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My wife has a 2001 Chevy Prizm, which as you probably know is just a Chevy branded Toyota Corolla.
Anyway, it has the 3 speed automatic, generally gets close to 35 MPG on the highway and about 27 MPG in town. She has 120K miles on it, and has been pretty consistent with that mileage until just the last couple of months. Now in town she is seeing only around 20-22 MPG, which seems jsut ridiculous for a car this small. Her driving habits are no different, all she does is drive the same route to and from work. It seems to run fine, no hesitation or lack of power, so I wouldn't think there was any problem with the injectors. Any ideas what would cause this? Would it be a good idea to have the spark plugs and wires replaced? The plugs were replaced 3 years ago (which for her light driving was only about 15K miles ago), if that matters.
The single most overlooked cause to low gas mpg is low tire PSI! Yep, driving on under inflated tires is like dragging a boat anchor behind the car due to the much increased rolling resistance. Believe it or not!!
IMO experience it's best to inflate your tires to within 3>4 lbs of max (to allow for expansion) as listed on the tires side wall to get the best gas MPG. Ride might suffer a bit but that will cost you nothing in the end like low tire PSI will!
The single most overlooked cause to low gas mpg is low tire PSI! Yep, driving on under inflated tires is like dragging a boat anchor behind the car due to the much increased rolling resistance. Believe it or not!!
IMO experience it's best to inflate your tires to within 3>4 lbs of max (to allow for expansion) as listed on the tires side wall to get the best gas MPG. Ride might suffer a bit but that will cost you nothing in the end like low tire PSI will!
Bad advice to say the least. Tire pressure should be what is indicated on the sticker on the drivers door, not the tire. Over inflation will cause premature tire wear and loss of traction due to reduced contact patch. Maybe you should stick to bicycles
The single most overlooked cause to low gas mpg is low tire PSI! Yep, driving on under inflated tires is like dragging a boat anchor behind the car due to the much increased rolling resistance. Believe it or not!!
IMO experience it's best to inflate your tires to within 3>4 lbs of max (to allow for expansion) as listed on the tires side wall to get the best gas MPG. Ride might suffer a bit but that will cost you nothing in the end like low tire PSI will!
I wouldn't exceed the manufacturer's spec for the tire pressure on this car.
Inflating these properly sized tires on the car will have it wear the center down much faster than the uniform wear that these cars regularly deliver for long tire life. On a car that's a little challenged to begin with for ride quality, it's not worth the incremental difference between the proper pressure vs overinflation.
The real issues here are more likely the car's condition/state of tune and the liklihood of 10% ethanol (or higher) fuel in the area.
Just reading through an AD letter from Cessna re ethanol fuel not being approved in their aircraft ... it states that their testing shows that changing from AVGAS to ethanol blend fuel (of 10%) requires a 40% increase in the fuel flow to match the same HP originally produced by the motor. That's the dirty little secret about 10% ethanol blends ... they are that much less for making horsepower.
Bad advice to say the least. Tire pressure should be what is indicated on the sticker on the drivers door, not the tire. Over inflation will cause premature tire wear and loss of traction due to reduced contact patch. Maybe you should stick to bicycles
Dead wrong bucko! The tire PSI on the door is what the car manufactures want you to run so you get the best ride NOT the best gas mpg. Harder tires roll easier with less resistance so there is less energy required to move the car i.e. better gas MPG.
"Overinflation" is when you inflate past the maximum sidewall tire PSI !
It's all about physics.............
Oh yes some should send your smart butt back to school..
IIRC there is a way to make a rough evaluation using an old school dwell meter. Something like 45 degrees of dwell means the sensor is working and the system (easy to probe. is responding - no?
If the CPU cannot make sense of the O2 sensor readings, it goes into open loop mode. Different manufacturers have varying criteria for going into open loop:
O2 must reach 0.9 V;
O2 must reach 0.1 V;
O2 must flop back and forth between 0.9 V and 0.1 V in so many milliseconds.
Generally speaking, the more the O2 voltage resembles a square wave, the better the O2 sensor.
Modern O2 sensors have four wires, two for the heater, two for the output voltage. In my 2005 Honda Accord, the output plug from the O2 sensor sits just above the engine block on the left-hand side and is easy to access. If one has a scope, it is easy to probe the O2 output voltage and watch it flip-flopping back and forth between between 0.9 V and 0.1 V (more less or less) .
The Honda O2 sensor itself can only be removed from below the engine.
Heated O2 sensor costs between $50-$100. Labor cost is about 0.7/hr to install. So if your mechanic charges $100/hr, the cost may come to $70+$75) or ≈$150.
Replacinging an O2 sensor is easier than replacing a spark plug.
One thing you got to consider is that when engines wear from years of service they just cant run as efficient as when they are new with tight compression. Wear caused friction.Friction causes the engine to work a little harder and thus more gas is needed.
One thing you got to consider is that when engines wear from years of service they just cant run as efficient as when they are new with tight compression. Wear caused friction.Friction causes the engine to work a little harder and thus more gas is needed.
