MPGs on a steady decline over past 4 years

[MaxMatt]

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:


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?

[MaxMatt]

Originally Posted by Old Tele man

...have you compared the average monthly temperature differences during that same period?

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

[jeff88]

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.

[Jasen]

air filter, fuel filter, PCV, MAF?

[MaxMatt]

Originally Posted by jeff88

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.

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.

[Jasen]

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.

[cbaber]

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.

[ksa8907]

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.

[2000mc]

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.

[YukonCornelius]

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