What is the logic behind the fuel reserve tank?

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JWJ
1983 MB 300D - > 430,000 miles - Deep Blue - Bilstein Comforts - 0-60 in 24 seconds - 27MPG
1985 MB 300TD (wagon) - > 275,000 miles - Manila Beige - Thule rack - 0-60 in 18 seconds - 22 - 25MPG
Biodiesel, Mobil Delvac 1, and Viton fuel lines!

There isn't one.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

So you don't get stranded when you run it down to empty, of course

There are times when you get stuck out in the middle of nowhere miles from the nearest filling station when the gauge hits empty.

Like getting stuck on the freeway between towns when the propane truck a few miles down the road ahead of you rolls over and catches fire and the fire crew stops all traffic well away while y'all wait for it to burn off, hopefully slowly, but possibly very, very fast (see "explode").

FYI, it takes ~5 hours for it to burn off slowly.

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

Why not a larger single tank?

__________________
JWJ
1983 MB 300D - > 430,000 miles - Deep Blue - Bilstein Comforts - 0-60 in 24 seconds - 27MPG
1985 MB 300TD (wagon) - > 275,000 miles - Manila Beige - Thule rack - 0-60 in 18 seconds - 22 - 25MPG
Biodiesel, Mobil Delvac 1, and Viton fuel lines!

I am pretty sure that's the point. It IS a larger single tank. They're just calling it a reserve for the psychological reason that if "empty" really meant "empty" and the car quit moving the moment the gauge bottomed out, some people would never refuel until too late. If you make people think they've already practically bottomed out but still have a "reserve" capacity to get them out of trouble, they'll watch for that moment when they feel like they're on borrowed fuel and start looking for a refill sooner.

"Reserve" just means "You've reached the point at which we think you should be filling up NOW if possible but we've decided to build in a little extra capacity just in case."

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

I usually just fill up when it gets down to a quarter tank.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

There is no reserve tank. The reserve light illuminates when you're down to the last 2-3 gallons............giving you ample warning of impending doom.

I fill up when it gets down to half tank (usually every two weeks on payday).

I run mine approx. 525 miles on the trip odometer which gets reset at every fillup and then start desperately looking for stations.

My gauge hits R at 450 miles or so on the tank but I have yet to EVER see the little orange light glow when it's supposed to so I don't trust the reserve system.

Based on my miles per gallon figures (never lower than 23, never higher than 26.5, checked at every fill up) I should be safe to take it 550 or 600 miles every single time. But having the gauge on R for THAT long makes me too nervous so I fill sooner. Invariably the car takes between 19 and 22 gallons when i finally stop to fuel, indicating that I always quit 50 or 75 miles or more before I'd have to have. I do remember a few tanks that I have run 580 miles before fueling.

Why do I stop so early now? Because one time I hit the road side completely dry at 528 miles showing, which just shouldn't have happened even with diminished economy due to some random cause. All I can figure is that that one time I must have filled up on a slope going the wrong way and not gotten the tank topped off like I usually do, but it's made me paranoid enough to call 525 my "better get fuel" point.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

If you are filling at 525 then you are running right to the last drop every time! I ran my car out on the trip home from buying it(faulty gauge still said nearly half!), and it only made it 514.4 miles. I've daringly gone as high as 465....and it took 20.3 gallons or so....real near empty. The tank only holds about 22-23 (maybe) on a SD/SDL.....maybe a bit more if you cram it. The gassers have a slightly larger 26-27 gallon tank.

My 300E lays down on "R" with only around 290-315 miles on the trip gauge.....and takes 14 gallons or so at that point. Meaning there's about 3-4 left since it can hold 18....

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I'm 100% sure I remember running 592 miles on one tank I drove for work one day a few summers ago. I admit it was on fumes when I got there.

*going out to check owners manual, because I'm curious now and don't want to post misinformation.*

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

According to the book, I've got a 23.8 gallon tank.

So at 25 mpg, 23.8*25 = 595 mile tank range which is just barely possible with every possible optimistic assumption made, which explains that one tank I remember. I got desperately lucky, and just happened to have a station nearby as it started to hesitate.

If we reduce that assumption down to 23 mpg, which is on average .5 to 1.5 lower than I usually record, that still gives me a 547 mile range.

So maybe 525 is a pretty good estimate after all. I DO fill up sooner if I don't know what area I'm in. Most of where I drive, I know where the stations are and there's normally diesel within twenty miles regardless.

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

I am surprised no one mentioned carrying can in the trunk?
I keep a one gal in the '99 & 2 1/2 gal in the '83. Used once in between fillups and twice to refill filter replacement done on the road. Poor fuel in the last instance. Even in the summer months, with the jugs capped tightly, I get no fumes inside. Would never do this with gasoline.

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Gary
Experience is something you get, Just after you needed it....
'99 E300 295K, '83 300D 220K, '75 240D 185K sold
'80 MCI Bus Conv DD 6V92TA, w/4 cyl. Kubota diesel GenSet 12.5KW

Man, you guys never sleep! (looking at the post times)

I'd get away from the computer for a couple of minutes (or hours) and fix the sending unit reserve light system......MUCH better than gettin' stranded and it's usually a easy, cost free fix due ti accumalated dirt inthe sending unit. Couple of threads on here how to do it.

The light and gauge work independent of each other. They are separate circuits. The light serves as a warning so you don't have to guess what the gauge is really reading.

Jaime
-who found out the light in his 79 300SD doesn't work, and had the engine stall right in front of the diesel pump a few weeks ago!