Sudden Drop in Gas Milage

I'm not much of a mechanic, but:

• the oxygen sensor (or its connections) is a good candidate--I think it would be picked up as a trouble code and light your check engine light, but I'm not sure
• maybe you've damaged something in the fuel line (including the filter) and have a small leak. (With a fuel injected engine, gas pressures are higher (that with old carbureted cars), so:
• you need to be cautious (about getting sprayed with gas)
• a small leak (in size) will leak more gas than you might expect
  

Take it to your local auto parts store, most of them now have handheld devices that will scan your codes and help find the problem. The most common reason for sudden drop in mileage is the Oxygen sensor. Running over a can could have knocked the wire loose if it is low enough on the pipes, or a mechanic could have bumped the wire. Because of the heat of exhaust pipes being so close to the Oxygen sensor wire they often become weak.

Another common problem is the air filter. I discovered that the previous owner had cut holes in the bottom of my air filter box and after driving through large puddles my engine cut out because the air filter was soaked. I had noticed inconsistent mileage variations but not if it was raining. Now I get consistent mileage.

You might not have holes in your air box, but if your air filter is old and hit some critical point and collapsed the folds (or something) it could be worth checking.

Drew

Thanks so much for your time and thoughts. Im hoping its the 02 sensor and not a leak in a fuel line. Your comment about the fuel filter getting wet in the rain may have prevented me from getting stuck today ITS POURING HERE i will wait until tomorrow to use the car! THANKYOU

Thanks so much for your reply. From your comments and others it seems it could be a problem of some gas not reaching the engine (leak) or a problem with the feedback loop from the o2 sensor (would mean shes running too rich?) The smell of gas was only on one occasion and was fleeting ,i dont know if that helps with diagnosis. Am i right in assuming if its a leak not to drive it anywhere but to mechanic and if o2 sensor i have a little time to get it looked at? Again thanks for your time

Yeah, but since one failure choice has the possibility of a catastrophic failure (fire) I'd bring it to the mechanic.

thanks man, had one car fire that was enough for me!

I would ask your mechanic to look for a gasoline leak. The smell of gasoline implies to me that your MPG loss is not from your engine under performing but from some of the gasoline not reaching your engine.

tHANKS your comment helped me to separate problem into 2 distinct possibilities. Im hoping a leak would mean i would be getting a more consistent smell of gas. No smell in the last few days.

I am leaning toward the Oxygen sensor being the culprit. When it is out the engine dumps in way too much fuel making it run rich. You would notice darkish exhaust fumes and they might smell of unburnt gasoline. A leaking petrol line would usually smell all the time and should drip on the ground too. Another (cant remember who) also mentioned you should have a check engine light illuminated with a bad Oxygen sensor (or wire).

When my truck had trouble with water, I was driving on partially flooded streets thinking I should be fine driving a 4X4 with 32 inch tires...but I wasn't.

If you don't smell raw petrol, head over to your nearest Autozone, or O-Rileys or whomever is closest and ask them to run your codes.

Drew thanks again. I haven't had the engine light come on and i haven't noticed any unusual fumes from the exhaust . I'm going to mechanic tomorrow and will be sure to post results a.s.a.p. My intuition is i knocked something out in the exhaust system when i hit the can.

you can easily rule out the leak , clogged filter or week fuel pump. just put a simple fuel gauge on the valve on the fuel rail.Thats what it is there for. it is as easy as checking tire pressure. also cars usually have 2 to 4 o2 sensors.

Check to make sure your tires are inflated to the proper pressure. Underinflated tires can significantly affect efficiency.

Here on the west coast USA, in winter, they do something to the gasoline - I think they "oxygenate" it. That's what I've heard anyway; and it's supposed to help eliminate air pollution.

I don't know if it eliminates air pollution, but I do know that it will cause your mileage to drop by 20-30% compared to summer gasoline. I monitor the mileage of my Civic every time I fill up and in the winter the mileage is noticeably lower than in the summer.

In your case, I'd visit the mechanic, tell him about the trash can incident, and tell him to look for a gas leak.

Hi

thanks for your reply. I haven't noticed any fall off in mileage secondary to the seasons,However my problem was preceded by my getting gas at a station Ive never gone to before. I have since gone to my regular station and will calculate mileage after this tankful. I think strong chance of gas leak has been ruled out(no odor,no evidence of drip etc.) I believe there is a good chance it may be debris in my air filter (see post by bob) thanks to all for your time !! :)

It is also possible that there was damage to the exhaust system from the "incident". A restrictive exhaust will drop mileage, and not trigger any "check engine" light, like an O2 sensor should.

