figure 12). Don't pour the coolant down the drain, instead take it to a recycling station. A note about coolant - it is highly toxic and dangerous to pets and small children. Used coolant should be packaged up right away. Less-than-intelligent animals seem to like the smell and taste of coolant, and will be attracted to it. Package it up and seal it off, otherwise you may find kitty dead in the garage one day - no joking. Rinse and dilute any contaminated areas with water.
Remove the small plastic bleed screw to the right of the radiator cap. On M3 engines, this screw is located on the thermostat housing. Using a large funnel in the expansion tank, slowly fill the car with new coolant. Use a 50/50 mixture of antifreeze and distilled water. Do not use tap water or spring water, as these have impurities that will contaminate your system. Distilled or ionized water is 100% H20, and does not contain any minerals, additives, or impurities. Fill the system very slowly, as it will take a bit of time for the coolant to get to all points in the system. On 4-cylinder cars, you will need about 7 quarts (6.5 liters). For the six cylinder cars, you will need 10.6 quarts (10 liters). M3 motors require 11.1 quarts (10.5 liters).
When the system is full of coolant, you will need to bleed air out of it. This applied only to the cars that have the expansion tank attached to the side of the radiator. Other BMWs with a separate, external expansion tank are self-bleeding. The procedure for bleeding these cars seems somewhat sloppy and not too slick. With the bleed screw removed, fill the expansion tank until fluid begins coming out of the bleed screw. Continue filling until there are no more bubbles coming out, as shown in Figure 13. This, of course will mean that plenty of coolant will spill all over your radiator, and down onto the ground. Have a catch pan ready to capture this coolant spill. When no more bubbles exit out of the bleed screw, tighten it to 8 Nm (71 in-lb).
Now, run the engine until it reaches it's operating temperature. Turn off the engine and let it cool down. Then, top off the coolant in the expansion tank to the appropriate level, if necessary.
The full write-up is at:
http://www.pelicanparts.com/BMW/techarticles/E36-Coolant_Flush/E36-Coolant-Flush...
As far as your "hiss" I'm guessing it's probably steam building in the coolant system, due to the air bubbles still inside. Pull the bleeder screw from the side of the radiator, and let all the bubbles out. It probably wouldn't hurt to start the car, run it for a while, then try and bleed some more. When you changed the water pump it was a great time to check your fan-clutch. those wear out, and are a huge source for heating problems as well.
Dont hesitate to ask anything else, I'll try to help in anyway possible,
Josh Fields
