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1984 300D Turbo
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9 Posts
Discussion Starter #1
Is it really possible to have zero vacuum from the pump? That's what it showed last night. Not a wiggle on the needle.

Could the pump check valve be stuck enough to do that?

Or, what could have broken in the pump that wouldn't have broken something else? Or at least be really noisy.

Or, maybe I used a plug and not an adapter on my Mityvac...


84 300D Turbo
265K miles
 

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Registered
1981 W123 300D non turbo, 1992 190E 1.8 <=> 2.0
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6,561 Posts
Is it really possible to have zero vacuum from the pump? That's what it showed last night. Not a wiggle on the needle.

Could the pump check valve be stuck enough to do that?

Or, what could have broken in the pump that wouldn't have broken something else? Or at least be really noisy.

Or, maybe I used a plug and not an adapter on my Mityvac...


84 300D Turbo
265K miles
Well using a plug instead of an adapter would do it!
 

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1984 300D
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5,070 Posts
Removing the Check Valve is easy enough.

I lost Vacuum once and it turned out the Check Valve was Gunked up. Cleaning it out with WD-40 cured the issue.
Others have found the innards of the Check Valve Broken.

I cannot recall if I lost all of my Vacuum.

I know that there is at least 3 little Disc Valves iniside of the Vacuum Pump.
 

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Registered
1984 300D Turbo
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9 Posts
Discussion Starter #4
I did have an adapter and not a plug on the Mityvac; the zero vacuum was correct. Today I learned why.

Here's what I saw when I removed the vacuum pump.
DSCF0715.jpg
While I've not seen inside one of these before, it wasn't hard to guess something wasn't wearing correctly.

I took off the front cover of the pump to see:
DSCF0718.jpg
Again, even as a vacuum pump virgin I was pretty sure there shouldn't be a hole in the piston. Let alone that kind of mess.

Here's what the piston looks like after cleaning.
DSCF0721.jpg

My guess is the bearing which rides on the drive cam gave out. Then when it wasn't driving the pump piston smoothly the piston failed.

On the good side, I think the pump drive cam seems ok.
DSCF0717.jpg

The main pump check valve was full of what seemed like diesel soot (makes sense now) but cleaned up well and works correctly.

So, I'm looking for a new pump.

While she's sidelined what else should I look at?



I'm only a 13 month old MB driver. This is my first time doing any real work on it but I can understand the respect for the engineering which is evident from reading this forum.
 

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1984 300D
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5,070 Posts
I think you were lucky; most often parts fall into the Timing Chain/Gears and something breaks and Pistons collide with Valves.

If the Cam area on the Timer is roughed up you are better of getting another from the Junk Yard.

Another issues is that if you have been having Vacuum Leaks it is believed that the Leaks cause the Vacuum Pump to over work itself.

Another twist on the issue:
New Vacuum Pump Ruined due to too much Timer/Intermediate Shaft End Play (the title from my notes).
second vacume pump, help! on a trip-Toronto - PeachParts Mercedes ShopForum
 

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Registered
1981 W123 300D non turbo, 1992 190E 1.8 <=> 2.0
Joined
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6,561 Posts
Check the track on the timing device - the roller coaster-like track

Clean it up really well. Look for wear marks, cracks, sharp edges etc

Next push hard up against the timing device and see if you can feel any movement.

If you can feel movement you should ideally measure it with a clock gauge / DTI. Officially you are meant to measure the play of the intermediate shaft that sits behind the timing device but to get to that you ideally need to remove the injection pump (on the other side of the intermediate shaft) as well as the timing device - which means timing chain removal etc.

This could turn into a pretty serious baptism of fire if you're not careful. I know you are not meant to check for play on the intermediate shaft this way - but I think it is best to just do a simple check first. Unless you have had lots of mechanical experience on other vehicles.
 
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