Om621 still no vacuum after pump rebuild
 

I tracked down a rebuild kit and rebuilt my vacuum pump on my 66 200D. It seemed pretty straightforward, diaphragm was torn, replaced that and the 2 check valves! Easy, but still no vacuum. I'm wondering if my driver plate is worn or the roller lifter is worn down? I included photos and hope someone has any experience with this!

Pictures aren't showing up. Not sure how silimat the om621 pump is to the early om617pump, but on the 617 there is also the external brass check valve in addition to the internal ones. Does yours have this? And if so, are they good.

just seen that. lets try again.... i do not see any check valves external to the pump. the only thing i can think of is if the bolt that holds the diaphragm to the "roller lifter" isnt long enough or not spaced correctly..........ugh

wait, are there suppose to be bolt heads or something on that rotating plate? the om617 engines have bolts on top of that plate and im wondering if theyre what actuates the diaphragm in and out? it also looks like the om617 and om621 pumps could possibly interchange??

It looks like your injection timing device is missing the vacuum pump drive plate. There is no chance that the pump is going to work with that part missing.

https://dq4zp01npifg0.cloudfront.net/sites/default/files/imagefield/product/dslvacpumpdrive.jpg

That looks like a om617 engine, I can't find any photos of an Om621 with the pump removed....

617 or not, if you don't have the rollercoaster cam attached to the injection pump, there's nothing to cause the cam follower to oscillate on the vacuum pump.

And looks like the timing device is worn as a result of the VP bearing having to slide along it. Hate to think of the swarf thats put into the oil...

I know, but where on earth did it go (previous hack got rid of it?), and what does the roller-coaster cam look like on the Om621, can only find 617 stuff still. Factory service manual is lacking in this area

I'd be willing to bet they're the same part numbers. Remember the 617 is just the bored, stroked and extra cylinder version of the 621.

Good now I just need to find one! Which is usually the hardest part in fixing these cars :(

Take a good look at the picture of your 621 timer. I am not seeing any bolt holes with which you could bolt on a Vacuum Pump Cam Drive Plate.

I found my Book; Service Manual Maintenance, Tuning, Unite Reploacment Diesel Engines OM 636 and OM 621.

None of the pictures in the book show a Vacuum Pump installed and none of the pics of the Timer show bolt holes for or the cam to actuate a vacuum pump.

That is strange..... How else could a diesel have power brakes beside hydroboost??

Gassers use manifold vacuum. I believe your Engine has a similar butterfly valve that creates manifold vacuum used for the Pneumatic Governor. That could also be used with the power brake booster if the manifold vacuum is high enough.

It is also possible that that Engine did not come with the Car. The pictures in my book are from 190Ds. Did 190Ds have power brakes?

I mentioned this in a post recently - there's no way to have high vacuum in a diesel and have it still have enough air to compress to run - if there was we wouldn't need VPs in the first place.

I do suspect either the engine was one which never came with a VP originally or the timing device.

You have a good point. After an extensive google search I can't find any references for a 1966 W110 200D having a brake booster. There is the possibility one was swapped in?

From the EPC diagram below, it looks like the vaccum pump is driven by a bushing that acts in concert with two springs.

Sorry I don't have any more info for you.

See if this helps. Looked up the same thing that Alec did, sometimes you have to look under weird things in the EPC to find what you want. Part numbers are shown on the left side. It looks like there are 2 different timing devices available.

Is there a way to use the vehicles VIN Number to find what Engine Number range is supposed to be in his vehicle?

ipernity: Throttle and venturi of a Mercedes-Benz diesel engine - by Michiel 2005

"From 1936 to 1979 Mercedes-Benz used a pneumatic governor on its diesel engines for passenger cars. The pneumatic governor used a throttle and a venturi in the air intake to govern the speed of the diesel engine."

If you look at the pic if the site you will see the Venturi in the Intake Manifold. However, if you look at the pic in the site you see the Venturi and the Butterfly Valve.

