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Old 03-16-2002, 07:43 PM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Rebuilding the M110 Vacuum Pump

You may recall from my post last week that my 280C started belching huge clouds of blue smoke. The diagnosis was that the vacuum pump had failed and was allowing oil to be sucked into the intake manifold.

The pump sits on the front of the cam cover and is driven by the exhaust cam. Two lines run to the pump. A black one from the intake manifold and a clear one from the brake booster.

Should something like this happen to you, the emergency fix is to connect the brake booster directly to the intake manifold with a three foot piece of 7/16 hose. It will slip over the existing fittings.

A replacement pump is $450- 500. However, there are two kits available for the pump on Fastlane. The diaphragm kit is about $8 and contains the large diaphragm and miscellaneous parts. The valve kit is $13 and contains the one-way valves and a seal.

I went for both.

The rebuild itself is painless. YOU DO NOT HAVE TO REMOVE THE PUMP! Disconnect the vacuum lines. Remove the screws around the perimeter and take off the diaphragm cover. Unbolt the diaphragm from the pump. Take the cover and the diaphragm to your workbench.

Clean everything up. Assemble the new diaphragm with the two existing washers and the new bolt. Unscrew the two screws on the top of the cover - equal bits at a time - and remove the top cover, minding the two springs. Replace the one-way valves and their washers. Replace the rubber seal on the top cover. Put the springs back in and install the top cover.

Coat the pump and top surfaces with the supplied sealant and reinstall the new diaphragm. There is a small O-ring that goes at the top of the threaded hole that receives the bolt that holds the diaphragm. Install the cover, connect the two vacuum lines, and you're in business.

An hour, tops.

In my case, I discovered that there was indeed a tear near the center of the diaphagm. My theory is that a little oil is allowed into the non-vacuum side of the pump for lubrication. When the tear occurred, this rendered the pump inoperable and allowed the engine vacuum to pull the oil into the intake manifold.
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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