Hi Wise Men,

I need some strength and guidence with a sucky problem! I have a 1991 350 SDL in wonderful condition - except that it suddenly has the following symptoms:

1) Transmission shifts hard in all gears. Sounds and feels terrible, like I need a new Tranny - makes me not want to even drive it for fear of breaking it worse.
2) My ventilation system works, except that the air only comes out the defroster outlet, not any of the other vents.
3) The passanger vent blows only cold air, even when the heater is on full.
4) When tunring off the ignition, the car sounds ever so slightly like it is "Dieseling". In other words, it does not turn off as quickly as it used to, and seems to sputer for a revolution or two more than I remember.

But... my power brakes AND my power locks seem fine.

I am thinking - vacuum leak? right?

I am awaiting a "MityVac", which I am hoping will help me diagnose the problem, but I am hoping that someone may recognize these symptoms and point me in a place to look. I think there is at least some suction coming from the vacuum pump oin the front of the engine. Best I can tell, if I pull off the little line (not the one to the power brakes) I can actually feel a little sucking on my finger. (oooh yeah!! cheap thrills!) I am assuming that the "Vacuum Pump" is this the only source of vacuum on this Diesel engine? Is this MityVac the right tool for troubleshooting and fixing this issue?

Unfortuanatly, I have not a clue how to troubleshoot past this point. I looked for obvious signs of brokenness or disconnect in the vacuum lines, but can not seem to see anything obvious.

With all these vacuum systems failing - less the brakes and power locks - where would I even begin to look?

PLease advise of guru's of the Benz.

This car is a very special one, being my Vegetable Car and all. Any advice, pictures, diagrams or Glossy Photos will be appreciated.

Thank you!

Stevels

__________________
Providence, RI
Current Fleet:
1991 MB 350 SDL running 2 tank Frybrid Waste Vegetable Oil
2005 Toyota Prius
1997 40' Bluebird Wanderlodge motorhome running running world's first 3 tank Frybrid Waste Vegetable Oil

Usual problem encounted after an oil change service...someone knocks/removes one of the vac' hoses/lines.
The reason the engine will not shut off quickly is due to low vacuum......somewhere something is stealing the vacuum or the pump is not putting out enough.


.

__________________
[http://languageandgrammar.com/2008/01/14/youve-got-problems-not-issues/ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

Vacuum operates or influences #1, #2 & #4.
#1 - vacuum 'softens' the shift.
#2 - vacuum allows switching from the default, unpowered, no vacuum setting of Defrost. Oh, by the way, warm air does NOT come out of the center vents, only cool or cold air. Warm air on the head and feet, cold air on the face for a while until the cabin cools down, then up the Defrost vents to avoid a chilly draft...
#3 - see below
#4 - a vacuum diaphragm pulls the fuel rack to cut off the fuel, stopping the engine. A leak will slow or stop vacuum buildup and thus slow, or prevent the fuel shut off.

As for #3, vacuum is not involved. The choice of air for rear passengers from the center vents is cold, cool or none. Heat for the rear is provided from very shallow, flat ducts under driver and passenger seat, which blow out through a gap in the carpeting. Don't give in to the 'bling' of plush, oversize "MBZ" logo floor mats for the rear, or you will cover these openings and the rear passengers will get cold...

Right! Only question now is, where? The vacuum components do not require a huge quantity of vacuum, and so your brakes may have enough even with a small leak. That is a Good Thing from a safety point of view.

Breaking News! The door and trunk locks do operate on vacuum, but have thier own separate electric vacuum pump in the trunk, under/near the spare. If you listen carefully, you can hear it cycle when you lock/unlock the car.

Sounds like the vacuum pump is good. The leak is probably somewhere 'downstream' in this 'little line' and you and your trusty companion MityVac will have to go find it

Yes, you will use it to apply vacuum to the 'little line' to see if it leaks down, and how fast. Then, you will trace the line, unplugging various components, plugging the connector, and applying vacuum the them to see if it still leaks. You will also test the component that you disconnected.

One by one, you will eliminate the items until you find a line, connector or device that won't hold vacuum. Sort of like testing electrical circuits, but no sparks if you mess up!

Yeppers, that vacuum pump is the source for everything except the door and trunk locks. On a gasser, engine manifold vacuum is the source. Diesels run unthrottled, so almost never develop a manifold vacuum, so M-B provides a vacuum pump as a source. The gasser and diesel transmission, climate controls and other sytems can then use a common design, with vacuum for their operating power.

Good Luck.

Jim H,

Thank you for the wonderfully written and informative response! I wish you were close to Providence and would work for beer!

I will get my MityVac and poke and prod as best I can.

Thanks again!!

Steve

__________________
Providence, RI
Current Fleet:
1991 MB 350 SDL running 2 tank Frybrid Waste Vegetable Oil
2005 Toyota Prius
1997 40' Bluebird Wanderlodge motorhome running running world's first 3 tank Frybrid Waste Vegetable Oil