Vacuum pump 1985 300SD reliability ? - Page 2 - PeachParts Mercedes-Benz Forum

mach4 · #16 09-30-2015, 11:10 PM

Quote:

Originally Posted by BillGrissom

The one test I didn't see is to block the VCV vent tube and verify you then saw the full 22"Hg vacuum. That would better assure that the VCV bleed was the problem and not a leak somewhere else (split hose, tranny modulator).

I wish I could find a <$20 vac pump to play with. All the used VW & Audi pumps I see on ebay are >$35 + shipping.

I know the system is tight as I run a full time vacuum gauge. The pumps are available in the PNP from time to time - that's where I got mine.

The other question that is unanswered is whether the pump can safely be removed without damaging the timing device on the IP. Some speculate that pressure from the vacuum pump spring is essential to maintaining tension.

kestreltom · #17 10-01-2015, 03:22 AM

Quote:

Originally Posted by mach4

Before you head down that path, you might want to read this - Electrical Vacuum Pump Conversion Project
The other question that is unanswered is whether the pump can safely be removed without damaging the timing device on the IP. Some speculate that pressure from the vacuum pump spring is essential to maintaining tension.

I read that. Very interesting, detailed, and thorough assessment of all the relevant issues facing an electric vacuum retrofit.

In my implementation, I am using the electric Hella UP28 as a backup to the mechanical Pierburg pump. My VCV resides upstream from a custom restrictor that is smaller than the normal brown restrictor that I installed years ago to get smooth shifting behavior from my 722.315 trans. I believe that this restrictor is making it possible for the electric pump to shut off most of the time, but I was also considering using a separate smaller full time electric pump just to service the VCV. That way I could pull the plug on the Pierburg and just install a gutted housing as a block-off plate. Mach4's question re: the need for pressure against the timing device is a good one. If anyone has a response to that I also would like to hear it.

kestreltom · #18 10-01-2015, 03:32 AM

Mercedes actually made a block-off plate for that purpose. There is even a part number somewhere, but I do not have it. I saw the picture of it on Land Cruiser retrofit post somewhere.
I do not know if there was a preload spring of some sort on the back side of it - but I sort of think not as it looked very low tech... sort of like a pie plate

BillGrissom · #19 10-04-2015, 01:03 PM

Yes, in the linked vac pump articles someone mentioned that the marine version of the OM917 engine came w/ a block-off plate, but those engines are rare. Didn't say if it has a spring loader underneath the plate. My suspicion is that since there is no thrust on the IP, and the timing chain has probably worn grooves into the plastic guides, it will probably run fine without the force from the vac pump spring. I bought a small pie plate on ebay to try someday. I need to figure a way to roll it another 1/4" in diameter to best fit.

Interesting that Kestreltom found that a smaller restrictor (brown) makes the transmission shift smoother, as a win-win. The 1985+ cars have the "blue saucer" vacuum amplifier (plus other functions) between VCV and tranny modulator, so the VCV doesn't require as much flow. I wonder if their restrictor and VCV are the same PN. I have a 1984 & 1985, so another variable to play with.