What Causes Gas Mileage to Get Worse?

I own a '99 Olds Alero GLS. I like the car a lot, it had a few minor issues that were covered under warranty, but all in all it’s been a solid performer.

However lately it seems that I’m getting really poor gas mileage out of it. When it was closer to new I could expect something close to 25 mpg. Over the years the gas mileage steadily got worse and now I’m getting close to 12 mpg.

Granted, I don’t do much highway driving these days so my numbers would certainly dip, but I’m curious what else could account for this.

I assume it’s due for a tune-up which would likely improve matters, but I’m curious if there’s any other mechanical issues which could cause this that I’m not aware of.

I had a mechanic check it out a couple years ago when getting something different serviced and he said the computer diagnostic didn’t show anything wrong. At that point the diminishing mileage wasn’t as pronounced as now so perhaps something was harder to diagnose then.

So, in addition to replacing the plugs/distributor, what type of issue should I look into?

Oxygen sensor would be a good thing to check, and when was the last time you changed the air cleaner in the car?

Other maintenance that may improve your gas mileage:
-fill tires to the proper air pressure (probably about 32 psi in your Olds but check your manual)
-change air filter
-have the wheel properly balance/aligned
-clean fuel injectors (perhaps during changing of spark plugs/wires/distributor)
-be sure oil and oil filter is changed every 3500 miles
-check and fill coolant and tranny fluid to ample levels

As Tuckerfan suggested, the oxygen sensor is possibly malfunctioning so I concur that you should have that checked out.


25 to 12 is a pretty good dip. I would get it checked out by a mechanic ASAP.

Possible problems, off the top of my head:

  • clogged fuel filter
  • bad spark plugs/wires/other ignition components
  • bad sensor screwing up the computer
  • computer’s gone south (most cars have a “cripple mode” which means they will run even if the computer dies, but you get really bad mileage, like, say, 12 instead of 25)
  • clogged exhaust (maybe caused by catalytic converter rusting apart on the inside - been there, done that)

I’m sure some of the real mechanics on the board can add to this list.

With some of these problems, the first thing you notice is a drop off in mileage, but if you don’t take care of the problem, other damage can result.

12 mpg?

I would suspect math error for that big of a swing.

If that is accurate, you need to get serious help.

As a comparison:

1988 Ford F-250, 460 ci, fully loaded, with trailer: 9 mpg
1965 Ford F-250, 390 ci, load independant: 10 mpg
1980 Fiat, gas leak, 80 mph everywhere: about 28 mpg
1997 BMW 8 cyl gas-guzzler, cruising at 75mph: 20.5 mpg
1996 Nissan pickup, 4 cyl, loaded and 65mph: 26 mpg
same vehicle, unloaded and 70+: 19 mpg

If you’re getting 12, without a math error, you are in serious shit.

I appreciate the feedback. As noted, I’m going to give her a tune-up this weekend myself. She’s woefully past due and I’m sure it’s a factor, though I’ll be curious to see how much of one. I’ll replace the plugs, the distributor, oil and filter, and check the rest of the fluids and afterwards re-evaluate gas mileage. Beyond that I’ll be interested to hear other more complex repairs to have investigated.

No, the math is right, though on a tank-by-tank basis it can vary up to 16 mpg. I should clarify that I live in downtown Chicago and nearly 100% of many tank fills are entirely comprised of stop-and-go trips under 3 miles. I’m essentially the worst case scenario for gas mileage.

Certainly I’m under-achieving from an efficiency standpoint, but I’m not ready to call it a likely catastophe yet. Also, while the car is a 2-door coupe, it’s fairly heavy and has the largest engine available. It’s factory rated at 26 mpg highway/20 mpg city, so it’s by no means a delicate flower.

As an interesting aside, the overnight radio program, coasttocoastam, has been having people call in and say that since the hurricane their gas mileage has dropped. Mine has dropped from 35mpg to 30.5mpg in my Mazda Protege. I’m going to get a tune-up and see if that helps, but I drove a Mazda 323 for nine years and had 225,000 miles on it when it died and it never dropped below 42mpg as old as it was; I changed the spark plugs myself and this included when I stipped one of them and it was running on three plugs.

Could it be something in the processing or the fact that some of the gas now being sold has been in reserves for who knows how long? Is there an engineer out there who might know?

That’s interesting. My mileage has been diminishing over time steadily, but it has seemed to get markedly worse since this summer. I never connected it to the hurricane but the timing is right.

Conspiracy theorists certain would blame the gas companies, accusing them of somehow watering down the gas or formulating it in a cheaper way which is less efficeint. Perhaps there’s a rash of local stations stretching their stocks in tighter times as well. Perhaps we’re supplementing our supply with lesser qualities from over seas or in Mexico.

Of course it very well could be all in our imaginations, but it’s a curious thought. Maybe a SDMB poll is in order in IMHO?

Delicate flower compared to a 65 F250 and an 88 F250 (by the way- SuperCab and 4 wheel drive!).

You really should not have dropped by a factor of nearly 2! Check that out!

The fuel-injection system adjusts the mixture ratio of air/fuel and spark timing based on the information provided by the oxygen sensor. The proper relative amounts of oxygen (air) and fuel maximize how well the car runs and minimizes pollution from what I understand. Failure of the oxygen sensor causes a car to run differently including a somewhat significant drop in fuel efficiency. it is very much possible that the oxygen sensor is a major contributor to your problem.

I suggest you pay a car mechanic to change the oxygen sensor if it has indeed failed. The sensor itself is located either inside the exhaust manifold or inside the exhaust system between the manifold and the catylitic converter. In other words, it’s a bitch to get to it. Removing the exhaust manifold is not a quick job.

