Gas mileage??? - Just drove from OK to OR - PeachParts Mercedes-Benz Forum

rgnprof · #1 11-22-2009, 10:10 PM

Just completed a 2000 mile trek in my '91 300CE and did not get as good of gas mileage as I was hoping. Car only has 63,000 miles on it and I bought it this summer. In June, I drove it back to Ohio and got 24-25 mpg and this trip I only got 18-19 mpg. I just replaced the fuel accumulator before we left to try and fix a warm start problem.

Question: We put a soft-side cargo bag on the roof - probably 45" X 32" X 18". Would that effect mileage this much? Any other suggestions? All interstate driving, 75 mph, no A/C - and car ran great!

One other, possibly related question: the accumulator does seem to have helped, but I did notice when removing the gas cap that I don't hear that pressure relief. Gas cap is new, but I bought it at a Mercedes repair shop and it looks like the black ones on the online parts shop - although the guy who sold it to me did say that it wasn't for my exact application??

Thanks, ryan

cliffmac · #2 11-22-2009, 11:04 PM

No, I don't think that's it...it's called altitude...you know, mountains...that makes a tremendous difference in mileage.....you'd be surprised (or maybe not).....

bsmuwk · #3 11-22-2009, 11:28 PM

Gas cap is most likely wrong and you SHOULD hear a SWOOOSH when opening the gas cap.

I do think that the cargo bag had a significant effect on the gas mileage. Combined with the altitude changes, there's your low gas mileage.

pawoSD · #4 11-22-2009, 11:42 PM

Quote:

Originally Posted by bsmuwk

Gas cap is most likely wrong and you SHOULD hear a SWOOOSH when opening the gas cap.

I do think that the cargo bag had a significant effect on the gas mileage. Combined with the altitude changes, there's your low gas mileage.

Exactly....these cars are quite aerodynamic....putting a big bag on the roof is like dragging a parachute behind the car. Huge MPG loss. Plus a naturally aspirated engine loses mpg's at altitude.

I second the gas cap....find one at a junk yard or just buy a proper new one....it should turn/click shut and have the "swoosh" when opening it. My car had an aftermarket one on it when I got it (did hold vacuum)....but I swapped it with a proper cap....works better, looks better.

cliffmac · #5 11-22-2009, 11:47 PM

Yep, you should hear a SWOOCH when opening the tank cap! Truth

rgnprof · #6 11-22-2009, 11:48 PM

I don't know about the altitude - I have driven thru the mountains before and have never noticed this kind of difference. Plus, the mileage from OKC thru Kansas was not any better - actually, it might have been a little worse!

Thank for the replies - I might try and track down an OE gas cap from a dealer here. No 'whoosh' means not enough fuel pressure, I guess? Would that make a gas mileage difference though?

ryan

lkchris · #7 11-23-2009, 11:46 AM

If you still have a warm start problem, the cause is most likely leaking injectors. New injectors should also make for better mpg.

And, high altitude always equals better fuel mileage, as you have less aero resistance (which you killed with your rooftop bag, of course) and lots of downhill running, which usually saves more than you spent climbing.