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Why did my fuel mileage suddenly drop substantially (from 30 mpg to 20 mpg)?
2004 Toyota Corolla

Why did my fuel mileage suddenly drop substantially (from 30 mpg to 20 mpg)?

(2004 Toyota Corolla)
This first happened at around 80,000 miles. I replaced the spark plugs and that fixed the issue.

At about 142,000 miles (beginning of this year), my CEL came on and car was vibrating especially when idle. The code (I don't remember what it was) suggested that ignition coil #1 was faulty, so I replaced it. That didn't fix it, so I brought it to a mechanic, who replaced the spark plugs and all was fine again.

At about 144,000 miles (a month or two ago), CEL came back on with no noticeable change in performance. Diagnostic code was P0171. I cleaned the MAF sensor with an MAF cleaner. That appears to have fixed that problem, as I've driven around 1k miles since then with no CEL.

Over the past two or three tanks of gas, however (since shortly after cleaning the MAF, if not at the same time), my gas mileage has dropped significantly, from the 29-30 mpg I've been getting for a decade, to around 21-22 mpg. This is the same symptom that first popped up at 80k, but since the current spark plugs are only a few months old, my assumption is that there's a different underlying cause this time around. There's been no change in driving habits, no change in the type of gasoline I buy, and no significant change in weather. I brought it to a mechanic this morning, who essentially told me that without more information, there's no way to know what's wrong, and starting to tackle possible causes one at a time could end up costing a fortune (not to mention it'll take forever, since I'll have to drive a decent distance between each attempt to determine if mileage has improved).

If it was your car, and you don't have a lot of background with cars but are comfortable with basic to intermediate DIY projects (but nothing too advanced), what would you try cleaning or replacing on your own before paying a mechanic to start replacing components?
1 answer & 2 comments
Popular Answer
on May 19, 2014
If you are certain this issue started after cleaning the maf sensor, then you should re-evaluate that component/location. When you installed the maf sensor, was it installed backwards? Did you (hopefully not) insert the tube from the maf cleaner inside the sensor air inlet hole causing internal maf sensor damage? Are all of the air cleaner tube clamps and screws tightened securely? Did a vacuum line to the air cleaner box get left disconnected? Did the air cleaner tube develop a crack at the ribbed section that would let air in past the maf sensor that the maf wouldn't see? Is the maf gasket sealing properly? , etc. It wouldn't hurt to clean the sensor again by spraying with maf sensor cleaner, letting the cleaner drain, and air drying before re-installation.
P0171 means system too lean bank #1 for v-6, and v-8 engines that is the cylinder bank that has the #1 cylinder, but on four cylinder engines it means the entire engine is lean. Lean means too much air, or not enough fuel whether real, or perceived by the pcm due to faulty sensor data. Other than that you might run some every 5,000 mile fuel injector cleaner through the fuel tank. Clean the throttle body plate, and intake air control valve (IAC Valve) passage in the throttle body. Check for vacuum leaks, check for cleanliness of air filter, and at some point replace your front oxygen sensors with new exact fit Denso brand sensors since they should be replaced every 100,000 miles for engine efficiency weather they are failing or not. You can purchase a Haynes manual for your car for around $25.00 that will have vehicle specific diagnostic and repair information also.
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on May 19, 2014
Thank you! I can't say for certain whether the MPG issue started before or after the MAF sensor cleaning, but it was definitely right around the same time, so I too assumed there was probably some link. I was extremely careful while cleaning the sensor to not touch the sensor or air inlet, and I was very careful reinstalling the sensor the same way I had taken it out, but I will check that and clean it again tonight. I will also run fuel injector cleaner through, clean throttle body plate and IAC valve. The air filter was replaced with my last oil change about 2k miles ago, so I'd be surprised if that's a problem. If I can get my hands on Denso brand O2 sensors, I'll work on that tonight as well.

