My gas mileage just started to drop like a rock
 

1982 W123 300TD. Solid maintenance. Regapped valves 6,000 miles ago, when I also had all filters changed.

Change oil every 3,000 miles.

I was getting 30-ish mpg on the highway, 26-ish in the city for a 28 overall mix since I bought the car.

Then about a month ago, the mileage between fillups started dropping like a rock. 24. Then 25. I just went to Salt Lake and back, and got 24, 22 and 23 on my tanks.

What gives?

Some variables I've considered...

Salt Lake is at high elevation. It's started to get kinda cold (car still starts fine, though).

One thing I did notice on my last tank fillup was some leakage out of the top of the tank cap. The cap is locked solidly, but it appears that while I'm driving for the first 90 miles or so on a fresh tank some diesel is leaking out of the cap, somehow, and down into the drain hose. There is also some splattered back on the right rear fender. I hadn't checked that when the mileage started dropping, so it might have something to do with it. But how is diesel leaking out and up through the gas cap when it is locked?

Any help is appreciated.

First off, welcome to the forum.

Second off, you shouldn't put gas into a diesel Mercedes (fuel mileage is the correct term). ;)

As for how fuel is leaking past your cap, like another member said in a similar thread just yesterday, check the rubber gasket on the cap. If it is deteriorated (due to age or using biodiesel) then every time you make a sharp turn to the left fuel sloshes to the right side, surges up the fuel filler tube, and leaks out from under the cap...

Sticking odometer.

x2

the latter mileage is about exactly what I get. averaged at 23.7 last I rounded them all up.

x3.

Could be that they switched to winter fuel and you are getting the lower BTU stuff that is less likely to turn to wax in your tank and filters.

Not likely since you are in Idaho and they should have been selling winter fuel for several months now.

-Jim

Winter fuel?

Hmmm. I'll have to check that against the mile markers on the next trip.

Do you think it's because of the cold? That's the only thing I can think of that's really changed over the least few weeks.

Thanks for all of the quick responses.

Sticking odometers are a known problem on the 123's. Don't know if cold exacerbates it but it might.

Every year from January to the end of March, I go from 23 mpg to 20 mpg. Warm winter or cold winter it's always the same, It's got to be the winter mix fuel.




'91 350SD 534,000
'87 300SDL 236,000
'94 SL500 40,000

Update on 'Mileage Dropping like a rock thread'
 

From a few days ago. If you recall, I was distressed at having my mileage drop from 28-ish in mixed driving to 23-ish over the past 6 weeks. That's about a 20% drop.

I drove from Idaho to L.A. over the past 2 days.

Checked the odometer against the mile markers. It spins a little slow. I marked 98.4 and 98.6 on the odometer (both main odometer and trip meter) against two separate 100 mile highway stints. About 1.5% too low...but not enough to cause the issue.

Filled up in Oregon, Nevada (twice). Both times at between 20-35 degrees F and 4000-5000 feet elevation. Tank mileage was 23, 24 and 23.

Filled up at the bottom of the drop from Reno to Tracy. That was 44 (!) but it was a downhill run and only about 2 gallons. I discarded that.

Then on two fillups to L.A. I got 27 and 26, respectively. So it climbed up again once I got to warmer weather and lower elevation. Although it was clocking 28-ish during the summer in Idaho, where I'm at 2800 feet or so.

So somethin's goin on. Different diesel grades for different weather? Elevation and temp? All of the above?

I think elevation is a big thing when it comes to fuel milage.
When I drove the FL car from Orlando to Denver last February
I was getting 31 in FL, GA and KY. As I started to climb, the
mileage dropped. In St Louis and KC, I was getting 27 or so.
By the time I reached the CO border I was down to about 25
and here in Denver that car gets about 23-24mpg on a regular
basis.
Summer fuel gave 1-2mpg more but not enough to make a big
deal.
Just my insights. . .YMMV.(pun intended)

theoretically wouldn't the thinner air decrease air resistance which is the main factor against your fuel mileage?

"Winter diesel" is indeed supposed to result in slightly lower fuel economy. With the thinner air in the Inter-mountain West (and your NA engine is limited to what it can suck into the cylinders with no help from a turbocharger), that might cause a drop of several mpg.

Is your trip odometer hanging up?

I loose 6 mpg in the winter,due to winter blends.

I recently went through the same thing and it was because my return fuel line was rotting back at the tank! I couldn't see anything with the engine off when I checked and it drove me nuts. Then one day purely by accident I left my car odling in my carport to go back into the house for something and when I came out.......drip..........drip.......drip and a little puddle from the worn out return line was getting soggy just enough to lose a little fuel when driving!:eek::D

I went to AutoZoo and purchased enough blue lined fuel injection hose to replace all my fuel lines and viola! My fuel economy returned! Hope this is your problem and if you haven't examined your fuel hoses yet, this is a good time.;)

6mpg is quite a lot!

Mine only goes from 27mpg to 25mpg.

Well to Forced like last weeks chill,I even mixed more kero in.K1 here in tn is $3.29 a gallon.I heat with a space heater in day time.
Oh yeah,ALDA in garage.Need to find a wrench.Or is it a air tight seal.

Have you ever used WVO or biodiesel in your tank?

I did once and the wastegate on my turbocharger begank to gradually get stuck farther and farther open, and my mileage dropped from (my guess at the time) 30Mpg to just a little over 18.

Have you noticed a drop in performance as the numbers go down?

A GPS unit would work also.

Update: I got the car back home after my long trip, and changed the oil, pre-filter and air filter. I had the K&N cleanable kind of air filter, but replaced it with the standard Mahle paper filter at the recommendation of Phil and others on this board.

The result: 27.8 mpg on the first tank! 29.5 on the second!

Whoa. Can that be right? The old filter didn't look that dirty. And the pre-filter was dirty, but it had only been in there for 6,000 miles.

A new air filter and pre-filter improving mileage like that? I'm going to change them every month if that's the case.

On my '80 SD, I had a little schedule going.

I would change oil, every 5000k, change air filter and secondary filter in 6 month intervals, and the prefilter every other oil change. every month is going to get pricey in a hurry.

Yes. It was sort of a joke.

What are you calling the 'secondary filter'? The spin-on fuel filter?

Yeah. the pre-filter (little clear plastic inline filter) filters the fuel going into the injection pump, and the secondary filter (the spin on one) filters the fuel returning to the tank from the injectors. There is also a pre-pre-filter of sorts...the strainer in the fitting at the bottom of the fuel tank. Lots of Filters...there's a power steering fluid filter too, but I never changed that, as they are hard to find locally.