1983 Mercedes Diesel - Loses Power Over Time

Question

I picked up this car as a gift, and on day one drove it the 370+ miles home. After the first 160 miles or so I noticed that the car was struggling to maintain highway speeds. So I pulled over into a rest stop, and took a short break. When I hit the highway again, it was back to normal performance. However after another stretch of driving (160 miles or so), I noticed it was again struggling to maintain 70mph. This time I pulled off the highway, turned the car off, then immediately back on and performance was back to normal.

It's my first diesel, and the first time driving this vehicle.

The car's got plenty of "quirks" but ran (mostly) fine other than this drop in power over time. Not really an issue for day to day driving, but something I'd like to address at some point. Hopefully it isn't indicative of impending near term doom.

The valves have been adjusted recently, and the maintenance seems mostly up to date. I'm not real familiar with diesel's in general, and so as an addendum here I would ask for any information that would allow me to utilize the manuals for keeping this old girl running.

Edit: Additional car specs.

300D Turbo Diesel

According to the Wikipedia article, the engine is: 3.0 L OM617 A I5 turbodiesel

Answer 1

Drop the fuel tank and inspect the contents, looking for jellyfish, fluff, water, dirt, and the bones of lizards...

Paulster2's initial comment was spot on. Although I doubt it is a single "obstruction", on a vehicle of this age (especially diesel fuel that can be inconsistent), you can collect quite the library of muckity stuff in the fuel tank.

A long trip provides the right amount of sloshing and constant "suck" that draws the naughty bits up against the fuel pickup sock/screen, eventually choking fuel supply and creating the loss of power condition. However, diesel fuel is slippery, and the bits fall off the screen as soon as the primary pump stops -- even at a short rest break.

So, if all the fuel filters have been replaced (same symptoms), my first line of attack would be to drop and thoroughly examine the tank contents and fuel pickup screen.

On edit: After watching @Zaid video link, It occurred to me that you could drive the tank close to empty, and then use one of those inexpensive flexible inspection cameras to inspect the tank without removing it. (Easy since a diesel has no filler neck restriction.) I got mine at Home Depot, it's a "Rigid SeeSnake" brand, but there are many other inexpensive Chinese units that are nearly identical (and it's a fun toy, errrrrr I mean a valuable tool to have anyway):

Answer 2

This sounds to me like the engine/ECU might have a "limp mode" that the ECU uses to keep the engine running, but with reduced power output, when it detects a lack of control over the engine's operating parameters – for example if the ECU detects and over boost condition from the turbocharger.

A mechanical problem, such as a clogged fuel filter, seems unlikely to respond immediately to a shutdown/restart cycle. That clue, to me at least, points very clearly to the ECU and to something that is transient or that requires an "accumulation of evidence" as the engine runs.

The first step in tracking this down is to get access to a service manual for the engine and to see if there is any kind of on-board diagnostics that might tell you something about why the engine is going into low-power mode (if that is indeed what is happening). Read up in the manual and on-line on how the engine is managed. My instinct says the issue could be related to the turbo charger and its management system.

The other thing that would be worth doing is to see what you can learn about the car's history. For example was it ever run on bio diesel or cooking oil?

Also, it sure wouldn't hurt to go through and replace consumables like filters.

Answer 3

Symptoms sound consistent with fuel starvation. Do replace the filters first, but you don't necessarily need to remove the tank as SteveRacer suggests. The strainer is accessible from beneath the car, and algicides like BioBor can be applied at the filler neck.