Thanks a lot! If you have any other suggestions, I'm all ears.
on May 19, 2014
When the spark plugs were removed, were they all a light tan color with well defined electrodes? If one was a different color such as black, or clean, then you would have more info to help diagnose the issue. I am thinking your OEM plugs should have lasted 100,000 miles easily. The fuel filter should be replaced every 35,000 miles. wipe clean the throttle body, throttle plate and the passage going toward the IAC valve from both sides of the throttle plate with a bent L shaped plastic tie wrap (ziptie) and vacuum out any debris before you do anything else and see how it acts after that.
A good mechanic with a professional scan tool $250.00+ type scanner could read live data from the pcm and specifically check the short term, and long term fuel trim and easily tell if the car is rich or lean, and how much so. Was the timing belt replaced on schedule? Your engine is most likely VVT correct? If the poor fuel economy and cell light happened before oil changes then my guess would be the VVT filters are clogging which would cause the valves to not be in sync with the rest of the engine. The remedy for that is an immediate oil change with synthetic oil, and the cleaning of the VVT filters. VVT = variable valve timing.
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on May 19, 2014
If you are certain this issue started after cleaning the maf sensor, then you should re-evaluate that component/location. When you installed the maf sensor, was it installed backwards? Did you (hopefully not) insert the tube from the maf cleaner inside the sensor air inlet hole causing internal maf sensor damage? Are all of the air cleaner tube clamps and screws tightened securely? Did a vacuum line to the air cleaner box get left disconnected? Did the air cleaner tube develop a crack at the ribbed section that would let air in past the maf sensor that the maf wouldn't see? Is the maf gasket sealing properly? , etc. It wouldn't hurt to clean the sensor again by spraying with maf sensor cleaner, letting the cleaner drain, and air drying before re-installation.
P0171 means system too lean bank #1 for v-6, and v-8 engines that is the cylinder bank that has the #1 cylinder, but on four cylinder engines it means the entire engine is lean. Lean means too much air, or not enough fuel whether real, or perceived by the pcm due to faulty sensor data. Other than that you might run some every 5,000 mile fuel injector cleaner through the fuel tank. Clean the throttle body plate, and intake air control valve (IAC Valve) passage in the throttle body. Check for vacuum leaks, check for cleanliness of air filter, and at some point replace your front oxygen sensors with new exact fit Denso brand sensors since they should be replaced every 100,000 miles for engine efficiency weather they are failing or not. You can purchase a Haynes manual for your car for around $25.00 that will have vehicle specific diagnostic and repair information also.
Sign in to reply
on May 19, 2014
Thank you! I can't say for certain whether the MPG issue started before or after the MAF sensor cleaning, but it was definitely right around the same time, so I too assumed there was probably some link. I was extremely careful while cleaning the sensor to not touch the sensor or air inlet, and I was very careful reinstalling the sensor the same way I had taken it out, but I will check that and clean it again tonight. I will also run fuel injector cleaner through, clean throttle body plate and IAC valve. The air filter was replaced with my last oil change about 2k miles ago, so I'd be surprised if that's a problem. If I can get my hands on Denso brand O2 sensors, I'll work on that tonight as well.

Thanks a lot! If you have any other suggestions, I'm all ears.
on May 19, 2014
When the spark plugs were removed, were they all a light tan color with well defined electrodes? If one was a different color such as black, or clean, then you would have more info to help diagnose the issue. I am thinking your OEM plugs should have lasted 100,000 miles easily. The fuel filter should be replaced every 35,000 miles. wipe clean the throttle body, throttle plate and the passage going toward the IAC valve from both sides of the throttle plate with a bent L shaped plastic tie wrap (ziptie) and vacuum out any debris before you do anything else and see how it acts after that.
A good mechanic with a professional scan tool $250.00+ type scanner could read live data from the pcm and specifically check the short term, and long term fuel trim and easily tell if the car is rich or lean, and how much so. Was the timing belt replaced on schedule? Your engine is most likely VVT correct? If the poor fuel economy and cell light happened before oil changes then my guess would be the VVT filters are clogging which would cause the valves to not be in sync with the rest of the engine. The remedy for that is an immediate oil change with synthetic oil, and the cleaning of the VVT filters. VVT = variable valve timing.

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