Greetings all...new to the site and hoping to glean some knowledge as well as contribute where possible.
I have an 87 300D with ~400K (odometer stopped at 323K four years ago). A few months back I had it in the shop and after getting the car back I noticed a significant lack of acceleration and mid-range punch. Pedal was to the floor and could barely approach 80mph. A couple days before I could easily hit 100 and beyond. I called the shop and asked if they had adjusted anything (this was unrelated to the reason I dropped the car off) and was told a "vacuum line going to the transmission" was disconnected so they connected it. I try to do most repairs on my MB but am still an admitted novice, and I'm not sure how/why this would have such a drastic impact on performance. Does this ring any bells? I've been lurking around the threads about the ALDA and I'm having similar performance problems...just not sure why the change would be so drastic if it's ALDA-related. Thanks for any help you can provide.

The only vacuum line that I know of that would go to the transmission would go from the VAV to the transmission modulator. That would affect the quality your shifts. With it unhooked you would get smooth up shifts but would get really clunky downshift. I do not know how any of this could affect the speed on your car. Do not know how hooking up that vacuum line could cause this problem.

Thanks much for your reply. Come to think of it, the shifting was a bit smoother post-shop. I need to find a vacuum line schematic for the car, which may help me at least eliminate some of the possibilities. This does appear fuel related...I may see whether shimming or removing the ALDA helps. Thanks again

Tops out at eighty probably is worth concentration on the boost circuit. Especially if it is sluggish in getting to eighty. You will find a line that goes from the intake manifold to an overboost protection device on the firewall and then to the alda.


This is not a vacuum line but a pressure line. If it is disconnected or pinched or blocked. I would expect your complaint. Now if the car was normal for some time after it was picked up. The fittings and the overboost device can soot up to the extent they are internally blocked.


A fairly common issue and you have enough miles on the car it should not be overlooked. In that case it would just be a coincidence that you had developed a milage developed failure shortly after getting the car back.


Welcome to the site incidentally. You will probably never be the same.

I believe this may be the diagram for your car:
OM603 Vacuum & EDS Diagram

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

It is also called a switchover. Will have two or three nipples and one of them is always vented to the air. Provides overboost protection to the ALDA. However when I replaced mine on a 300SD I got the hoses mixed up and could not get over 30MPH. Switched the around the right way and all the power came right back.

So I do not see how you could get to 80mph with a switchover problem; maybe so if one of the hoses is partially blocked. Or one side of the switchover got partially blocked.

The boost signal to the ALDA is shared with the transmission vacuum amplifier, the round blue thing by the glow plug relay which is also called the blue flying saucer or BFS. If the line comes off or if the BFS leaks, you won't get a proper boost signal to the injection pump. Maybe the line was capped because the BFS leaks and reconnecting the line reintroduced the leak.

Sixto
83 300SD
98 E320 wagon

See attachments.

Attached Thumbnails

         

Thanks all for the helpful responses, including the diagram and pics. I'll go through the items mentioned and report back on any improvement.

I would personally just run the boost line directly to the ALDA. Everything (boost signal, BFS, OBPV, etc) has to be working perfectly in order to have good power. Having a direct/clean boost signal from the manifold to the ALDA is the goal in my opinion. A marginal or inoperable unit in the system will diminish performance. When new, all the components worked together to provide smooth power and shifting. In your 25+ year old car, chances are one or more units have failed. I also like a firm shifting transmission. I feel it will last longer. The BFS modifies the vac signal to the trans modulator and ALDA to provide a smooth shift. It also dumps the boost signal before shifting to lower the power from the engine. Again, it is all well and good if everything is new and/or working properly......Rich

__________________

All Diesel Fleet
1985 R107 300SLD TURBODIESEL
2005 E320 CDI (daily)

LOTS of parts for sale!


EGR block kit
http://www.peachparts.com/shopforum/diesel-discussion/355250-sale-egr-delete-block-off-plate-kit.html

1985 CA emissions 617 owners- You Need This!
Sanden style A/C Compressor Mounting Kit for your 616/ 617 For Sale + Install Inst.

Sanden Instalation Guide (post 11):
http://www.peachparts.com/shopforum/diesel-discussion/367883-sanden-retrofit-installation-guide.html

Tightened the boost sensor (quite loose) and the fitting (nearly plugged), but the car remains quite sluggish to 60 and above. Anyone not having driven the car prior to this issue would think there's nothing wrong; just a normal, slow diesel. On to the switchovers and checking the boost circuitry for impediments. Thanks again for your advice.

*Temporarily* jump your manifold port straight to the ALDA and see what happens.

Just be sure you're not confusing the ALDA (square can on top of the IP) and the shut off (round can near the rear of the IP).

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Bypassing the split to the BFS.

Sixto
83 300SD
98 E320 wagon

Yes take one fresh piece of hose, with fresh rubber pieces, and go directly from the boost tap on the manifold to the ALDA. Do not pass go, do not collect $200...

Note your transmission will shift funny without the BFS connected. We are trying to establish why you lost your boost.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Check condition of all vacuum lines carefully. Flex them and check for brittleness. Check all soft-rubber connections to vac lines for a tight fit and flex them, checking for holes and splits- often these won't be easily seen unless you physically manipulate the line.

I had similar issue to yours and the problem was hidden splits in two flexible connections in the ALDA vacuum circuit.