This is another area not well understood by many ... while it sounds logical that normal engine component wear will result in a loss of volumetric efficiency, hence loss of fuel economy ....
Extensive testing in carefully monitored fleet operations has shown that absent an almost "dead" cylinder on compression, the ability to make power from an internal combustion motor is not greatly affected by in-service wear on street motors at typical horsepower/cu in ratings. As long as the piston can suck in the air/fuel mixture, compress it to some extent ... even if not as completely as when the motor was new ... and contain the combustion to drive the piston ... it won't have a great loss of fuel economy. Keep in mind, we're visiting street tuned motors at manufacturer's regular output in normal street service and not a racing motor tuned to within a fraction of it's life with a goal of maximum performance just long enough to finish a race.
The OP posted a loss of about 20% in fuel economy. That's not what you'd get for a loss from a slightly worn motor, and they've only turned a bit over 100K miles on a motor that's typically mechanically sound for many more miles than that. Absent a completely dead cylinder, which would result in a miss-fire and setting trouble codes, the engine internal wear isn't likely to be the problem. More likely would be fuel systems, electronic controls/sensors, exhaust system obstructions, ignition problems, or the poor quality of the fuel.
I've done a lot of leakdown testing of automotive engines through the years, and the results can help pinpoint why a given motor may be an "oil burner", but not necessarily a low power or low fuel mileage motor ... even with worn rings or valves not sealing as well as they should anymore.
Thanks for the info! I am not mechanically inclined at all, but being able to scan and at least see what the problems are would help a LOT, so I may look into this. So the "check engine" light on the wife's car is no longer on (was only on for a couple days), but she's still getting the same rather poor gas mileage- is it possible the Oxygen sensor could be out/going out without triggering the "check engine" light?
Absolutely. It didn't activate the check engine light on my car but I replaced the oxygen sensor myself for $50 from Oxygensensors.com when I thought that was the problem. In my case it was a bad contact at my coolant temp sensor causing the ecu to default to that rich fuel setting so I wound up replacing the o2sensor unnesessarily and that's before I bought the scantool. With the scantool I would've saved the time and money of replacing a good part. Bad Oxygen sensors don't always trigger a check engine light for sure and that bad contact at coolant sensor didn't activate it either so again with a scantool I undoubtedly would've pinpointed the problem in no time.
Now I have a scantool to pull trouble codes and it tells me about intermittant problems so yeah I do recomend getting one. I also bought a used factory service manual for my car off ebay for cheap but a Chilton or Haynes manual specific to your car will do because the scantool will tell you what's wrong and the manual will show you where the sensors and other parts are. Another thing is you can buy good used parts off ebay for a fraction of the price of what new costs, although in some cases like o2 sensors it's better imo to buy new.
You can type in obscure part #'s on ebay like MD166255 and there's a good chance someone's selling that part. Btw, that's the part # of the ecu for my car which I did buy used off ebay for around $100 and the dealership wanted $1200 for a new one
One other thing, there are forums dedicated to just about every car out there so if you own a 2004 Honda Accord for example type that in google along with 'forum'. You will not only get great advice but many forums have a used parts section,. I've been using 2 national forums specific to my car (90-94' eclipse/talon/laser) and along with the advice, scantool, service manual, and used parts availability I have saved a small fortune over the past 5 yrs or so because of them
Absolutely. It didn't activate the check engine light on my car but I replaced the oxygen sensor myself for $50 from Oxygensensors.com when I rhought that was the problem. In my case it was bad contacts at my coolant temp sensor causing the ecu to default to that rich fuel setting so I wound up replaced the o2sensor unnesessarily. Bad Oxygen sensors don't always trigger a check engine light either though.
(snip):
This is why I like to get a signal from various components displayed on my o-scope or output readings displayed on my scanner, looking for any that may be out of line with "normal".
While it's a good practice to replace aging oxy sensors as a general rule ...
It's better to diagnose the problem, or take general measures to "clean up" the electronics in these late model cars than to guess and simply replace parts.
It's amazing what you can do on a lot of cars by simply cleaning up all of the electronics and electrical connections. Taking the sensor connectors apart and cleaning them with spray contact cleaner can help a lot of times. Dirty intake airflow sensors can many times be cleaned up to improve their performance. And engine to frame to ground circuits for computers are especially sensitive wiring circuits for reference voltages/signals. The computers can only control what they can "see" ... and if the signals are messed up, the output is messed up, too.
On Jan 1, every gas station in the U.S. is mandated to buy 10% ethanol blend gasoline. Maybe in your area ethanol was not being used and you are now feeling the effects of it. Older vehicles get less mpg with ethanol.
My old Honda civic use to get 35-37 mpg. Now I'm lucky if I get 31-33 mpg.
On behalf of boaters I'd like to thank the environmentalist for ruining our gasoline. Can't find a non-ethanol station anywhere in our parish.
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