How to stop declining gas mileage?

September 23, 2009 8:24 AM

Gas mileage dropping on my 2003 Chrysler mini-van with V6. Used to get 17city/21hwy. Last couple years has slowly but steadily declined to now getting about 15/18. Tune-up didn't help, at all, and "scoping" engine indicates no problems. Would like suggestions on what, if anything, can be done to get the old mileage back?
posted by unclemike to technology (18 comments total)

Is the gasoline you are putting in car 100% gasoline, or is it 10% ethanol? This would be marked on the pump.

It may be suspension related. Tire inflation affects mileage, as can alignment. Have you kept up on routine maintenance, that would have by now involved changing the oil in the transmission and differential?

Also, the posted mileages when you buy a car are almost never right, so if 17city is simply what was on the sticker, you probably weren't getting 17mpg.
posted by Pastabagel at 8:51 AM on September 23

Tires would be the first thing I'd check. Proper pressure? Aligned? Unevenly worn?
posted by unixrat at 9:00 AM on September 23

I am assuming that "Tune-up didn't help" means you went to a mechanic or dealer and asked for a tune-up. Do you know what work they actually did at that time?
posted by fritley at 9:10 AM on September 23

Seconding unixrat, and also, check your air filter. A dirty air filter will rob you of mileage.
posted by jclovebrew at 9:12 AM on September 23

Beyond the tire pressure and ethanol points already raised, Carbon build-up in the engine? How many miles on it? If its at least 100,000 try a product called "Seam Foam". It breaks down all the carbon build-up in the combustion chambers and burns it off. Smokes like crazy though- go somewhere remote to do it.
posted by moleodonuts at 9:13 AM on September 23

17/21 to 15/18 is not a significant loss for a car that's six years old.

That said, tire pressure, replace fluids, fuel filter, air filter, PCV valve, spark plugs, ignition wires, oxygen sensor ... does this car have a distributor cap? Replace that and the rotor. Maybe see if you can flash the ECU. None of this will hurt, except for the cost.
posted by Cool Papa Bell at 9:16 AM on September 23

If its at least 100,000 try a product called "Seam Foam".

s/Seam/Sea
posted by unixrat at 9:22 AM on September 23

Over time, fuel injectors can develop a varnish of gasoline deposits.
An injector cleaning involves removing the fuel-supply line to the engine and substituting a pressurized volume of high strength solvent mixed with gasoline and running the car on that mixture, forcing the solvent through the injectors and cleaning them out.
I've had some success smoothing out rough idles and improving performance after performing an injector service.

Also, something not yet mentioned, is the Mass Air Flow sensor (MAF). That sensor measures the mass of air entering the engine by heating a wire or film element and measuring its resistance. As the air passes over that element, its temperature and resistance changes within a range detectable by the engine computer. It uses that change to calculate the air mass.
I have seen, on many cars with higher mileage, crud and tiny dust bunnies accumulate on that heating element. That can generate a condition whereby the engine always thinks there's more air than there really is and delivers an increased volume of fuel correspondingly. You won't get a check engine light because the sensor will still be within the accepted range even though it's delivering slightly incorrect data.
They make a special spray solvent (safe to use on MAF sensors) and you can delicately apply that spray to the heating element.

Unless you're really mechanically inclined, I'd leave both of these procedures up to a professional. These are two more advanced tune up procedures for late model fuel injected cars.
posted by Jon-o at 9:44 AM on September 23

Seconding unixrat, and also, check your air filter. A dirty air filter will rob you of mileage

Fortunately for the OP, this can actually be ruled out as a possible cause. Dirty air filters do not reduce MPG on any modern car, because the clogged filter simply appears to the engine control system as more restrictive throttle valve, and it will open the valve further and/or reduce fuel delivery to the cylinders to compensate (and maintain the proper air-to-fuel ratio, or AFR).

What it WILL effect (affect?), however, is acceleration performance.

Source: http://www.fueleconomy.gov/feg/pdfs/Air_Filter_Effects_02_26_2009.pdf
posted by lohmannn at 9:56 AM on September 23

There's a lot of stuff that can mess with fuel economy. The car is running too rich OR it's not burning gas well OR it's got something blocking the gases on their way out the tailpipe OR you've got some rolling resistance.

So, in terms of being too rich, it could be a bad sensor (fouled oxygen sensor, coolant temp sensor, throttle position switch, or MAF). Or it could be a leaking fuel injector. I'd guess O2 sensor if I had to start somewhere.

Could be carbon buildup (seems unlikely) or bad ignition stuff (plugs/wires/coil(s)). Or, depending on miles, it could be that the engine is losing compression. Or it could be a clogged catalytic converter (easy enough check with a vacuum gauge).

