MPGs on a steady decline over past 4 years
 

I like to think that my driving style is quite consistant. I keep my car in shape and improve my eco-driving knowledge all the time.

Yet somehow my MPGs are steadily declining over the last 4 years:
http://ecomodder.com/forum/fe-graphs/graph2083.gif

I used to get as high as 26-27 MPG and now I can hardly get over 25.
Most of the time it's around 22 or less.

That made me think. What's wrong? Is it me or is it my car?

My 3 prime suspects are:
1. Catalytic converter - it may be getting clogged up after 20 years.
2. O2 sensor - AFAIK it has never been replaced - might be getting some off readings which get worse over time.
3. MAF sensor - same as with the O2.

What do you guys think? Am I on to something? Is it a thing that the fuel economy gets worse as some parts get older?
Or is it just me?

I thought about it. But no. The temperature makes no real difference.

You could also look at how clean the inside of the engine is. When's the last time the throttle body and intake manifold were cleaned? Is there sludge on the oil cap? Maybe there is a build up of sludge inside the cylinder or the oil control rings are wearing down. Have you done a compression test? Maybe the compression rings are going bad.

I'm no expert, but with a 20 year old car, I can imagine you could have these issues.

air filter, fuel filter, PCV, MAF?

Thanks for your input, Jeff.

I cleaned the throttle body and the intake manifold with proper carb cleaner a few times over the last few years. I have also cleaned it while dissassembled, but to speak the truth - it was all nice and clean in the first place.

I am using a fully synthetic oil since I bought the car. There are no deposits whatsoever. I had the oil pan off and it was impressively clean for a 20 y/o car. The compression is spot on and it doesn't use up any measurable quantities of oil between oil changes.

I've done most of the things, which I can do with just my collection of tools and a lift. I am looking for a second opinion before I start throwing $$$ at it just to get MPG up a notch.

Is there a consensus about the catalytic converters? They don't help the fuel economy, but than they keep the enviroment from harm. What about the 20 y/o ones - do they still get their job done?
Since an original Mercedes CAT is very expensive I am thinking about replacing it with a metallic converter.

20 years is getting on the long side for a CAT. I'm guessing it's the old pan full of marbles style, not the free flow honeycomb. Although it is still with in the ECU's perimeter's, it's probably not at optimal performance.

CAT's can hurt fuel economy when they become clogged. If it's legal, replace it with a new one. I found a Magnaflow Hi-Flow CAT for about $80 on ebay. I paid $40 to have an exhaust shop weld it in. I'm not concerned about the "hi flow" part, but it was 2-3 times cheaper than the OEM style CAT's, and it has worked well for almost 2 years now.

I suspect your O2 sensor could be going bad. Many sources I found explained that their usual lifespan is only 100k miles. Even if it wasn't the primary cause of your bad MPG, replacing it can be insurance for later on. After all, the O2 sensor helps determine how much fuel your motor needs to burn.

I would also do a complete tune-up of the engine. Before spending money on parts though, make sure to do a compression check for each cylinder. You want to make sure the engine is in good condition before blowing money on tuning it up. If the compression is high and even across all cylinders, do the full tune up. Plugs, wires, dist. cap/rotor, fuel filter, air filter, motor oil, transmission oil, etc. Following all that, make sure to check that the timing is right on the motor.

That would be a start. And remember, make sure your tires are pumped up to the correct pressure.

Two other things that you may look into, the hydraulic hoses going to your brake calipers. As the vehicle ages the alcohol based brake fluid breaks down the rubber, the brakes feel fine when applied but because the hose has swollen it keeps some pressure in the caliper. #2 would be your alignment.

Do you log all of your tanks? If so, I'd think it would be a result of how little you're driving. If not, are you making all the same trips as you were a few years ago, or have things changed? Your last tank is also right at your lifetime average, so you might try just watching your next couple tanks.

I lean towards cat/O2 sensor. But I would repack the wheel bearings with a synthetic grease.

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.

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

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

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

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.

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.

Long shot, but...

Check for plugged / nonfunctional PCV or EGR system.

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.

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

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

I owned my 03 TL since new. its a decade old now and is giving me its best consumption ever on original sensors. all my cars (usually imports) power and efficiency improve with time. I had an acura rdx which started putting more power to the wheels year by year until it hit 60k miles. it never lost power or MPG though. I could still get 25mpg on highway with its 2 ton curb weight and nearly 200 horses at the 4 wheels just by using cruise control at 70mph. this was after 60k miles of mostly stop and go driving.

if youe o2 sensors had problems you would get a code and CEL...ECM monitors both o2 sensors (downstream and upstream) readings and if they are the same or similar it will throw p0420 code. then you can change your o2 or cat depending.

Thanks for input ever_green!

However, with all due respect to your experience, bear in mind that the car in question is 20 y/o and has over 120K miles on it.
There is no CEL, there is only one o2 sensor - in front of the cat. It's not an OBD II car, it has a 4-speed auto transmission with no electronic control whatsoever.

I also have a 25 y/o car with over 400K miles on it and one that is over 40 y/o.

I love older cars and I do my best to keep them in top condition. However, technology advance is now faster than ever. You cannot compare a 40 y/o car with a 20 y/o one and a 20 y/o one with a 10 y/o one.

There's an informative article about o2 sensors here:
Oxygen Sensors & Emissions - Facts & Repair Advice

My sensor is a little bit overdue for replacement.

I am sorry. I had no idea it was an old car. thought it was a decade old max.