No, I doubt it. The math is not complicated. Such a steep drop in mileage is plausible. For example, I had a friend who negleted regular maintenance and absolutely ‘gave hell’ (as he described it) to his old V6 Blazer and it was getting 6 mpg by the time he junked it.

Yes, this is true.

I am much more familiar with pre-1995 autos which rely less on computers, so I don’t have any experience with “cripple mode” problems. engineer_comp_geek may have identified your problem here.

Lastly, I haven’t noticed any change in gasoline quality here in my locale since Katrina. Maybe you could speak with other friends/drivers in your area to see if they have noticed some drop in fuel efficiency.

I meant catalytic convertor :smack:

I typed too quickly

If you have a 99, and an O2 sensor fails, the “check engine” light will come on. You can take the car to a Kragen/Whatever and they will most likely hook up an analyzer ( called “E something”- give me a break! I’m drinking!), for free, which will then read out a code telling you exactly what, if any sensor has gone defective.

(Trust me on this: BMW has about 12 sensors, both pre and post catalytic converter that fail with depressing regularity. And since they fail so often and are DAMNDED expensive, I did a little research. By not replacing them, the net effect on gas milage was NEGLIGABLE! Certainly not in the range of a factor of 2! Depending on driving condition, I went from 26.5mpg to 21.2mpg. This includes lots of high-speed driving and mountains.)

I doubt that O2 sensors could account for halving your gas milage.

gatopescado, I understand what you’re saying but I think you’re over estimating the amount of change I’d discussing. I should have been more clear.

When the car was new I commuted via highway and was getting 25+ mpg. I am not the type to monitor my gas mileage religiously, though I do always reset the trip meter at each refilling. Certainly those numbers varied based on loads, weather and driving habits. That 25 mpg ballpark figure was in line with what the manufacturer specified.

Today, I do virtually zero highway driving. So we’re not comparing apple to apples when discussing the 25 mpg number to the 12 mpg figure. Certainly I should be getting better than 12 mpg, but even if the car were new I wouldn’t expect substantially better than 18-20 mpg based on my driving habits.

Also, I don’t mean to imply that the change in mileage changed suddenly. It’s slowly diminished over the span of a couple years, just recent reaching a point that looked to me to be past an tolerable level.

If I had to lay odds, a simple tune-up will recover most of the loss. Still I want to be prepared for possible further action in case.

You live in Chicago*, it’s January, you are driving stop and go trips of less than 3 miles, and you are complaining of poor gas mileage? :eek: :confused:
You poor car is probably never getting out of cold start and warm up mode. Until the engine is almost fully warm, the computer will ignore signals from the oxygen sensors, and will by necessity inject extra fuel. Lots of extra fuel. The colder it is the more extra fuel it will inject.
If Chicago is one of the areas in the country that uses oxygenated fuel in the winter, your fuel mileage may also dip a few mpg from that.
While some routine maintenance is like chicken soup for a cold IOW it can’t hurt, I am more inclined to go with the dip in gas mileage is weather, fuel and short trip related.

*I have always heard that the way Chicago got started was a bunch of New Yorkers were standing around talking “Well we like the crime, we like the overcrowding, it’s just not cold enough here.” :eek:

I inflated my tires from soft to hard and increased 2 mpg.
I removed a 10-ream box of printer paper from my trunk and increased it another 2 mpg.
If you do not park overnight in your own garage, have you got a lock on your gas filler door?

I’m inclined to generally agree with this. Though, like I said, I want to prepare for other possibilties. Still, I had the same driving regimen 12 months ago and the mileage is a measurable amount worse. So perhaps the plugs and filters are just that much more beyond where they needed to be replaced.

Also, yeah, I only fill my tank like once every 2 months so it’s not my most pressing concern. That’s why this issue has been lingering without attention for as long as it has.

Always was one of my fave Chicago jokes.

Ignatz, that thought occured to me because I do not have a gas cap lock. Though my car gets street parked, it’s in a pretty high visibility/high traffic area 24/7, so I think the odds of it are reduced quite a bit. Also, this has been happening over the span of many tank fills so I think it’s an unlikely factor.

I’m not a pro, though I was 25 years ago. I still have a pretty good shop & tools & still mess with cars from time to time.

Seems to me that before buying $50 in parts and spending a weekend doing amateur mechanic-ing in the cold, you should fill the tank and take a drive on the interstate to, say, Milwaukee. Heck, even going someplace 30 miles away & returning would be enough.

Then refill the tank & do the math. Due to the short run you’ll want to be sure you fill the tank as close to the same level as possible both times. If the highway mileage computes out at anything between, say, 22 & 30, you’ll know the car is getting about the same mileage as before, net of the uncertainly in how full is “full” on the two fillups.

I bet that for the expenditure of $5 in gas and an hour of time in your nice warm car you’ll find there is nothing wrong except what Rick said; driving a car in freezing temps such that it that never warms up will deliver very crappy mileage.

I’d also be cautious about replacing the “distributor” as you said you’d do. Unlike the old days (ie pre 1980) with points and such, there is very little inside a modern distributor that won’t last longer than the engine. You can really trash the tune beyond the ability of bolt-on repairs to fix by messing w/a distributor & inadvertantly bunging up the timing or the wiring, particularly in freezing conditions. If you have a shop, and timing tools, and manuals and all the rest, go for it. But if you had all that, you wouldn’t be asking this sort of question.

Good luck & let us know what you find out.

It is interesting as I have noticed about a 2 mpg drop since.