Just drove my '87 300TD wagon from Los Angeles to Lincoln, NE where my wife and I recently moved. We cruised at 85 mph the whole time with good power and shifting. The car began to struggle at high altitude (9000 ft.+) going through Colorado. Accelerating off the line was pitiful although highway speeds were possible once it got going. This also happened on previous road trip through the same area, and once we got below 3000 ft. everything was fine again. However, this time the sluggish acceleration never went away, and our cruise speed of 85 mph was only possible by keeping the car in 3rd gear at 4000RPM going through Wyoming into Nebraska on I-80. The next day was even worse...super slow acceleration and pedal-to-the-metal I could only achieve 65mph in 3rd gear with a quite a big of shift flare. Towards the end of the day, I was struggling to reach 55mph as we limped into Linclon, NE.

I found a torn 90-degree rubber vacuum fitting with the black hose disconnected on the VCV. I tried pushing it back in, and didn't notice any improvements. What do you guys think? Fuel filters? Did my ALDA valve get stuck and the car still thinks it's in the mountains?

When I floor it, I don't get any noticeable black smoke out of the exhaust. No oil is being consumed. It feels like a fuel-lean scenario, but I'm a little perplexed on where to begin....hence the mechanic recommendation! Please advise...

Cheers,
Marc

Fuel filters and air filter would be a good start. Mann, Hengst and Mahle are all good brands.

Pre-fill the main filter with clean diesel fuel before installing the bolt. Hint: this looks like a spin-on filter, but you loosen and remove and install via the 17mm bolt on the top of the filter holder. When you take it off, you may spill some fuel. Put the new filter in place, fill it up via the hole where that 17mm bolt was (its hollow and has a couple o-rings that you need to replace, they should come in the box with the new fuel filter). Be sure the fuel you fill it with is clean - you are bypassing all the filters, so any contamination could damage the injection pump and the injectors. I like to remove the return line in the engine bay and route that into a clean container, then start the engine and use the filters in place to produce some clean filtered fuel.

If new filters don't do the trick, you may have a clogged strainer in the tank. Fill the tank, and reverse the fuel lines in the engine bay to bypass that strainer, and see if performance is restored.

Last would be an ALDA adjustment, they do tend to lean out with age, but such a dramatic change during a drive may mean that the ALDA has failed. Finding a replacement may be tough, they are not sold individually by MB, you'll either have to find a used ALDA or find a Bosch service shop that can sell you one.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

The banjo bolt on the boost sense line of the intake manifold gets clogged with soot.
This prevents the ALDA from sensing boost and functionally turns your turbo off.
It's a simple DIY job.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

I wholeheartedly agree with all of the above that Maxbumpo just stated. You may have gotten a dirty batch of fuel that clogged things up, possibly the strainer, primary and secondary fuel filters. It is easy to visually check the primary fuel filter, as it is clear and you can see if it is dirty just by opening the hood and looking at it. (Although I am thinking of my 300D. I am not familiar with the setup in the 87 wagon.) The primary clear plastic filter is cheap and it is a good idea to keep a spare with you just in case you are on a trip and get a dirty batch of fuel. My guess is that this is the problem given the sudden onset of the symptoms. Good luck. I will be curious to know what you find.

__________________
85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
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98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

That's also a good idea to check, cheap and easy. Not a bolt though, it is simply a fitting on the intake manifold that sends pressure to the ALDA via the over-boost protection valve on the firewall near the left front fender.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

First I'd take the over boost protection valve out of the loop. When the valve fails you loose the turbo. Can fail slowly. Also check the connectors for leaks.


Run the vacuum line directly from the manifold to the Alda connector. If the car runs better then leave it that way or replace the over boost protection valve. Like others note: clean the manifold / vacuum line connection out.


If no change then go for the fuel swap out.

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1986 300SDL 440,xxx

Good idea, just note this line is a pressure line, not a vacuum. The pressure line from the intake manifold is provided to the ALDA, which then increases the amount of fuel being injected to match the increased air from the turbo.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

If you have tools and time, remove the ALDA and see if that helps. At least you'll know where to focus.

If the ALDA and fuel check out, is it possible your oxidation catalyst is clogged?

Sixto
98 E320s sedan and wagon

I'd run through some biocide first. You may have gotten some fuel with bacteria in it. it will show up as black dots in your small plastic fuel filter. The biocide will kill the bacteria and render it small enough it will pass through the filter and be no further problem.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Thanks maxbumpo...pressure line! Good correction.


I think it will also dump boost pressure if the line, connector, or over boost valve is toast.


Definitely not a vacuum line as I misstated.

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1986 300SDL 440,xxx

Thank you all for the insightful responses! I can't tell you how much I appreciate each and everyone one of you for taking the time to write...because with your expertise I FIXED IT!!! (along with the help of an '87 300TD sedan at a local junkyard in town)

It ended up being a bad ALDA valve. Here's what I did...

1) Replaced both fuel filters in the engine bay with Mercedes filters. No change in performance.

2) Replaced the VCV valve I pulled from an '87 300TD sedan that had an intact rubber vacuum boot. Mine was torn, and while I'm not sure if the tear was causing any leaks, I wanted to eliminate it as a variable. No change in performance.

3) Filled the tank full and swapped the fuel lines near the left fender to eliminate the strainer filter. No change in performance

4) Pulled the vacuum (pressure?) line going from the manifold to the overboost protection valve on left fender / firewall and plugged directly into ALDA valve (and capped the two-way vacuum line I pulled off the ALDA valve). --Not sure if this is what @86-300SDL meant-- No change in performance.

5) I swapped out my ALDA valve with one I pulled from the donor car in the junkyard. And BOOM! It was like the car woke up from a coma! I noticed a difference as soon as I stepped on the pedal to pull out of the driveway. The car is fast again, and it is shifting normally (no more shift-flare).

The crazy part is that the junkyard only charged me $5 for the ALDA valve!

Thank you guys so much!

-marc

Excellent! That is a rare failure and not at all common.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC