Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help

Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Diesel Discussion

LinkBack Thread Tools Display Modes
Old 12-20-2006, 12:36 PM
Registered User
Join Date: Dec 2005
Posts: 2,360
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?

Reply With Quote
Old 12-20-2006, 12:38 PM
junior member
Join Date: Oct 2001
Posts: 592
diesel engine has .........

no throttle. ergo no vacuum.
Reply With Quote
Old 12-20-2006, 12:39 PM
TheDon's Avatar
Ghost of Diesels Past
Join Date: Oct 2005
Posts: 13,282
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
Reply With Quote
Old 12-20-2006, 12:43 PM
rrgrassi's Avatar
mmmmmm Diesel...
Join Date: Jun 2006
Location: Royse City Tx
Posts: 5,122
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.


70's Southern Pacific #5608 Fairmont A-4 MOW car

06 VW Jetta TDI 133K miles-replaced the other '06 that quit after 300k miles. (did not catch the bad cam in time)

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 195K

90 Dodge D250 5.9 Cummins/5 speed. 400K
Reply With Quote
Old 12-20-2006, 12:47 PM
Registered User
Join Date: Jun 2004
Location: West of Ft. Worth. TX
Posts: 4,186
Wink 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.

84 300SD 350K+ miles ( Blue Belle )
Reply With Quote
Old 12-20-2006, 01:17 PM
vstech's Avatar
Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,486
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 HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
Reply With Quote
Old 12-20-2006, 03:23 PM
Shorebilly's Avatar
Marine Engineer (retired)
Join Date: Aug 2006
Location: West Virginia
Posts: 1,268
Boy do times change.....!!!

Originally Posted by tangofox007 View Post
Diesels don't have throttles to restrict air flow into the intake manifold. Therefore, the is not much vacuum in said manifold. Even less in a turbocharged diesel.
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.......


'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG
Reply With Quote
Old 12-20-2006, 03:31 PM
Registered User
Join Date: Aug 2005
Location: Buford, GA
Posts: 438
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..
87 300SDL - 215K Miles !!
99 F-350CC Dually PSD - 190K
86 300SDL - 189K
All on B-100
Reply With Quote
Old 12-20-2006, 04:00 PM
Registered User
Join Date: Oct 2003
Posts: 237
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.

Last edited by 97E300D; 12-20-2006 at 04:18 PM.
Reply With Quote
Old 12-20-2006, 04:06 PM
Registered User
Join Date: Sep 2004
Posts: 8,933
Originally Posted by 97E300D View Post

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 32lbs\sq inch that pushes the air in to fill the non-air bubble or low pressure area.
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.
Reply With Quote
Old 12-20-2006, 04:08 PM
Registered User
Join Date: Oct 2003
Posts: 237
Actaully it 14.7 I think right one atmosphre..Right
Reply With Quote
Old 12-20-2006, 04:11 PM
Registered User
Join Date: Oct 2003
Posts: 237
Yep it is 14.7 sorry for the disinformation in my tirade... I thinks it's water presure that build at that rate every 10 meters or something.....I forget
Reply With Quote
Old 12-20-2006, 04:24 PM
Registered User
Join Date: Oct 2003
Posts: 237
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
Reply With Quote
Old 12-20-2006, 04:43 PM
rg2098's Avatar
Detailing Moderator
Join Date: Jun 2004
Location: Rochester Hills, MI
Posts: 2,415
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.
Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section
Reply With Quote
Old 12-20-2006, 10:24 PM
Mustang_man298's Avatar
Man of the fire
Join Date: May 2006
Location: Shingletown,Ca
Posts: 941
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?

Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On

All times are GMT -4. The time now is 10:54 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2022, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page