1999 300D Fuel Milage Question
 

I had been regularly getting between 35 - 37 mpg on a bi-monthly trip back and forth between Pennsylvania and North Carolina in my 1999 300D. Last summer, the exhaust system blew out at the muffler inlet weld joint, while I was in North Carolina. Of course, this sort of thing ALWAYS happens just as I have to leave to come back to Pennsylvania. So I drove the car 410 miles back to PA with the hole in the exhaust. But here's the weird part. During that trip, my fuel milage calculated out to approx. 45 mpg. I made sure the tank was topped off before I left N.C. and immediately when I returned to PA as I always did, and I triple checked all calculations. There did not appear to be any mistakes. Then, after having the muffler replaced at Midas Muffler with a German made unit, my milage dropped to 33 - 35 mpg for the same route, and that's where it's been ever since.

My questions are:

1. Can the "backpressure" caused by a muffler have that much of an affect on fuel milage on a diesel, assuming I did not miscalculate or something?

2. Are there any mufflers out there that might safely reduce the exhaust backpressure in order to increase milage without damaging any exhaust converters etc.?

Thanks
Jack525

Hard to say.... I drove a 600 mile per week commute and was happy when I got 600 miles per tank. To expect more than that, you'd have to be in a tail wind, driving downhill, both, or not measure things right.

The 45 mpg tank was a fluke. If MB left that much for mpg on the table, then shame on them. These cars get 30mpg with the hammer down on the highway, 33 with a light foot, and maybe little better if conditions are perfect. Anything better, you just didn't measure right.

Well, I don't know how big a hole you had but it certainly is possible to see some increase in power and fuel economy if you can reduce or eliminate the backpressure caused by the exhaust system. Having said that if you regularly get 35-37 MPG you are doing quite well and the 45 was probably a fluke, but I could see an extra 10% not being out of the question...so maybe 40? Also, your lower mileage now might be cause not by the new exhaust system but more likely from the effects of winter blended fuel.

I suspect the pump turned off a couple of gals early for whatever reason and that while the math wasnt in error, the result was.

1999 300D Fuel Milage Question
 

I thought it had to be some sort of error too, but wasn't sure if someone had a logical, mechanical explanation for it.

Thanks for responding.

Jack525

I would think that the reduced back pressure would let the turbo have a few more spins and have better combustion. it's oxygen we burn. The diesel justs sets it off. I think.

I think you need to do a control experiment. Maybe you can have a flange joint welded up to your mating ends and open it for a long trip. refasten it if you need to.

Nope, oxygen burns the diesel. Diesel is a fuel, oxygen is an oxidizer, when you burn something you oxidize it. Nothing is being set of by anything, its just the oxygen reacting with the diesel to create oxides

98 - 99 E300D Anticipated Mileage
 

I finally got my 98 E300D earlier this week and I'm still on my first tank... One of the first things I did was connect my ScangaugeII to the car to monitor the Fuel Mileage... So far it looks like I am averaging about 28MPG on a mixed Commute of 75% highway & 25% city... Some of that is stop and go but for the most part it is an open drive of 45 to 75 mph... I've been reading the boards and doing research searches since October and I've seen some great MPG estimates but would like to know what I can expect realistically for the winter and summer...

I like the ScangaugeII is anyone else using it??

Joe

I think that a lot of the really high mpg reports are just that - estimates or a report on one tank of fuel. If you just round off your mileage and round off the gallons you put in and just do it on one tank of fuel, you can come up with some very unbelievable and very wrong estimates for mpg. I calculate mine on every tankful and do the gallons to three decimals and the mileage to one decimal and don't worry too much about one bad calculation or crow too much about one good one. I average them all out and when all is said and done I'm at somewhere between 28 and 29 mpg. I've seen as high as 40 mpg and and low as 22 mpg, but that's been on a single tankful and is not accurate.

I get about 25 mpg in local short drives in the winter, and about 28mpg in the summer. As for over the road, I have gone as high as 35 mpg at a steady 60-70 mph. I have a 96 non turbo e300d. As for the ScangaugeII, it was one of the best investments I have made in a long time. Scans and clears codes, and gives you an exact real time readout of the parameters you decide to set it for. In regards to my fuel ups, I also use a small pad to keep track of my usage as a backup for the Scangauge. With all that said, the Scangauge is usually within one half a gallon of my own manual calculations. The only thing that does not work in my car is the fuel gauge, which has been diagnosed as "defective" by Mercedes. The sender is working properly, but the gauge is not responding. Good luck with you "new" vehicle!!

One possibility is the noise from the leaking exhaust modified (temporarily) your driving habits.

I have run fuel computers on my cars since the Prince On Board Computer in the early '80s, and learned a lot about how driving habits affect fuel mileage. If you have a loud exhaust, you will hear the increase in power, that and possibly not wanting your car to sound like a Clampett vehicle will keep your foot lighter.

On my big Cats I always liked hearing the turbo, I would moderate my foot by the sound/RPM of the turbo. I have also driven by the boost gauge, tried to keep it under 6-8psi, rather than letting it run up over 15, knowing it is burning less fuel.

The sound of a broken exhaust is similar to the MPG gauge in the '80s gas M-B cars, which was basically a vacuum-gauge, the more pedal the further into the red the gauge went. Keep the foot light, the gauge stays in the high-MPG range, keep the exhaust quiet, the better the MPG.

I must admit I never knew you could monitor fuel economy through the OBDII port on our cars...that's great information. Is there more than one model of ScanGuageII? I see it goes for about $170...if it does all it says it does that is not too bad.

I'd have a closer look at the calculator!

Some of us get the opposite effect. We get loud exhaust on a vehicle and start keeping our foot further into it more of the time because we like the noise.