I've had my 1987 300D for a few years now and it used to move down the road pretty well.
About 2 years ago, I noticed that it would start to lose power and shift much harder until a new air filter was put in, and then it would regain performance, but I would have to change filters 4 times a year.
I just installed a K&N filter, hoping that this was the problem, but getting it out on the road, I still can't get over 85 on the highway and the power is still less than what it should be.
I've heard of several theories, and had a mercedes mechanic look at it a couple of times over the last couple of years, but no resolution.
Anybody have any solution for this?

DUMP a whole container of Startron in the almost empty Tank.Fill Tank.Run.

O.K. I'll Bite.(Although your post reads like a "Spoof")
(Poor performance shouldn't cause harsh shifting)
(And changing an Air Filter Will not Cure shifting Harshness.)

How OLD ,DIRTY and Nasty is the Tranny Fluid?

1.Lose the K+N. The Factory filter does a much better job and will not allow damage to the engine like the K+N will.
2. How Many Miles? (on the Engine?)
3.What's the Compression Like?
4.At this alleged 85MPH,which gear is the Tranny in and What's the RPMs.
5.Four (4) Air Filter Changes Per Annum? 'You live in the Sahara?

Diesels are VERY simple:
Air
Fuel
Compression
>GO.

'Need More Information...Like what did This "Mercedes" Mechanic Say? Is She/He "Sighted" or "Challenged"?
[And what's her/his CV (Curriculum Vitae) ( Qualifications)?]

Reading between the Lines,It's possible to Prematurely "Wear Out" one of these engines By allowing a DIRTY intake.
[AND You've got Zomezing Going on with yours.]

__________________
'84 300SD sold
124.128

I am with you on this C.I.

If not, then welcome to the forum.
4 air filters a year has to be an exaggeration !! we dont replace them that quick on our earth moving gear !!

Members on here spot BS very quick.
Location?
other details in your signature /profile?

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Could be a clogged oxidation catalyst.

Sixto
87 300D

Sorry and thanks for responding,

My 300D has about 165,000 miles, and the trans fluid is about a year old because I had the shifting mechanism (forget the nomenclature) that connects to the trans changed at a pretty good expense when it was diagnosed as a trans problem.

I've noticed that vacuum issues can play havoc with this car, and by this I mean that the turbo won't kick in at normal times, the power feels drained and the shifting is sluggish and harder. When it acts like this, you have to let off the accelerator a little to get it to change gears.

I don't change the filter 4 times a year, but I would have to in order to keep the performance up. I know that these cars shift hard in normal conditions, but it's hard for me to believe that it needs to go to 4,500 rpm from 3rd to 4th before it kicks to the next gear.

Also, the kick down, when you floor the motor, runs at high rpm but kicks the trans down to lower gears so when you use the kickdown, it actually goes slower on the highway.
I don't know, what's an oxidation catalyst?
But my old mechanic worked primarily on mercedes and was regionally famous among owners because he was the only one around for miles, but the problem never really was fixed.
I've dumped alot of money on this car but have been stumped.
Thanks for any help.

As far as my profile, I didn't realize it was such an issue. I just registered because of this specific problem. I'll fix it soon as I figure out the site.

Thanks, please reference my reply below. Any help is appreciated.

Oh, the miles sit at around 165,000, the compression is between 2 and 3 on the gage and it's been tested, but I don't know the exact compression ratio, and the rpms will be around 3,000 to 3,500 while the trans is in 4th gear(?). I honestly don't think it's an engine problem, but a vacuum issue but I'm stumped.
My old mechanic was this guy out of Topeka Kansas who was pretty well known for his work, but he never really figured this out.
I read a post about an H shaped vacuum fitting that can be replaced with an F shaped fitting, but don't know the story behind that as the performance used to be much better when I lived in Colorado!
Thanks for replying.

Finally found it! After replacing the fuel filters, etc...and not getting any change, I started to dig into the vacuum system and found that the line from the switch over valve coming from the ALDA to the intake manifold was clogged. I cleaned out the fitting in the manifold and blew through the vacuum line and the switch over valve and took the car out on the road. What a difference! Drive's like a whole new car!

Great that you found the restriction in the boost line to the ALDA, that will sure make a difference. However I believe that the line in question is not a vacuum line but under pressure. Also the switch-over valve that you mentioned, is an over boost protection switch.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150