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M112 - Issue with water pumps and pulleys on earlier engines
The issue is that at some point Mercedes made a change in the water pump pulley and the front flange of the water pump for the M112. The newer pulley (1122020110) is about 42.5mm wide and the earlier one (1122020010) 44.5mm wide. 2mm difference. There was also a change in the pump flange. Number changed from 1122010039 to 1122010139. Don't know how this changed. This is part of the pump and may or may not be an issue. New and old seem same on my pumps. This is documented here: EPC showing changes
This change was made early on in M112 production, but the Mercedes EPC does not say at which engine number. It only says that the old pulley number only can be used on pump 1122000201. But the pumps don't have a part number on them. Just a casting number which is not the same thing!
How I found this out, was in buying an aftermarket pump for my 98 E320. The same issue could have occurred with a reman MB pump. Even more so in my case, because EPC gives wrong number for the pump for my car even with VIN and Engine number input. How would MB know which pump to send me?
If you are unaware of this, the problem shows up after you have installed the replacement pump when you go to fit the original water pump impeller. At this point, you may not notice that the rear edge of the pulley is in contact with the pump casting. In my case the pulley would not turn and cut grooves in the pump casting when I started the engine!
Only solution for me, was to change the pump pulley. A new MB pulley costs over US$200 - More than a pump! URO have one for less if you are happy with that! My option was a used pulley of a later engine. Be aware: If you have an early M112, you may need a pump AND a pulley!
There is one key dimension on the pump that should be checked. This is distance from face of the front pump flange to the back face where gasket fits. On my 98 OE pump, that distance is 85.5mm. On the aftermarket pump I bought, it is 83.5mm. That 2mm difference is cause of the problem.
In order to determine if all aftermarket and MB reman pumps have this problem, it would be very useful if anyone who has a new or used M112 water pump on hand could measure the face to face offset of their pumps and post the make of pump. Current part numbers are 1122001401 with oil cooler nozzle or 1122001501 without. The attached drawing shows the dimension needed.
It should be noted, that this issue applies to early 98 E320s and other models with same M112 engine, at least up to engine number 11294130026246. Offset Measurement
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