The Governor actually runs off of the Venturi Vacuum. However, it is hard to believe that with the Butterfly Vlave partly blocking the air way there is no vacuum in the Intake Maiflod beyond the Venturi Vacuum.

Also you can have a partial vacuum and still have Air Flow. But, I have not ever read anything one way or another on how the vacuum effects compression in Diesel Engines.

Just a random thought, could you have inadvertently installed a check valve (or two) backwards?
That's something I could see myself acomplishing.:eek:
It could account for your symptoms of no vacuum after a rebuild of a previously working vacuum pump.
Do you know if the pump ever produced vacuum before you rebuilt it?

I really don't know what " of mercury etc they use to work. But no way a diesel which relies on compressing air to ~1/21(th?) of its original volume would work with a throttle plate blocking most air getting in.

We already worked out why he has no vacuum :rolleyes: Next mystery is why there's no ramp on the timing device....

There is a Butterfly Valve/Throttle Plate in the manifold of this particular Diesel.

It also takes X amount of heat to ingited the injected Fuel. However, that heat does not have to come entirly from the heat of compression the compressed Air.

I don't have the specific starting instructions for a 200D. But, I somewhat remember what people said and last night I read the instructions from another Diesel Car that has a Pneumatic Governor and the instructions were the same as people on the Forum have said.

You pre-glow then you step on the Accelerator Pedal (that would open the Butterfly Valve/Throttle allowing plenty of air to pass buy) and crank the Engine.

So we have the Glow Plugs and plenty of Air for Compression. At the same time the Pneumatic Governor in the face of no venturi vacuum (because of the wide open throttle) is going to go to full fuel so that you have plenty of Fuel.

When the Engine is going good you can take your foot off of the Accelerator Pedal and the Throttle Plate obviously partly closes to some idle position. Even though the Throttle Plate is mostly closed there is apparently enough Air and therefore enough compression to keep the Engine going at idle.

When you accelerate you open the Throttle Valve and remove the restriction to the Flow so you have plenty of Air for compression.

Once combustion starts in the Precombustion Chamber I bet that Ball Pin that the Injector Sprays down on gets hot really fast (and stays hot as there is limited area for heat to conduct away from it). That means once that Ball Pin is hot if any raw Fuel sprays on it the Ball Pin itself is sufficient to ignite the Fuel. If the compression was for some reason low once the Ball Pin is hot enough the Fuel is still going to ignite.

So that is my understanding on the above stuff.

In the past someone also mentioned that direct injection Engines have about a 17 to one compression ration and that the mercedes had a 20+ to one compression ratio. That would seem to indicate that 17 to one compration is sufficient to ignite the Diesel Fuel and if the Mercedes suffered some compression loss it could drop some and still work.

Maybe someone with a 200D or 240D that has a Penumatic Governor on it can disconnect the Vacuum Pump Connection at the Intake Maifold and connect a Vacuum Gauge to that port and we can find out for sure if there is any Vacuum in the Manifold of those particular Engines that have a Throttle Plate in the Intake.

I a curious to know that since this particular Engine does not seem to be setup for a Vacuum Pump if the Intake Manual has the port open where intake hose on the Vacuum Pump attaches.
If there is no open port on the Intake Manifold I would belive that that patricular Engine was not made to have a Vacuum Pump.

When looking up the 621 and 636's, it seems that they were very common in watercraft, probably more common there than in automotive applications and obviously have no vacuum pump. The timing device is different for the various uses, it would be interesting to know if this car has a watercraft engine or timing device fitted. Would certainly explain a lot...

Wow this thread really blew up! Awesome information for me, the more technical the better. I specialize in repairing modern European cars and diesel trucks so I'm basically figuring out the mechanical operations that modern cars accomplish by computer systems. Let's give an update, I ordered a injector pump gear with vacuum pump cam actuator online. Once it gets here I will scrutinize the 2 parts to confirm they are similar in design.

As a side note, I shipped my brake Booster out to pwrbrakes.com and they are just about done rebuilding that.

Where could I find any identifying casting numbers?

