Vacuum pump differences

[Stugist]

I've seen and heard a few things about the vacuum pumps on the W123s, mainly having to do with the difference between the 1-valve and 2-valve models. I'm just outright confused now. The two-valve version has a line coming from the air filter to the pump, then a line going out. The 1-valve just has the one line going out. What gives? Does the single-valve model pull unfiltered air straight from the block? Furthermore, my car (see sig) has the 2-valve while my girlfriend's car, also a 79 but with an auto trans, has the single.

[NC-Diesel]

I have cars with both and have wondered this myself.... Although I think with it being a vacuum pump the air is pulled from the vacuum lines and exhausted to the air cleaner. So it may make sense if they exhaust the air to the block where it could get PCV'd back into the intake without an extra hose...

[qwerty]

Originally Posted by Stugist

What gives? Does the single-valve model pull unfiltered air straight from the block?

No. It pumps air into the block. And, it's single-hose, not single valve.

[BillGrissom]

The old style has 2 tubes and the newer style 1-tube. The old type has a large rubber diaphragm. The new type has an aluminum piston with elastic seal (O-ring w/ polymer or paper ring). The old type tends to fail easier, but rebuild kits are only ~$15 vs ~$110 for the new type. The new type has 2 outlet check valves in parallel, 2 inlet check valves in series one of which is external where the tube connects. They seem to fail or degrade ~120,000 miles. With low vacuum, your engine won't shut off, door locks won't work, and the transmission shifts hard.

The air flows from the vents towards the vacuum pump. The vent is a tube going into the cabin. If you accidentally pull a thick black vacuum tube out of the rubber grommet and think you disconnected something, you didn't. It is supposed to be open at the end, but sucks clean air from the cabin. In my 85, it sucks thru a clear plastic filter. In my 84, there is no filter (will add one). Components that suck from the vent are the transmission controls (white plastic valve on top of injection pump, plus blue valve in 85's), EGR valves, and the "engine on/off" vacuum switch on the key (already in the cabin).

[mbzr4ever]

I'm trying to decide if I should swap the diaphragm type in my 240D to the piston type.

Is there going to be a whole lot of modifications involved?
The person selling me the piston type is asking if I need the "cams and advance mechanism that goes on the injection pump"; don't know if I do.

But the pros for changing (no more diaphragm to tear and replace, better vacuum) are leading me to make the switch.

[Govert]

The vacuum isn't better from a piston-type pump (and diaphragms don't tear that easily: 150,000 miles or more for the diaphragm is quite normal).

[mbzr4ever]

Originally Posted by Govert

(and diaphragms don't tear that easily: 150,000 miles or more for the diaphragm is quite normal).

Good point.
At this time in the 240D's life (208K miles), am really going to replace the diaphragm again?

But on the flip side....
I never dreamed 30 years ago I would still be driving this vehicle - and almost on a daily basis!
And who knows how much longer I will?

[Stevo]

A diaphragm is allot cheaper and easier if you have to rebuild the pump and it makes just as good vacuum as the later piston pumps. I would stick with the diaphragm type.

[mbzr4ever]

Think I'm convinced with just repairing the diaphragm (and a few connectors/valves), even though it sounded like a good idea to swap it with the piston type.

Easier, for one. Don't know the specifics to adapt.
Cheaper - parts and related shipping.

Most likely faster, too.

Thanks, everyone, for your help.