Of course, once you start just replacing parts for the hell of it, you'll be spending hundreds or thousands of dollars, so I vote for ignoring it.
posted by paanta at 11:34 AM on September 23

Have your driving habits changed? Have your tires changed? Either of these can have a big affect on mileage. Some manufacturers ship cars with tires whose only positive attribute is very low rolling resistance, so that they can get good mileage numbers.
posted by zippy at 11:44 AM on September 23

All of the above are good suggestions, tire pressure, air filter, MAF, fuel quality, etc. I'm a Volkswagen TDI owner and as a group we tend to geek out on mileage. Whenever somebody complains that they are not getting the mileage they expect the first thing I recommend is that they benchmark their mileage.

1. Go find a gas station located just off a freeway. Set your tires to the maximum pressure listed on the sidewall. Don't skip this step. Don't bring the whole family along for this. It should just be you and an unloaded vehicle.

2. Fill your gas tank and note your mileage.

3. Get on the freeway and drive long enough to burn at least 1 gallon of fuel. Lock your cruise control on 55mph or 65mph. Don't tailgate people and constantly brake. Keep your windows up and the air conditioning off. Do this at a time when there is relatively little traffic so you don't get stuck in a stop and go rush hour or construction. Plan your route so that you burn 1 or 2 gallons of fuel and end up at the same gas station that you initially filled up at. Refill your tank from the same pump and calculate mileage.

This is obviously not characteristic of normal driving but it is a very cheap diagnostic method that only costs at most the price of 2 gallons of fuel and you get to find out how the vehicle will perform under nearly ideal conditions. If your mileage is still below expected then you can start looking at filters, MAF, injection cleaning, etc.
posted by well_balanced at 1:57 PM on September 23

Tune-up didn't help
Definitely find out what they did for the "tune up". There really isn't such a thing as a traditional "tune up" for most modern vehicles. Unless there is something horribly obvious, they hook up the computer and read any trouble codes it may have stored.

Have you ever had your plugs changed? That, alone, can have a great effect on fuel milage. Worn tires, too. Pastabagel's comment about ethanol content in gas is right-on, too.
posted by Thorzdad at 3:14 PM on September 23

Set your tires to the maximum pressure listed on the sidewall.
Do not do this. This is the maximum safe pressure of the tire. Set your pressure to whatever specification is detailed on the driver's door placard. Overfilling your tires WILL cause irregular and premature wear and is not worth the small percentage of milage gain.
The maximum safe tire pressure listed on the tire sidewall may be 50psi while the recommended tire pressure is 32. That is a huge difference and it's always best to go with the vehicle manufacturer's spec.
posted by Jon-o at 5:32 PM on September 23

I might offer up oxygen sensors. This could be causing the engine to run rich or lean. Not sure if there are cam position sensors, but those could be worth a look.

Then I'd check spark plugs and the MAF.
posted by luckypozzo at 7:13 PM on September 23

Do not do this. This is the maximum safe pressure of the tire.

I was recommending this for a short benchmarking test run. I believe maximum inflation pressure is calculated by filling the tire till it blows off the rim and then halving that value. A max cold pressure of 44psi tire exploded at 88psi. Play it safe if you prefer but still check pressure immediately before the test run.
posted by well_balanced at 5:05 PM on September 24

Oh, ok. I misread. It's not going to do any real short term harm, beyond decreasing the actual amount of tire touching the road. However, it's not a good practice over a couple thousand miles.
I had a customer bring their brand new car in for its first oil change only to find that all of their tires were completely bald. Cars are transported with overinflated tires to prevent flat-spotting during storage and, upon delivery at the dealer, someone had forgotten to deflate the tires to the normal pressure. They were completely wiped out within 2500 miles.
But, for a benchmark test, there shouldn't be any problem. Carry on!
posted by Jon-o at 5:45 PM on September 24

This drop in mileage has been gradual and consistant. Formerly used gasahol but changed exclusively to non-alcohol gas to try to improve mileage about none months ago. Current mileage I've reported is on regular gas and would be worse on gasohol. Tune-up about 90,000 miles was new plugs and scopeing engine that found no error codes. I've had car aligned and changed tires about 4 months ago with no change in gas mileage. MAF sensor makes some sense and will try to find some "Seam Foam". I did have Wal-Mart do their "Gum-Out" treatment to the car and six tanks of gas since then with no improvement. Thanks for all your help and I'll try some of these suggestions to see if it helps. About 107,000 miles and I don't think a 15%+ drop in gas mileage due to miles on the engine is routine. Runs and accelerates well.
posted by unclemike at 8:23 PM on September 24