Sudden drop in MPG!
 

W210 1998 MB E300TD 281k miles

Couple weeks ago I was enjoying the 600+ miles per tank....though on the next fill up, i got less than 400 miles before the gauge was pointing at 0. I thought there was something wrong with the fuel level sensor, but then when I filled it up, it took 19 gallons, as usual.

I thought because I added a stage 1 tune to it---since before the tune, I had 600 miles. Immediately after tune, 400 miles. So I thought that was the cause.... So I removed it and returned it to stock/factory tune (with EGR disabled).

Same problem.

Oil change, all fuel lines at the IP replaced, all filters (yes, including the tank strainer) replaced.

Thought it was a faulty waste gate transducer, so I swapped it with the EGR transducer. Same problem.

I serviced the transmission and differential fluids about 2000-3000 miles ago

Maybe bad batch of diesel? I tried out a bunch of gas stations, including ones that 18 wheelers use. Same problem.

Tires are brand new Michelin with proper pressure.

So what do you guys think? Fuel injectors? The PO says he never had them serviced, and I'm led to believe him. It looks original.

The only way to measure mileage accurately is to record the miles travelled between fillup's and record the gallons pumped. Average over several cycles since pumps cut-off at different levels. Verify your odometer against freeway mileage posts and/or a GPS. Some people actually think the tic marks on a dash fuel gage have some type of quantitative meaning, like the half mark means you have 1/2 the tank volume left. Never driven a car where that is even close to true.

I got the car a year and half ago with 266k on the clock. From day 1 to now, I've been religiously monitoring the car performance through my phone app. When I input the current mileage, amount of fuel I pumped in and the app calculates the true mpg from the last entry. I also reset the mileage trip between each fill up, that way I know how far the car goes.

From that day up until two months ago the average was a solid 28.9 mpg. The fuel gauge in the cluster is actually accurate -- each quarter mark on the gauge was exactly 150 miles -- as in I drive 150 miles, the gauge would drop 1 full quarter. At 300, it would be half tank, and so on. At 600 miles, give or take, the car would take between 18.5 to 19.4 gallon. I could set my clock to this, it was so accurate.

Then up two months ago, immediately after I installed the stage 1 tune, poof. 200 miles gone. I went from 28-29 mpg down to 22-23 mpg virtually overnight.

I'm thinking of redoing all O rings with viton.

Viton for Mercedes Engines at FRYERPOWER HOME

Is this the only source? Is that still available?

Was your EGR disabled before? That can make a difference.

You bring up a good point. Before, EGR was not disabled. It was on factory/stock tune. 600+ miles range.

When I flashed it to stage 1 tune, that one had the EGR disabled. ~400 miles.

When I panicked and flashed it back to stock, I left the EGR marker as disabled. Basically the car is on factory tune with disabled EGR. Still ~400.

Hmm, I'll have to flash the unmodified tune after a fill up and drive again.

I thought the gas tank had a float which was completely submerged when the tank is full. So you have to burn off quite a bit of gas before the float begins to sink and therefore it registers full long after several gallons have been used. This would mean that the ticks would not register the same amount of mileage. Is it not so?

Check for a dragging brake caliper