Any explanation as to why it would only park 2 out of 5 times?

I rigged up another test using the vac pump in my 85 as the vac source. It only pulled 22" Hg, not enough to move and park the vac pump under test. I took a pic of the setup and will post it later. In the mean time I will look for a window air cond compressor for a stronger vac source.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I thought a new Check Valve came with the new Vacuum Pump?

I had a loss of Vacuum Issue and it turned out to be a gunked up Check Valve. If I had put it on a new Pump I think I would have had the same problem.

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84 300D, 82 Volvo 244Gl Diesel

It does come with one, I didn't take notice of what exactly comes with the pump. I have a new, updated style on the way from fastlane.

If you pulled a Vacuum with an outside Vacuum Pump the Vacuum from there would close the Check Valve and the Vacuum would not enter the Vacuum Pump.

I hate to use this term because I do not know if there is such a thing but the flow of Vacuum; the Air/Vacuum flow goes towards/into the Vacuum Pump.
Vacuum exerted in front of the last Valve; the check Valve reverses the flow and closes the Valve.

All you need to do is connect a Vacuum Gauge to the Check Valve; you can use that setion of Plastic Hose; and, crank the Engine (or compress the lever enough to work the Pump) until the stock Vacuum Pump pulles the desired Vacuum you want to test at.

Another comment: Even if the highest possible Vacuum is not enough to Pull the Arm away from the Timer; having Vacuum on one side of the Piston would still mean it takes less force on the Arm to move it. That should mean less stress on the Bearing and the Arm.

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84 300D, 82 Volvo 244Gl Diesel

Here's my test setup. An old plastic vacuum line connecting the 2 vac pumps.



The wire clamped to the pump is a gauge to indicate if the cam follower moved or not. 22" Hg was insufficient to move it.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

My best guess:
* Valves forming a less than perfect seal.
* Piston seal ring forming a less than perfect seal.


.

The Green Arrow shows direction of the Vacuum Flow. Pulling vacuum on that side will close the Check Valve where the Red Arrow is. That will prevent Vacuum from entering the that Vacuum Pump.

Even after you get past the Check Vlave there is other Valves inside of the Pump that are one way valves like the check Valve.

In fact if you use the Mighty Vac to create the Vacuum it will close both the Check Valve on both Vacuum Pumps.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

Thanks D911 for pointing that out. What changes do I have to make in order to run the test? whunter, how did you do your test?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Funola
I’m sorry to have to tell you that you are wasting your time with that set up. All you are doing is testing the pump's suction plate valve for leaks. If Whunter was using a piston pump and had even the slightest movement of the rocker then he has proven that the suction plate valve is u/s and leaking. It is impossible to check the piston pump like that unless you draw out of the exhaust valves and block the suction pipe - that is an obvious no-go on a piston pump.

It is however possible to check a diaphragm pump and I checked a very old one I have yesterday by simply connecting the booster connection to the pump exhaust and blocking the suction pipe with my finger. It pulled the roller back 9mm which is not bad for an old relic like that at 2000ft! If there are still any “Doubting Thomas’s” left may I suggest that you try that.

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Beagle

It is interesting to see how this thread has developed.

Are most people (or even all?) now convinced that the piston parks?

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I think the diaphragm pump has the ability to park, still not convinced it happens much on the piston pump. Diaphragm pump has a lot more area and no rings for the vacuum to leak around.

Maybe that's why we know of a history of failures with the piston pump but I don't recall any bearing failures on the diaphragm pump reported on here. Does anyone else?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

No problem Beagle. I spent maybe 20 minutes on the test and it was not a total waste. In that I did not blow anything up and that it held vacuum indicating the plate valves in both pumps are ok.

Now I am curious how whunter was able to park his NOS piston pump, but only 2 out of 5 times.

Any suggestions on testing a piston pump whether it is parking? What do you think of drilling a hole and get a visual with a borescope? Installing a proximity sensor on the rocker arm to detect movement? Or just listening with a stethoscope? I haven't tried that yet but will do so before doing anything else.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Wow, is this what Al Gore invented the internet for, or not?

I have a friend that found the telltale balls in his oil pump screen from an early VP failure, I think it was an early pump, I'll find out.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Good question! Maybe we should post a pole to find out. I would be more interested in how many now plan to replace the bearings and valves.

As far as I am concerned that is a side issue and a No Contest. Whether you believe it parks or believe it runs continuously is irrelevant. The fact is that these pumps are over 30 years old and should have the bearings and valves replaced soonest unless you want to join the thousands around the world who have had them disintegrate. The object of this thread is to try and convince others of that - looks like I have failed.

Over the last 40 years I have seen in the course of my business, at a wild guess, a couple of dozen failures. Certainly many of them were diaphragm bearing failures. I think the reason most of recent failures are piston is that there are now many more piston (post ’79) than diaphragm still on the road. The vast majority were from bearing failure, a few from timer bolt breakage or loosening and others piston jamming (nut coming loose).

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Beagle

The diaphram may have more area but the piston has a longer stroke? I would assume the piston ring has no gap and does not leak, at least not more than a diaphram. Why did Mercedes replace the diaphragm pump with a piston pump? I would assume it must have the same or better specs.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked