Doorman50

I have a 99 ML320. When I got my car service, the pump was noisy and I was told it was functioning ok if I could put up with the whining. I couldn't, went cheap and put in an overhauled pump. This was a mistake. I returned this pump and installed an OEM pump. It is better but still noisy. I also changed the fluid not only in the reservoir but by removing the cooler hose at the rad, starting the car for a second, refilling the reservoir multiple times. Of course I bled the system after I was done. This is also marginally better. When the engine is cold, the pump is quiet, as it warms up, it becomes noisy. I rechecked the fluid when the engine was warm and noticed air bubbles in the reservoir. I have bled the system, have no leaks and am puzzled where the air could be coming from. Is it possible the washer on the pump/resvoir connection is letting air in even though it doesn't leak fluid? Any help would be appreciated.

Maj. Dundee

Your problem is not with the pump but with the reservoir. Inside the res. is a mesh screen which eliminates noise. When it fails, the res. must be replaced.

Step one is to order a reservoir, 2 O-rings (in case you damage the first one on installation) and locking C-clip. If you do not order OE parts from the dealer you will have leaks at the res. and the screen will fail again.

Step two, after receiving the parts, is to choose a location where you can install the new res. and be able to raise both front wheels off the ground. You cannot start the eng. after installing the new res.

Suction out all of the fluid in the res. Then remove the C-clip and follow instructions for removal & installation.

After the installation and partial filling do not start the engine. Raise front wheels off the ground, turn the key to pos. #2 (DO NOT START THE ENGINE) and follow bleeding procedure in pdf with aid of someone looking into the open reservoir until all bubble are eliminated.

Doorman50

Thank you very much. I will get an OEM reservoir from the dealer today, already have the o-ring as I thought maybe it could be this. I am done with buying aftermarket parts, I've had nothing but bad experiences with them. I have all equipment needed to do the job and am quite surprised the MB dealer didn't know about this. I was thinking of changing the reservoir but I couldn't imaging what could be wrong with it but did not know there was a screen inside. I will let you know the results.

Doorman50

Maj. Dundee you are the man. It was the problem. I am still really surprised that the MB dealer didn't know this could be the issue. I now question if the original pump was bad at all. The vehicle only has 90k miles on it and has always been serviced by MB. Thanks for you help!

Maj. Dundee

You must understand that all dealers (every make and model) will tell you the worst when diagnosing a problem. If you would have allowed the dealer to do this job, they would have replaced both; the reservoir to correct the problem and the pump to separate money from your pocket. That's just the way it is.