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MPG's
With gas prices going threw the roof, never to go down to where they were before- mpgs are of interest.
But even if gas went back to 25¢ a gallon- mpgs are always a concern.
Why? Bad mpgs usually is a sign that something is not working as it should. A bad O2 sensor, fuel injector, running rich- its easier on your engine to run the way it should. And a properly tuned engine gives you the best power.
Look at emissions as wasted hp. Why woud you want your engine to run like crud? You dont.
So what can cause bad mpgs?
Well, lets start with the basics.
Gas cap
Believe it or not, some people steal gasoline out of other people's cars. You should always have a locking gas cap, just to be safe. Plus it rules out that possibility when troubleshooting your engine anyway- so better to have one than not. They're cheap, usually under $5.
Air filter
Prolly one of the most overlooked causes for mpg loss. The more dirty your filter is, the worse your mpgs will be. This also wastes power- you can pickup a bit of Hp by merely messing around with airfilters. This is why throwing in a large cone airfilter can pickup 5-10hp in some situations.
Oxygen Sensor
This is the big one. Oxygen sensors are VERY important to these cars. The oxygen sensor is used by the PCM in non-WOT conditions to fine tune the engine. I forget the exact % of fuel that it can fine tune, but I want to say as much as 20%. If your oxygen sensor is saying your car is lean or if the PCM cant find the O2 sensor, the PCM will literally dump fuel into the engine.
This will make the engine run rich, gum up your spark plugs and in time- kill the cat converter. Not to mention waste some power.
The PCM will/should throw a code when the O2 sensor is dead or unconnected, however not always will. The O2 sensors commonly die over time, thus making it possible for an O2 to need to be replaced for months or even year before the PCM finally throws a error code!
On my car, the O2 sensor slowly died over a period of half a year. In that process first the O2 sensor got lazy- would give fewer readings per sec/minute/etc, then after a while it would only give lean readings... then it stopped making readings altogether. All the while the PCM never threw a code. I can even unplug my oxygen sensor and have the PCM not notice!
The O2 sensor typically lasts abt 75-100k miles and it is possible for its wiring to go bad (open, short, etc) and cause mpg problems with the sensor still being good.
Although bosche invented the oxygen senor used in these cars- most people I have talked to have said they have horrible reliablility and sometimes only last 40-55k miles.
Tire psi
An other overlooked item that may rob your mpgs is how much pressure is in your tires, underinflated tires hurt mpgs quite a bit (tho more so on highway use than city use). Also as a safety issue, its always a good idea to make sure your tires have the correct pressure.
Leaky fuel injectors
Fuel injectors get dirty, leak, die- and in the process this can lead to too much fuel in one or more cylinders. The PCM will sometimes notice extreme examples of bad fuel injector, but in the mean time- it is something to consider if everything else has been ruled out.
And then there are the usual sensors to check when all else check ok, MAP, TBPS, coolant temp sender, etc.
Dont forget to ensure the gas tank doesnt have a rock hole or leaky fuel lines while you're at it.