I don't know if this was ever posted here so I will throw it in. Now first a little backround: The original vacuum pumps on the 1986-87 300SDL/D were a flawed desgin that should have been replaced by now. The original design had exposed bearings that would lodge in the timing chain and destroy the engine if the pump goes south! I compared the new one and can see how they enclosed the bearings so they can't escape. My local MB dealer said they updated the pump 5 times! A quick way to tell if you have the good one is to look at the cover, if it has 4 screws on it your in good shape you have one of the safe ones. If you see a smooth cover like I did, them replace that pump asap! It will cost you $350 but it is cheaper then a new engine.

I didn't know this a few weeks ago! Thanks to this forum I now know about this potential problem.

Replacing the pump is a pretty simple job now these instructions are for a 300SDL a 300D could be different.

1. I un clipped the plastic fan shroud.

2. Remove the fan it takes a 8mm allen wrench.

3. Take them out and set them aside.

4. Stick a rod/screw driver in the top plastic part of the belt tensioner. Take the tension off of the bolt that holds tension on the spring. Back the bolt out and relax the spring. I took the the top bolt out of the shock too.

5. Remover the belt from the top pulley's and push it down to get it out of your way.

6. Now disconnect the top two vacuum lines from the pump.

7. Remember the pump can be under tension from the large spring inside so remove and losen the bolts evenly. They are 10mm.

8. Once the old pump is off make sure you get the old gasket off and clean everything up.

9. Compare the new and old pumps you will see what I am talking about.

10. Now here is the important part you can not just stick the new pump on, it could be under tension if the "lobe" that causes it to pump is in the right position. I used a breaker bar and a large socket on the crank pulley to turn the engine clock wise until the lobe is out of the way. By doing this the spring in the pump is not compressed. I was afraid of cracking the housing it is just cast aluminum. By doing this their was no tension on the pump as I tightened it.

9. I put some oil on the new pump so it wouldn't start dry.

10. You have to play with the pump and gasket to get them to sit right. The gasket is metal and keeps wanting to fall down.

11. Tighten everything up and reconnect the two vacuum lines.

12. I started the car after I put the belt back on and put a vac guage on the new pump to make sure it was working right. I also listened for any weird sounds and checked for oil leaks. The fan gets in the way leave it off to check the pump.

Thats about it; total cost of oe parts from my local dealer:
$339 for the pump
$10 for the gasket.

It didn't take me to long maybe a little over an hour.

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