Basically, the test is to check the vacuum directly at the pump. If it's not there then you'll need a rebuild kit - cheap and fairly easy to do with the pump on the car. The kit comes with hardware, three check valves, and an O-ring and sliding seal for the piston.

I'd plug everything but the brake booster and check vacuum. If you don't have decent vacuum (~20 psi) then pull the booster pipe off the pump and check directly at the vacuum pump. The other checks are good to make sure everything else is connected/operating correctly, but you need to verify vacuum is available first before looking for other problems.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

Ok.... I have pretty much done everything but that now. I replaced all the vacuum tubing (short pieces) under the hood and a couple in the car under the dash. I found one on the ignition switch that had a small tear in it. I checked the VCV and it is good.

Also, I hooked my mighty vac up to the main line before the check valve (the little nipple) and plugged the nipple on the other side. The gauge is only reading around 10 or 11 in. HG. The needle on the gauge is bouncing around wildly. Revving the engine brings it up to about 14 in. HG.

I'll go out and pull the main line and check the pump now. Not sure how to hook up to it though?

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AJ

1985 300D (SOLD)

Bouncing gauge makes me suspect the check valve. It may be possible to remove it and clean it or swap in another one. Not sure you'd get any vacuum at all if the pump had failed.

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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

Do you have a vac reading from just the pump alone? I think you need to determine if you pump is pulling adequate vac or not and if not you will need to rebuild or replace.

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'95 E300D ("Tank") - 231,000 miles
'79 240D ("Biscuit") - 197,250 miles (Sold)
'83 240D ("Ding-Ding") - 217,000 miles (Death by deer)
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"Back off, man. I’m a scientist” ~ Peter Venkman

Thanks guys... I'm guessing that there is a vacuum pump overhaul thread somewhere? Kerry, you are talking about the check valve "IN" the pump. Right?

I pulled the main vacuum line, plugged the nipples on the check valve and the main line and pulled vacuum with the mighty vac. That line is good too. I also pulled some vacuum on the trans. modulator again and was getting a really slow leak. So, I pulled the line and checked that one too. No leaks. So, I re-installed it and now it holds vacuum. No leaks at all.

I checked vacuum at the pump and it seems to be around 15 in. HG. The damn needle bounces around so much it is hard to read. If I pull the handle on the mighty vac it stays steady right at 15 in. HG with the engine running.

I'm not really sure what to do now? I went for a drive after I did all of this. I stopped a couple times and it shut right off and the brakes worked good. BUT, when I got home it shut off a little slow. But, it did shut off.

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AJ

1985 300D (SOLD)

Yes, check valve in the pump. I suspect when you pull the vacuum with the Mity Vac it is helping the check valve stay closed. Just a hypothesis but I don't know what else would cause a fluctuating gauge.

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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

I believe you have the same vacuum pump as mine. Believe me, it's very simple and straight forward. The hardest part is pulling all of those small hex drive screws holding the pump on the block. (The check valve portion can be rebuilt with the pump on the engine but I removed it to inspect the major mechanicals of the pump.)

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Sam

84 300SD 350K+ miles ( Blue Belle )

Bouncing guage is due to bad check valves in the pump. It should be steady once it builds pressure but the check valves are allowing vacuum to escape. As SDBlue said it can be done with pump on car, just remove the front plate (flat blade screws.) Hardest part is placing gaskets properly while pump face is vertcal.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

The car pulled this crap 2 more times today. I got the rear end fixed, so we have been driving it a lot more. I put about 200 miles on it today and it was slow to shut off 2 times out of about 20 along with weak brakes.

What is involved if I pull the pump off the block? I want to take it off and inspect the entire pump. I see that the kit does not come with that particular gasket (block to pump gasket). Should I go ahead and order that gasket separately? Or can I reuse the old one? Is there any kind of special alignment I need to do for removal and re-installation?

I went to whunters recent re-post on vacuum pumps and went through a bunch of threads. I found one that describes the procedure pretty clear on how to handle the rebuilding the pump itself. No pictures but sounds pretty easy.

