I've always kept track of the fuel economy in my 1998 Camry since I bought it in 2003. For a long time, my in-town gas mileage ranged from 23-26 MPG. However, just recently, it has dropped to 18-20 MPG, and I'm curious if anyone has ideas why this could be the case.

The car has 167,000 miles on it, and I just had the timing belt replaced for the 2nd time. It broke while I was driving 1000 miles ago and I had to get towed. I also bought new tires about 500 miles ago. Could either of these changes be related to a big drop in fuel economy? What about the fact that I haven't had a full alignment?

Going back further, I thought I had transmission issues about 6 months ago, as my car had stopped using 1st gear and O/D. It turned out to be a solenoid that needed replacing, and the tranny seems okay now.

I'm sort of just grasping at straws because I can't figure out why my car is turning into a gas guzzler. Any ideas? Thanks.

 

If a timing belt problem the engine would have operational issues.

Some tires contribute to higher or lower MPG but not as much as you are having and would have showed up from the start. If the alignment were bad enough to cause this much drop in MPG tire wear might be effected.

Check for correct tire pressure and abnormal tire wear.

How does the engine start and otherwise operate?

If no change other then MPG might be a 02-AF sensor failing.

The car having ODBII someone with the proper code reader can watch many of the engines components operate in real time. If an O2 sensor or something else the engine data output on the reader can pin help point the problem. This is better then guessing.

On the off chance check the engine air filter. Typically if plugged this bad engine performance would be effected but perhaps never know, a mouse built a nest.

When was the last time the plugs were changed?

Has there been any change to the gasoline you buy such as the addition of ethanol which can lower MPG?

 

Thanks for all the ideas, Toyomoho. I'll try to address as many as I can:
Timing belt: yes, I don't think it's a problem anymore, and seems to have been installed properly because there are no vibrations, clatters, or odd sounds coming from the engine.

The tires are Pirelli Four Seasons. Consumer reports marked them slightly down (if I recall correctly) for fuel economy, but it shouldn't drop mileage by 20% like you say. The tire shop inflated to 30ppi, but the specs say max load is 44pst. Should I pump them up a bit more? Too soon to see if there's abnormal wear, and the car doesn't pull hard to either side of the road - maybe a slight drift to the left, as I can only take my hands of the wheel for ~5 seconds on the interstate before correcting back into the center of the lane.

The engine starts and runs well. Sometimes I feel like it's missing some of the power it used to have going up hills, but it is a 14 year old car. I think the air filter was changed last time I had an oil change (~500 miles ago). Spark plugs haven't been changed in 20k miles, which happens to be the last time I had a tune-up.

I don't know what to say about all the potential sensor issues, but I do know that my mechanic had error codes the last time I needed to pass emissions. Once he reset the codes, the car was able to pass, but I remember him saying that eventually I might need to replace something.... could have been the O2 sensor.

I always buy 87 octane fuel, nothing weird there.

So maybe I'll try a higher air pressure in my tires, but it sounds like an internal sensor somehow related to fuel injection or O2 could be another place to check?

 

If the drift is constant on any road this is not normal. Check for uniform tire pressure. 44 psi is the max but pressure varies with tire load. The higher the load (up to max load printed on tire), the higher pressure required.

Suggest no more then 35 psi cold tire.

Was the check engine light on before the codes were reset? Do you know what the codes were? The codes may lead to the problem area.

The O2-AF sensors are in part responsible for setting the air/fuel ratio. It is possible some other sensor related to the fuel ratio is bad but the 02-AF sensor fail the most often.

Might have the computer checked for pending codes. This being a trouble condition that has not reached the state the check light comes on.

Plugs should be OK.

Very much doubt tire pressure is the problem.

 

Okay, so I'm home and can check my documentation on the car now... funny you ask about the check engine light because that's been quirky for a while.

So, in September 2010, when I needed to pass emissions, my check engine light had been on for almost 2 years. The mechanic had looked at it once and let me know that it was a sensor issue and I didn't need to worry about it, unless I wanted to spend a few hundred dollars to take care of the sensor.

To pass emissions, the mechanic had to "Reset OBD 11 computer monitors" and the diagnosis was either P0441 or P0446. That got the check engine light to turn off, and it passed. He told me the engine light would probably come back on, and it did a day later. It stayed on for another solid 8 months.

This past summer, there were a few times that my fuel tank got really, really low. Both times, when I filled my tank, for some reason the check engine light turned itself off for a couple days. It would come back on before long, but it was the first time that light was off in almost 3 years. I had the solenoid fixed, then I had the timing belt repaired. And ever since then, the check engine light has been off continuously. It seems like a strange pattern to me, and made me wonder if all that time the check engine light was on, it was actually telling me something was going on in the solenoid.

But - and sorry to ramble - the original error code was Reset OBD 11 - P0441/0446, and I was told that the vapor pressure sensor in the Evap system was the culprit. Does that give you any clues?

 

It could have been both codes.

As you state these are EVAP codes. The pressure sensor measures pressure in the EVAP system for the computer to compare with atmospheric pressure.

Typically one pressure is different then the other, if they are same pressures the computer thinks there is a problem, typically an air leak in the EVAP system and sets a code. Or a bad solenoid. Or the pressure sensor itself can be bad.

The EVAP system stores fuel tank vapors in a charcoal canister, then burns them in the engine once warmed up.

Doubt if these codes are the issue for the lower MPG.

 

Well, I'll try putting a little air in the tires and replacing the engine air filter. I thought it had been done recently, but it's actually been quite a while - 25k miles. Maybe that's why my acceleration seems poorer than it used to be.

 

If the air filter is still clean and showing white paper don't bother. It takes a very plugged filter to drop the MPG this much. Tire air can effect MPG 1-3%.

 

My wife's 98 Camry 4 cyl has the same poor economy problem. We noticed it this winter (- my wife doesn't track her mileage.) At first, I thought it was just my wife's driving. She has a heavier foot than I when it comes to accelerating and decelerating. But even on the highway our mileage dropped. We used to be able to get 30+ mpg highway and I think we may be 20-25 mpg highway now and ~15mpg local driving. Even allowing for colder weather (PA), we don't remember this kind of mpg drop. The car has ~ 125k miles and we've owned it since new.

I've tried replacing plugs and the air filter. New tires were put on last summer. Tires are rotated and checked every 5k miles by the shop (freebee service.) Timing belt was replaced ~ 100k miles. We haven't seen a "check engine" light. My mechanic said the computer checks for an O2 range and we could be just inside a limit; therfore no light. I haven't had it checked for codes as we haven't had the light come on. We fuel up at a variety of stations (87 octane).

I'm at the point of trying to replace the O2 sensors, but at $60 minimum each (2), and difficult access to the one after the cat converter, I'm hesitant and thought I'd check if others have any other advice.

Any suggestions?

 

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