I recently bought this 250K mile car and I swear an old 240D is quicker. Acceleration is very slow but once up to speed it cruises fine on the highway. It idles smooth, starts easy and doesn’t smoke even at full throttle. One thing I noticed was an absence of that turbo whoosh that these car have. It has a new air and fuel filter and I disconnected and plugged the EGR line. It doesn’t have a trap oxidizer. Also, the transmission is reluctant to upshift unless I momentarily lift off the throttle and then it shifts firmly (although not as harsh since I fixed some broken vacuum lines). Where do I start the diagnosis? I had the same car a few years ago and it ran like stink so I know how well they should go.

The sluggishness sounds like the symptoms of plugged intake pressure line/switchover valve to Alda on injection pump. Oily crud from intake manifold occludes the system and needs to be cleaned out with brake cleaner. Use the "search" function using some of the above words and you will find alot of information on the topic.

You should try running some diesel purge through. Also, do you have a gauge that you can hook up to measure the turbo boost? You should be able to do a search here to find out waht the boost should be at. I don't own a car with that engine so my advice is rather vague, sorry.
Thanks
David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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I checked the lines and valves and they are clear. I hooked up a vacuum/pressure gauge at the alda and got about 4 psi in 3rd gear at around 3500 rpm with the throttle floored. Is this starting to look like a stuck open wastegate?

You should have ~12.5 PSI of boost at full throttle at that engine speed. It sounds like you have a busted over-boost protection valve or a clogged line from the intake manifold to the ALDA. A malfunctioning wastegate is also a possibility, but I think the other scenario is more likely.

There's plenty of info available about this if you search.

I disconnected the rod to the wastegate and the flapper is free so I can eliminate the wastegate system as a problem. How can I bypass the over-boost protection valve to see if that may be the problem? I cleaned all lines with no change.

You should be able to simply pull the electrical connecter off of the overboost protecter. It is set to engage at 1.2 bars, or maybe alot less if it is defective-easy test.

To temporarily bypass the switchover valve for troubleshooting, disconnect the line that runs from the intake manifold to the switchover valve, and connect it directly from the intake manifold to the ALDA. If you're back to full power, you know that you need to replace the switchover valve. (edit: see last paragraph)

On my SDL, the switchover valve is located on the driver's side of the firewall... it will be in a similar location on your 300D. You should be able to find it easily by following vacuum lines.

IIRC, there is an electrical connector on the valve that runs to a pressure sensor located in the side of the intake manifold. It is also possible that this sensor is bad. Someone please correct me if this is wrong.

Good luck.

I bypassed the over-boost valve and disconnected the wire to the over-boost switch and no change. Also removed and cleaned the intake crossover tube and still nada. It's got 6 month old (at least) diesel in the tank and I'll fill up tomorrow but I don't expect any change here either.

The fuel pick-up screen in the tank may be plugged with algae and sludge.

Fuel flow as seen through the clear filter is strong. Time to adjust the ALDA?

Adjusting the Alda will increase fueling but have no effect on boost. Consider hooking up the boost gauge directly to the intake manifold, it would eliminate switchover valve and tubing leaks possibly clouding the issue.