Look at the pavement where the car is parked for stains on the ground. This could be caused by fuel leakage. This would be very serious.

If you missed the garbage can because of the amount of leaves, could leaves have been sucked up into the air intake? Try inspecting your air filter for signs of this. Good luck.

Hey Bob,

since my original post , I had the car inspected (pass) no comments from the mechanic about anything amiss. The gas odor that i experienced once i believe was not coming from my car. There is no evidence of gas leak odor or drip.My report of Decreased gas mileage was based on a visual estimate of the gas gauge ,and might have been overestimated at 20%. I Am now in the process of measuring this last tankful with the trip odometer with a tank of gas from my regular station.A damaged exhaust might have come up on the inspection? I already had doubts about my having a gas leak and didn't report any possible problem to the mechanic to keep things objective. I think it is very possible that i have a clogged air filter from parking over piles of leaves in the weeks preceding the decrease in mileage.I will look up where my air filter is and how to go about inspecting it> Thanks so much for your insight!!

Fuel gauges are notorious for lacking accuracy. 1/2 the fuel gauge seldom is half of the volume in the fuel tank. Gallons put in the tank vs. miles driven will yield better results. Especially if done with multiple fill ups.

Hi bob

After my visual estimation of a decrease in mileage i set the trip odometer and went thru half a tank.Rounded down slightly it looks like I'm getting only 20 mpg. The problem is i never checked to see (when the car was knew) what my mpg was. According to the manual the car gets 38 highway and about 25 city so it averages to about 31 mixed driving. Based on this half tank I'm getting 20 mpg only. Taking into account the cars age 8y.o. I could understand not getting 31 but i think 20 is too low. I'm comparing the distance the needle moves relative to the distance it used to move with the same amount of driving. I haven't had a chance to research all the possible causes of a sudden decrease in mileage but am keen on learning. What makes me think it might not be totally do the cars age is the sudden onset of the decrease.Looking into possibility of the air filter being filled with leaves and maybe its time for some type of maintenance on fuel system.

thanks again :)

You would have gotten a much more valid fuel consumption reading if you judge the gallons consumed by the gas pump's reading of how many gallons it takes to refill your tank after driving an odometer measured distance. Well assuming that your gas gauge is giving you a consistent reading (a very big assumption) there are several simple things that one can check to see if something is wrong with your vehicle.

• Check the pressure in your tires. A tire that is supposed to be inflated to 32 PSI will put a considerable drag on a vehicle if it is instead at 15 PSI. One of the drawbacks of radial belted tires is that the belts are typically stiff enough that only a flat tire will look like a problem.
• Immediately after driving your car around town place your hand next to the hubs of your tires. If one wheel is excessively warm then you may have a seizing brake. (This is not likely since the repair shop should have caught this problem as a safety issue. I'm just trying to be thorough.)
• The earlier mentioned air filter.
• In the spirit of some situational comedy shows, are you sure that someone is not siphoning your tank?
• Most of my other ideas would have likely been found by the shop checking the computer's diagnostic report. (timing, EGR, etc.)

Thanks red

You hit on some possibilities.I haven't checked the tire pressure but i do have one tire thats chronically low. I'm pretty sure no one is siphoning. I can see exaggerated gas gauge movement with one trip of moderate distance. I will defiantly check the hub temp.When i get some time i will try to find air filter and make sure its clean of debris,this way i could eliminate that as a cause.

Interesting note about the tires . I was do for new ones and figured the one wich was always low might be a bad tire. All 4 tires changed and this tire position still needs to be filled with air every month or so

thanks again :)

I read something (a long time ago and not found again since) in a magazine where aluminum rims could allow air to migrate through the rim itself. I think it had to do with the way that type of rim was manufactured. You could do a google search for deflating tires in your type of car.

Damage to the contact point between the rim and the tire could also allow air to escape. Also, the valve-stem could leak because not all shops change valve-stems when they change the tires. If you spray a mixture of dish detergent and water on the bead and the valve-stem you should see bubbles if it is leaking.

If you recently changed your tires, you might take it back to the shop and they should have a water tank they lower the tire into to find the leaking bubbles.

Additionally, Redfred (i think it was redfred) described the best method I know of for checking gas mileage. I once kept a log in a car that was having inconsistent fuel consumption. Even after discovering the problem was in the fuel injection system I kept the log for over a year developing some interesting trends between my driving and my (ex)wife's.

Drew