If there's a venturi, that will provide the vacuum required without having to cut flow off dramatically. Remember if you cut 1/2 the airflow essentially you're cutting the CR down to 10:1. Now if these engines started and ran with 10:1 compression with working glow plugs yadda yadda they wouldn't build them with 21:1.... that's my understanding of it anyway.

Only trouble with disconnecting the injection pump pipe from the manifold is that it will run away once started, so would have to be a Tee. I might try it one day on my Granddads '75 HJ45...


Block casting number is likely to be behind the IP. Should be 621 918 apparently

I think you guys are mixing up compression ratios and air fuel mixtures. The compression ratio isn't a dynamic number (except the new Nissan variable compression engine). When you momentarily block off an intake, the engine will only run richer due to the remaining air being used up faster then it is replaced. The engine will then stall once the rotating inertia isn't enough to overcome the vacuum in the manifold, or when the mixture is too rich and is unable to burn. Modern diesels have throttle bodies on them for emissions reasons, in certain drive cycles the TB will partially close to better have egr gasses pushed into the intake

I am wishing someone would actually hook a up vacuum gauge; just curiosity.

I will when I get everything situated with the vacuum pump

When you do test it, connect the vacuum gage directly to the pump source, with nothing else in the car connected. That is the only way to know if the vacuum pump alone is not doing its job.

The dynamic compression ratio is dependent on volumetric efficiency, as normal static compression ratio is based on a full cylinder worth of air (I should have been clearer). As soon as your cylinder has less than that (which it almost always will to a certain degree as our engines aren't anywhere near 100% VE NA) the effective ratio decreases which is my whole point as restricting too much air into a diesel will stop it working. Nothing to do with A/F ratios as we know diesels will still work spewing huge amounts of smoke :)





....(sorry to digress)

Just came across this thread. I am very familiar with this engine so I can shed some light.

First, if the picture you posted early on showing the front of the engine where the vacuum pump is attached is of your engine, then you are definitely missing something. The front of the injection timing device needs to have the roller coaster track in order to run the pump. The track is integral to the visible part of the timing device, not bolted on. The ball bearing rides on the track, which in turn moves the diaphragm up and down pumping air. The two check valves in the pump keep air flowing the correct direction, from the brake booster to the intake manifold. Thus they must be inserted correctly so the air flows in that direction.

Second, early 190Dc cars (OM621.912 engines) did not have a vacuum pump. All 200D cars (OM621.918 engines) did. The change was done in the mid 1963 model year. This is because early cars did not have front disc brakes, but had drum on all four corners. No power booster was used on these cars so no pump. In place of the pump was a cover, and most likely there was no roller coaster track as well. Front disc brakes may have been an option up to 1963 but not sure. So either your car does not have front disc brakes or the engine is probably from a 190Dc (or other vehicle whether it be a truck, boat, or power unit). The engine model and serial number is stamped on the block and ID plate at the top left rear of the engine looking from the driver's seat.

As for the manifold vacuum, yes there is a butterfly in the intake manifold to create a small amount of vacuum to control the vacuum operated governor. When closed it pulls vacuum to reduce fuel injected into the engine by the injection pump. As the throttle opens, the vacuum is reduced and more fuel is injected. This is unique for diesels as the time as most did not use a butterfly or vacuum and fuel was controlled at the pump itself mechanically. The amount of vacuum created by the engine with the butterfly is closed is not enough to power the brake booster and provides very little braking assist. And as soon as the butterfly is opened up a little, then the boost is back to zero and you have no boost. You can trust me on this as I have had first hand experience.

So if you do have front disc brakes, you do need the vacuum pump and you need the parts to fix the injection timer to add the ramp. If you do take the timer out, make sure to install the small bronze washer between the timer and engine block/injection pump drive shaft to space the timer and sprocket correctly with the other sprockets in the timing chain system. Of course the camshaft and injection pump will need to retimed to the crankshaft.

Thanks for your experience that provides clerification of the manifold vacuum issue.