MPGs on a steady decline over past 4 years

[MaxMatt]

I think I'll stick with O2 and cat. O2 sensor is not THAT expensive after all and I think I'll remove the cat for inspection. If it's plugged, Ill replace with a metallic one.

Thanks for all the great input. I see we are all thinking in a similar way.

Brake hoses on my car are a few years old - new teflon braided in stainless steel - Goodridge of UK. I check the alignment in the front 2 times a year and once a year for the whole chassis (the rear axle has some alignment of its own in this car).

I register every single tank. Whenever I can, I ride my bike, so my car mileage is little. I do a lot of driving in other cars. This one is mine - personal and I am saving it
When I do drive this car - the route is almost always the same. Hasn't changed in years - unfortunately.

[2000mc]

11-22-09 according to your fuel log you put in new plugs, and looks like that's when mileage tanked, highest tank after that you could disregard because you said you had a vacuum leak. The 27 after that would have most likely been all highway because it was only 3 days later
Might only help if you put in some sort of aftermarket plugs, then I'd suggest getting plugs from your dealer

[jeff88]

Could it be that he put in the wrong temperature plugs?

[2000mc]

Could be a lot of things, heat range, differences in plugs between manufacturers, plug gap, problem induced with a plug wire when changing plugs...

[MaxMatt]

The plugs are correct Bosch ones from the Mercedes dealer. These cars are very sensitive to plugs, so no cutting corners here.

Must be a coincidence. If the plugs were wrong, I guess I would get poor F/E form that time on. But the tendency is that it's getting worse and worse.

I am also contemplating replacing the temperature sensors. There are 3-4 temp sensors on this car and they DO affect the A/R ratio thus F/E.

[jeff88]

Plugs are cheap, sensors are *usually* not. Maybe change the plugs and see what you get out of them. If at the worst, you have a backup set and that way you can rule them (and any coincidences) out.

[JRMichler]

Long shot, but...

Check for plugged / nonfunctional PCV or EGR system.

[brucepick]

I would change the oxy sensor as others have mentioned. Not a lot of money, not nearly as much as the cat converter.

Depending on where it threads in you might be concerned about corroded or locked up screw threads. Dose it with PB Blaster or your own preferred rust-eater 2-3x daily for a few days before removing sensor.

[YukonCornelius]

In the domestic performance car world Bosch is regarded as one of the worst brand spark plugs you can buy. Most don't know a reason other than it being general knowledge not to use them. I tried to find the test or article that I'm recalling this info from but to my knowledge Bosch plugs don't transfer the spark at the speed of other brands resulting in the equivalent of a timing delay. I remember it was 1-1.5 degrees equivalent.

A previous car I owned, in my avatar, was very sensitive to spark plugs. I had to become a plug expert when modding because the car had no knock sensors and you wouldn't hear pinging over the supercharger. SVT couldn't even get it right and made multiple changes resulting in 3 different OEM plugs in 2003.

I found this on MBworld.com.

As a professional MB mechanic for over 30 years my preference has always been an NGK. They have always provided a superior spark plug which has never failed me. I can't tell you which number to use, but your parts store should have an NGK catalog to look up the proper plug. Don't waste your money on platinum, because your car does not require it.

Read more: 300e 24v spark plugs? - MBWorld.org Forums

[user removed]

NGK plugs and an 02 sensor would be my first steps. As the sensors hit their lifespan limit they get lazy and their response times slow down. By the time you get any kind of malfunction indicator your mileage can drop close to 25%.
Sounds like your maintanance is covered otherwise, even less than the intervals I would use myself, but that may be environment (driving) related. I have not used Bosch plugs in decades.

regards
Mech