Why do diesels need a vacuum pump?
 

Does the engine not make enough vacuum?Gas cars usually tie into the intake for the use of vacuum needs ,Why does the diesel engine differ?

diesel engine has .........
 

no throttle. ergo no vacuum.

plus a mechanical vacuum pump lasts longer than an electric version... reason for vacuum actuated.. everything on the W123... just look at the W116's servo control AC.. the servos fetch big money if OEM replacement parts still in the box

You would have to hook up before the turbo, or onto the air intake for vacuum, but the air intake.

Probably on a non turboed car you could tap into the intake like a gasser.

Now I'm curious to see if I could make a vacuum device work like a siphon type like on a garden hose sprayer.

Hmmm....

A generic answer without a lot of details........
 

Gas engines have to generate a vacuum in order to overcome the throttle of the intake. This determines their air/ fuel mixture.

Diesels generate little to no vacuum in their intake, if fact, most of the time there is pressure. They use as much air as you can pump into them. That is what makes them a more efficient design.

Yeah, due to the restriction in the airstream in a gasser, you get vacumme unless you are flooring the pedal. However, high performance motors, like a race car, the cam profile is too large, and the motor makes poor vacumme. also in a gasser, vacume aids in the evaporation of the fuel spray, to get as much fuel atomized for a more efficent burn. a Diesel has it's fuel directly injected into the cylinder at huge pressures, into highly compressed air. this ignites the fuel as it is sprayed in, no mixture or evaporation needed.
basically, unless there is a butterfly flap to restrict air as it is pulled by the pistons, there will be no vacumme to speak of. the intake is open completely to atmosphere.
John

Boy do times change.....!!!
 

How about a pressure in a Turbocharged Diesel!!!

My first car had vacuum operated windshield wipers, as does my ol' power wagon.....I wonder how many of y'all have ever had to deal with gettin' off the gas to increase wiper speed.......:D

SB

Dunno about the vacuum wipers but my 69 Lincoln had hydraulic wipers that ran off of the power steering pump. You did not want the wiper blades to hit your hand. They did have some real power.. :o

Hows this for going off on a tangent....
 

Our cars breath in a sort of reverse aquarium

It's funny how people think of vacuum or "suction"... The pistons don't really pull air in through the intake. It's the weight of the atmosphere *14.7* lbs\sq inch that pushes the air in to fill the non-air bubble or low pressure area. A Diesel will produce a small vacuum and it will build as RPMs increase, but as stated it is not enough to power things.

I always like to think of the benefits of a supercharger or turbo in this way. As the cylinders request more and more air in the same give time the atmosphere can't push it in through the intake plumbing fast enough. By the time the intake valve closes more air is depleted then can rush in. sort of like pushing allot of water through a small pipe or high amps through a thin cable.

Turbos can help to over come this... even if the pressure never exceeds the relative atmosphere, power builds as the cylinders fire twice as fast in the same time, yet can't get as much air as it could without the forced induction of air. Remember the equation work = weight moved over time.

Got a reference for that 32 psi figure?

If atmospheric pressure was that high, we would not need turbochargers!!!

But you're right, it's funny how people think. And sometimes even funnier what they think.

Actaully it 14.7 I think right one atmosphre..Right

Yep it is 14.7 sorry for the disinformation in my tirade...
http://en.wikipedia.org/wiki/Atmospheric_pressure. I thinks it's water presure that build at that rate every 10 meters or something.....I forget

Actually we would still....need turbos ( I think) It wouldn't matter it's all relative to the surrounding pressure.
That's a good one gota think about that one...

And we would be fighting to over come the increased air resistance. The drag Coefficient increases with the sq of speed. Four time the drag on a car doing 40 as doing 20 not twice as much.

But what the heck are we talking about.... Ha

Some new diesels are now throttled, but not to the same extent as a gasser. Fuel mixture is now just as crucial as a gasser for emissions. They throttle the intake to create some vac to pull the EGR gasses through.

You know, its interesting, the 617.912 seems to me to be a 616 with an extra cylinder. No other real differences other than firing order and timing of course to account for the 5th cylinder. Now heres the interesting part, my 80 240 had a throttle plate, the 617 I put in doesnt. They both share the same kind of injector pump operating in the same manner, so whats the point of the throttle on the 616? Or was that some kind of emissions brainstorm?
-Chris

I used to drive my old boss's 57 fairlane, the old doublebarrel shotgun tail light v8 312... vacuum door locks and wiper motor, something else was vacuum too... don't remember though. that car had rear pump in the auto tranny too. my boss loved to roll it down the hill to speed start it. he liked to save the load on the starter... riot.
John

It is all relative. There are two different ways of reading pressure in our atmosphere, absolute pressure and gauge pressure. Absolute pressure takes into account the pressure of the atmosphere. So if you said your turbocharged diesel was getting 25 psi at the intake, you'd be correct, in absolute pressure. Gauge pressure is pressure relative to the outside of the gauge. 99% of the time this means the difference between the pressure inside a system and the pressure outside, which is usually the atmosphere.

Not quite...
 

There is some vacuum in the intake manifold or the air would get sucked into the engine for combustion on the engines without turbos... but you are are entirely correct that the lack of a throttle valve [as ther is in gas engines]... the lack of such a valve means there is much less vacuum [almost none at idel ]. IF you were to monitor the vacuum in the intake manifold of a naturally asperated diesel, you would NOT see the very distinct fluctuations as there are in a gas engine.