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AJ

1985 300D (SOLD)

Here's the problem with removing the entire pump. You have to push against a stiff spring in order to get the piston out. The manual calls out a special tool to mount the pump onto for this purpose. If the pump is left on the car, you can rotate the engine to do the same task. If you're bent on removing the entire pump, here's what I would do. Pull the front cover (flat head screws). Turn over the engine until the piston is just past its apex and is starting to return. Remove the nut holding it in place and then keep turning the engine so the bolt withdraws then just begins to come through the center of the piston again. Jack the piston over just a bit so the screw gently wedges in the hole and pushes the piston out of the cylinder far enough to remove it. Replace the O-ring and sleeve, wipe out the cylinder bore, lubricate it and the piston with clean oil, and return to piston to the pump and secure the nut in place. Now remove the pump and replace all the check valves and gasket and reassemble. You will need a new gasket for the pump, wouldn't rely on the old gasket - too much chance for a leak and it's cheap insurance.

I don't know how hard it is to push against the pump spring, maybe it's not that bad. If the vacuum guage is bouncing like you say, then your piston mechanism is fine - spring, piston, etc., so removing the pump from the engine will likely not reveal any surprises. It's a little tricky to replace the check valves and gasket while the pump is installed because of the vertical face, but if you put the car up on ramps the pump/engine will be tilted enough for everything to stay in place. It may save you time and money if you can do it that way.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

Ok.... Thanks for the info. I'll do it on the car if my back holds out that long. I did not know I could replace the piston seals without removing the pump. That was my main concern. That and cleaning/lubing the bore.

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AJ

1985 300D (SOLD)

I think it's only seven flat head screws to remove for the front plate. In my case the check valves sat in their grooves fine, but the gasket kept moving when I tried to put it in place (it's a round rubber gasket). If you have any trouble keeping the front plate gasket in place, use a light coat of grease to make it sticky. The interior of the pump should be clean when you open it, so you shouldn't have anything to worry about there. And with the car up on ramps your back will be less stressed than if it were on the ground. Probably less work this way than removing the pump - the screws on the front are much more accessible than the allen bolts that mount the pump.

Good luck! Let us know if this solves your vacuum problem.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

If you DO decide to do this job with the pump on the car (and actually, this advice is true even if you do it OFF the car too) MAKE SURE you note which way the check valves are installed.
What I mean by this is that the little round check valves (2 of them) are asymmetrical - they have to be installed a specific way. Also, one of them is installed one way and the other valve is installed the opposite. With the pump still on the car, it might be difficult to know which way they're installed because in all likelihood, they'll simply fall out once you remove the front plate and you'll not know which way they were originally installed.

Also make sure that the external check valve is in good shape. On my car, which exhibited intermittent vacuum issues like yours, it turned out that the external check valve had fallen apart into its individual components...which apparently is a fairly common scenario. The good part is that replacing that valve is simple....the bad part is that stoopid little thing is $50-60! On mine, I went ahead and just rebuilt the pump anyway.

FYI, I just did this job about 2 weeks ago (pulled pump off though) and it was one of the easier jobs I've ever done on the car.
The mention of the spring inside the pump DOES mean you have to have a way to compress the spring though...so that you can remove the piston and replace its seal/ring. Putting it back onto the block will also mean that you have to find a way to compress the spring while you get the first two bolts started. I used a carefully placed broom stick handle.

My car is in 'dry-dock' right now though so I took my time doing all this. But even still, if I had to go through front start to end, I probably could've done it in about 2hrs. I had purchased a kit though that included all the valves/seals, gaskets, and a little board that had the bolt holes in it so that I could mount the pump TO the board...and thereby compressing the spring. VERY handy.

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Chris
1985 300SD - 'Grace' (198K mi.)
2018 Honda Civic Sport
2018 Honda CRV LX
2010 Honda Fit Sport (RIP)
2013 Honda Accord Sport (Sold)
1996 Lexus LS400 (Retired)
1995 Ford Contour SE (Retired)
1976 Porsche 914 (Sold)
1972 Datsun 240Z (RIP)

That is one of the reasons I was looking for a "picture tutorial". I was wondering about the check valves and which way they go? Is there a picture of that somewhere? Or a diagram? I can picture my self dropping them on the ground and then having no idea how to put them back in.....

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AJ

1985 300D (SOLD)

If you haven't done this already, remove the end of the main vacuum line that connects to the pump and pull it aside so that you can look down into the external check valve. On mine, it was like the check valve was a hollow tube (all the little parts had come apart and fell inside the pump).
See if you see a little disc sitting inside there. If you're lucky/unlucky, yours will appear hollow like mine...which simply means you need a new external check valve. The unfortunate part about this is that it STILL means that you have to pull the front cover off in order to retrieve the 3-4 little parts that used to be inside the external check valve.

As for a diagram, I can't find the doc that came with the kit right now, but it did a full walk-through of the process...including which way to install the valves.