300SD power decrease as fuel level decreases?
 

Hi all

Am doing long commutes with my 82 300SD (job transfer). Filled up before I left for home on Friday. Driving from south of Fort Wayne, In to Watertown, Wi. About Chicago, I seemed to notice a bit of a loss of power. Did not get any worse. When I got to Milwaukee, climbing the hill past Rte 100, I really noticed the difference and the car would not go above 60 up the hill, but seemed ok the rest of the way home (A total of about 370 miles). The car sat over the weekend. Filled up Monday, drove back today. Everything was fine till I hit Chicago again (sorry, must go through Chicago), and I noticed the loss of power again. At Rte 30 in Indiana, I pulled off Rte 65 and topped up the tank. Jiggled the plastic filter around. Drove off, full power, buried the speedo as a test further down the road. After an hour or so, started loosing power again. Could not get her to 65 on a hill, but could on flatland. Got to where I was going. A few hours later, took her for a spin. Seems fine again.

Are these the symptoms of a fuel filter starting to plug up? Or what else could it be?

Thanks
Fred Konchan

Just a thought.. Most of the fuel that goes throught the IP goes back into the tank through the fuel return lines. That fuel gets quite warm. I am trying to think if that would cause a fuel filter to slow delivery?
Maybe someone knows. I think warmed diesel improves the power though.

Hell yes, it sounds like bad fuel filters.... could also be fuel strainer in the tank - a 'top hat' shaped screen in the tank.

Sounds like an algae problem? The ratio of algae-to-fuel increases as the quantity of fuel decreases, reducing the flow through the in-tank filter. Algacide and filter cleaning may do the trick.

I concur w/ 240DieselDog. Replace your spin-on and inline filters and see if that fixes the problem. After that, you'll have to start looking at the rest of the fuel system (ie, fuel pump).

Ditto on the fuel filters.

Fred,

When you remove the filler cap on the tank, does it seem like you have a vacuum? I know that sometimes the tank vent clogs and as you use fuel the system goes below atmospheric pressure. This can collapse portions of the tank, and it will eventually shut the car down.

The fix in my experience was to replace the soft rubber connections as they had become gummy over time and just got kind of glued together on the inside. Good luck, and I hope this helps, Jim

Fred,
Following up to what Jim Smith said, if there is a vacuum when you release the fuel cap, try operating with the cap loosened. When I bought mine it had a similar problem it would run out of power after about an hour of driving. I noticed the vacuum when I would fuel up, I left the cap loose and the car ran fine. I found the tank vent which is under the car was plugged and the hoses were soft and gummy. I replaced the hoses and cleaned out the vent. I check the tank vent everytime that I change the fuel filters. Hope this helps.
Tom
82 SD
86 SDL

Hi everybody
Ther first thing I did when I pulled off the road was check for vacuum at the cap. No vacuum or airflow either way.
I'll change the filters tonight or tomorrow and see what happens. Just wanted to see what else it could be. I suspected the fuel filters.
I really doubt the algae thing because it has been cold, and I use about 30 gallons of fuel a week, and do let it run to 5 o0r 6 gallons left.
Thanks
Fred

You mean that the fuel filler cap's not supposed to have vacuum when you open it up? Boy, you learn something new every day...I haven't had any power loss problems like what has been described, but every time I open the filler cap, I get a vacuum, and had assumed that that was normal...sounds like I may be heading that way. I'll have to check out the tank vent...very interesting...

The Warden,

Early Diesels did not have a system for recovering evaporative emissions, and therefore they do not generate over or under pressure in the tank. Diesel fuel is a bit less likely to evaporate than gasoline, and apparently they did not need such a system to pass the evaporative emissions testing of the day. Later cars, like my 1998 E300D Turbo have an evaporative emissions system, and like gas cars, you get the expected "whoosh" when the cap comes off, but I think it is air escaping, not rushing into the tank.

In any case, the intent of the venting system on the W123 and W126 vintage cars is to maintain at least atmospheric pressure in the tank, much like the old gas caps of yore when they had a vent hole in them. Jim

Hi all,
It was the fuel filters.
Thanks everybody
Fred