Terrible gas mileage: 17 mpg mixed driving
 

Hi,

the gas mileage of my 1996 E320 dropped from 21 mpg (which was already not the best) down to 17 mpg in mixed driving 35% city 65% freeway.

According to many tips from the forum I have replaced thermostat, air filter, spark plugs, ignition coils including ignition cables, O2 sensors, MAF (after cleaning the old one did not help).

On the maintenance side I've got coolant system and transmission flushed, oil change every 4000 miles with synthetic oil.

During driving I actually can see the fuel gauge move a bit after every 15 miles as if the fuel was leaking along the road but a fuel leak is not likeky because I smell no gas, even when the car is left unused several days in the garage.

Can it be leaking only when driving? I just don't what more to do now except taking the car to the dealership or an indy.

Any suggestion would be appreciated.

Tire pressure? Parking brake adjusted correctly?
If it were a mixture issue I think you'd get a CEL.

That is a big drop. Do you know if the fuel blend changed? More ethanol or a lower grade will decrease milage.
Ed

Hirnbeiss, Yes, tire pressure is checked regularly (also tips from the forum) I put 32 Psi on the tires.

Actually the code P0400 (something with EGR) came up once then I replaced the EGR tube and it's gone for some months but once in a while it resurfaces although it can be erased every time. At the moment I don't have CEL and I just scanned yesterday with no code.

Ed, I use only premium gas.

cat
 

This could be an early sign of a clogged cat.

Ben

I would try a "scientific" test such as:

Fill up in the AM. Drive a reasonable distance on the highway. Fill up again that day at the same station and pump. This removes a lot of variables. See what mileage you get on that test. Make sure your odometer shows the correct distance also.

Thanks mpolli for the input.
I'd not had been desperate if it had happened recently.
I drive to work every day the same route and the drop has been observed (the mileage calculated) for a long time, at least 2 months and 6 tank fillings. In the good days I used to reach 200 miles until the half tank mark, now only 160-170 at most.

Benz, it there any sure diagnose for a clogged Cat, I just don't want to blindly replace part anymore.

Have your gotten new tires in the last 8 months? If so, what size.

I have had the car since January and changed the rear tires in May with almost new tires (stock size 215/65/16, remaining thread 10/32), but tires and tire pressure cannot account for the loss of 4 mpg, can they?

Confirm the tire it is suppose to have. My 99 uses 215/55/16's. Yours are taller which will effect mph and mpg computations.

How's the rolling resistance? With that big an MPG change you should feel a difference if that's the cause.

Sorry typo, the size is of course 215/55/16's.

Hey Tex I am experiencing the same problem on my wifes '93 300 te with the m104 engine. We just purchased the car about 3 weeks ago and noticed that the milage is very low, around the 20mpg range. I replaced the thermostat, with no help as well and my air filter is clean. I inspected everything, and I am thinking its probally our cats. I don't have emissions testing where I live, so I am probally going to take it out, it gotta help! Let me know if you find anything!

Dirtyhands

Not sure if your model has an EZL ignition module that has a vacuum line, but on our 93 500E, the vacuum line was broken underneath the intake manifold and was not easily seen. I averaged 16-17mpg (which I thought was normal) with the broken line. After replacing it I started averaging 20 mpg. I went from 330 mi per tank to 400 miles.

Not only did mileage improve, but power as well. Without the proper ignition timing advance, it seems power and fuel economy suffered. Wish I discovered that broken line 2 years ago...

DieselGuy, your 500E has 8 cyl, 5 liter engine, if it can do 20 mpg than mine 16 mpg is a shame.

Anyone knows anything about the vacuum line on 1996 E320, where it is? Picture would be great!