Possible Sidebar Reason - The 1930s/1940s generation of American automatic transmissions that MBZ copied in the 1930(s) when they first started building "slush trannies", well these were for gas engines and they were designed to shift based upon that same dramatically fluctuating intake manifold vacuum which is also a very good way to both measure the load on the engine and the "go" requests of the driver which needs to be communicated to the tranny.

Thus the MBZ's engineer's ingenius design for a vacuum control system on their diesels which evolved over the years up through about 1985.
Another Reason - This also allowed them to use basically the same trannies on GAS as well as Diesel engines. Much of the changes in the latter 1980(s) resulted from the engineers trying to appease the California State environmental "wackoes". If you don't believe me, take a look at the diagram for the 1985 California diesels at: http://www.peterschmid.com/vacuum/1977_1985/617_95/1985_cal.jpg.
Thanks goodness they never made a "Federal" version of this system... at least I don't think they did?... in the SDL(s)???
Regards,

The butterfly valve is for emissions. It restricts the airflow creating a pressure differential to make EGR gasses flow into the intake. Disable the EGR and get rid of that butterfly valve.

Yes, I remember seeing this on Diametricalbenz's 240D w114. Been wondering why it had a throttle plate, when diesels ran on fuel input and not on throttle (for lack of a better term)

He was talking about his 80 (W123) 240D. On the preW123 bodies, it's there because the injection pump is pneumatically governed instead of mechanically like our 617's.

200D, 220D, and early 240D tuning guide.

Why do diesels need a vacuum pump?
 

I find it hard to believe where the POSTs on this THREAD have gone when the original question was a perfectly GOOD one that I'm sure many have wondered about !...
I'm out of here!

Merry Christmas !

Regards,

I own an 84 190d ,the 4 cylinder runs great but the vacuum pump has long gone.When test driving it last week I kind of found this out by emergency needs,the guy selling it failed to let me know.Ive gotten use to breaking hard and can live with that for the mean time its the turning the car off that im trying to remedy .The line from the key to the IP has been guided via anothe rhose up to my console and when I need to shut it down i pull vacuum with my mity vac .Vacuum dosnt seem to happen except in very small amounts,the intake is were you can get it from a gasser.I was considering teeing into the line right before the windshield wiper reservior,that has to create vacuum ,how much is the question.The car performs well just looking for a cheaper fix for now.

I knew our diesels needed a vacuum pump because the there was no vacuum generated (or not enough) on the intake manifold, but never really knew why This post answered the *why* in my opinion quite well. Anyway, what's the big deal with a little shop talk?

Heres a ghetto fix for the shutoff that I discovered when looking at a 300D to buy - The guy had taken a length of wire, tied it to the shutoff lever on the injection pump, ran it around the front of the battery and into the passenger compartment through a grommet by the glove box. The he would hak on the wire to shut it down. I had to pass on that 300D, there were tons of "creative" repairs like that.

rebuild kits for VPs are not terribly expensive. I think you can damage the brake booster if you drive much with out vacuum. also vacuum is needed for the automatic transmission to shift properly. door locks and engine shut off as well.
John

As an interesting ( at least to me) note....
Ford did exactly the same thing. Their automatics--C6 and others used engine vacuum to signal the tranny when to shift.
When they installed a C6 tranny in a diesel pickup, they needed an "Transmission vacuum modulator" to convert the vacuum from the pump to a proper signal for the tranny. Its basically a variable bleed that reduces the vacuum to the tranny when the throttle is suddenly opened. Works well when it works. If it breaks, all shifts are like at WOT, meaning high in the engine's rev range and quite firm.
Not many Ford-Mercedes similarity, but this is one.

ditto GM with their infamous 5.7L motors VP was driven off the cam, the pump looks like an HEI distributer without all the wires....
John

Why do diesels have vacuum pumps? Well, not all diesels do have vacuum pumps but the ones that do generally have the pumps to provide vacuum to operate a myriad of control functions and accessories. Vacuum would seem to have been considered more reliable than electronics, etc. in engineering some vehicles from the not to distant past. MB liked to use vacuum for brakes, transmission control, emissions control, locks, ECC functions and shutting the engine off. Other manufacturers have used vacuum for the above stuff plus wipers, headlight doors, etc. RT

I agree with some of the others... this THREAD is too laid back... you should take your very real vacuum problem and make it the subject of a separate THREAD... one titled something like... Vacuum Shutdown Problem: YR/Model of car. While on the subject, also include the YR/Model in your signature, something like what I have shown a the bottom of this POST... AND don't forget to POST on this THREAD what the THREAD is that you stard
Regards,

Don't forget the 6.2/6.5s, they're ran the same way-the early ones like mine just had a diaphragm sticking up for generating vacuum, then they went to a belt driven one on the later ones and used the "distributor" for an engine speed sender.

Uh-oh, thread hijack alert!!!! BY ME!!!! They have vacuum pumps so they can power all those things that leak and drive the owner nuts!:silly: Like door locks, climate controls, vent pods, anything else on a Mercedes!

Double Hijack! My '93 6.5TD Chevy has no vacuum pump! Stick shift, servomotors for the accessories. Last year without computers, all mechanical. I love it. RT

Thanks for the clarification, I'm not yet well versed with the 240D's (114 or 123) :o

I would be extremely hesitant to continue running that engine if the vac pump is/has died, they are driven by the timing chain and are notorious causes of sudden timing chain breakage failures, from the pumps coming apart when they fail, causing severe engine damage.

Forcedinduction, Not to worry my friend, that 616 engine with the plate had #1 rod sticking out the side when I got the car, so all that stuff is out now anyway. The 617 is happy in its new home.
_chris 80 240 w/617 engine