My 300D Turbo Diesel doesn't seem to go faster than 70 mph. Though I know this is faster than most posted speed limits, this shouldn't be. The trans goes through all it's gears but it seems to rev very high. Naturally, the tach doesn't work. The turbo still works though, according to my mechanic and the pickup is OK for what it is, I don't know why the car has to work so hard to maintain highway speed.

Take this to the diesel forum- you got classic 'slow D' symptoms easily overcome.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)

I'll help as best as I can...

I assume you've checked fuel filters? Replace them anyways. Have you checked for flow from the tank strainer? You need to pull the rubber line off at the tank and if fuel doesn't come out in a steady stream, you need to pull the strainer and clean it out. Don't forget to order the gasket for the strainer. Do you see any black dots in your clear in-line filter? If so, you have an algae problem and will have to treat with Biobar or similar product. Come back if you have algae and we'll go into that further.

Are you new to this car? Those machines sound loud on the interstate, that's normal. Either your trans is slipping on the interstate (very unlikely), your still in 3rd gear, or, it's normal. I think you should be turning around 3000 rpms at 70? I can't be sure as I don't have a 123 chasis 300D. This is a 123 chasis correct (81-85)? Do you have any thick smoke coming out when driving?

How about a valve adjustment? I could sit here for 30 minutes and give possible problems. Why don't you go check the clear fuel filter then report back and we'll be glad to help. If you have low power, it's most likely a fuel starvation problem.

Thanks
David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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I'd be willing to bet you have a fuel enrichment problem at boost. Your turbo may be boosting, but the pressure signal from the manifold that goes to an electric switchover valve mounted to the firewall behind the brake booster then to the ALDA on the injection pump may not be making it through. So in effect your ALDA is receiving zero boost signal thereby not adding more fuel as boost increases as it was intended to do. You could have a plugged up banjo bolt, plugged pressure tubing, or plugged/inoperative firewall switchover valve. Also, the boost pressure sensor on the intake manifold could be faulty. Replacing these is easy and fixed my problem which was almost exactly like yours.
Can you feel the effect of the turbocharger at around 2000 rpm?

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)

Yeah, I can hear the turbo working, and I can sort of keep up with regular traffic on secondary roads. I don't have a non-turbo car to compare pickup with, So I'll assume that the turbo isn't the problem. The tach doesn't work, but the tranny seems to rev too high, higher than Mercedes intended. I did look at the clear line filter and it's black as night, even though it was changed a few months ago.
I'll change the filters (for the 3rd time in 2 years!) but I don't think that will solve my problem. It's always been like this.

yeah I could hear mine working too- trouble is the signal wasn't enriching the fuel, therefore i heard it working i just never felt it. the turbo's surge can be felt at around 2000 rpm- it's a boost in power. checking it out is easy and doesn't cost anything. your prefilter should show only clear golden diesel fuel with no air bubbles. if it's black and it's always been that way you could have a chronic fungus issue.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)

It shouldn't be black. Are you using a clear filter? If it's filled with black specs than you have algae in your tank and need to add an additive to get rid of it, that's the only way.

Thanks
David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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Mercedes makes the pre-filter now a solid color. No more clear. Now you can't see if it's clogged. Anyway, I changed out the filters but I think I'm going to have to get the tank cleaned out. This is the 3rd time I'm doing this. If you run the tank low and then fill up. a day or so later you're stuck on the side of the road.

Actually if you look on Fastlane they offer about 4-5 pre filters. The official MB ones are white but every one elses is clear.

You have algae I would buy 1/2 a dozen fuel filters and a bunch of Biobor. As soon as you dump the Biobor in you will be changing filters like crazy if it is bad. Once you have killed it then you need to pull the tank strainer and clean it.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Biobor and Fastlane. I'll look them up.