Poor gas mileage? (traction, spark plug, tires, MPG)
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2) Oxygen sensors may be "lazy" or failing, but still giving adequate signals to the computer which doesn't trigger a fault. The system is designed to trigger "faults" which could adversely affect emissions, but not so sensitive as to trigger codes when it's simply fuel economy at risk.
The above statement is "spot-on."
A DTC is set for a pre-cat O2 sensor if (1) the output is too high or remains high for a period of time; (2) the output is too low or remains low for a period of time or (3) the number of "zero crossings" (where "zero" is about 0.45 V) per unit time is less than expected (the signal should oscillate at ≈1 to 7 hz, depending on the engine). Fuel economy is best when the signal flips sharply between full rich and full lean, so that the output resembles a square wave. Over time deposits accumulate on the electrodes and the signal starts to resemble a sine wave. That is, the rise and fall times of the O2 sensor output increases.
The most common DTC for an O2 sensor is caused by a malfunction in the sensor's heater circuit. At cold temperatures there is no output from the sensor, but the ECU waits a short period of time, about 30-60 sec, for the sensor to heat up. If no signal is received by that time, the ECU assumes the heater is kaput and sets a DTC.
Pre-cat O2 sensors should be replaced at about 75,000 miles to maintain good fuel economy.
I'd also take a look at the long term fuel trim numbers, if your scan tool gives them, to see by how much the engine is running rich.
BTW, the term "O2 sensor" is a misnomer. The sensor actually measures the quantity of oxygen ions, O⁻⁻², which always occur where ever O₂ is present, whether it be in the air or in the engine exhaust. When an engine is running rich, there is little O₂ and consequently little O⁻⁻² in the exhaust, as compared to the amount of O₂ and O⁻⁻² in the atmosphere. When the atmospheric air strikes the externally vented electrode, more O⁻⁻² strikes and "sticks" to that electrode than to the electrode exposed to the exhaust gases. Since the O⁻⁻² carries a negative charge (the oxygen ion has picked up two electrons) the air-exposed electrode becomes negative relative to the exhaust-exposed electrode. So strictly speaking, the name "O2 sensor" is incorrect; they are called lambda sensors in the rest of the world. Lambda sensors were first developed by Bosch in the late 1950s.
While we are at it, IIRC the air filter is not very important to fuel economy assuming the rest of a closed-loop fuel injection system is operating correctly, right? I read that in Consumer Reports. In my own experience (2 cars) changing a fuel filter that has not been changed in many miles (cars bought used and both were a bit neglected) however *did* improve MPG - I guess because of better atomization of the fuel. BTW don't let anyone tell you the O2 sensor on old CIS (K-Jetronic) cars is not important. I put a new one on the 82 Scirocco after fixing a head gasket failure, the old sensor must have worked to some extent because the frequency valve on the fuel distributor buzzed periodically, IIRC it responds only to the O2 sensor - anyway went from roughly 34-35 MPG to 36-38, with no change in driving style.
While we are at it, IIRC the air filter is not very important to fuel economy assuming the rest of a closed-loop fuel injection system is operating correctly, right? I read that in Consumer Reports. In my own experience (2 cars) changing a fuel filter that has not been changed in many miles (cars bought used and both were a bit neglected) however *did* improve MPG - I guess because of better atomization of the fuel. BTW don't let anyone tell you the O2 sensor on old CIS (K-Jetronic) cars is not important. I put a new one on the 82 Scirocco after fixing a head gasket failure, the old sensor must have worked to some extent because the frequency valve on the fuel distributor buzzed periodically, IIRC it responds only to the O2 sensor - anyway went from roughly 34-35 MPG to 36-38, with no change in driving style.
The air filter is extremely important to all of the FI system proper functioning.
The mapping of the system assumes certain manifold pressure ranges at various throttle openings. An obstructed air filter reduces the airflow, and hence ... to get the power desired, the driver presses further on the throttle pedal. So the computer gets a wider throttle opening sensor at a faster rate to opening, which will also affect the mapping for an electronically controlled automatic transmission (wider throttle opening demands staying longer in lower gears). This is why higher altitude operation will find a lot of cars "hunting" between gears; the result is you accelerate to a road cruising speed, and lift your right foot as you no longer wish to accelerate. The computer upshifts, but then the motor cannot deliver the required horsepower at the RPM so the driver again presses ever so slightly on the throttle to maintain road speed ... and the car downshifts again to accelerate back to the desired road cruise. This same car, at lower altittudes, may have no problem with power/throttle opening/shifting because of the greater volumetric efficiency there.
The best response for a driver at altitude is to manually place the transmission into the lower gear and hold it there while needed so that the car doesn't upshift ... similarly, I see a lot of manual transmission cars driven at altitude where they cannot keep up with traffic flow on the mountain roads. And yet, that same car will have no difficulty if the driver would simply downshift and accept the higher RPM for the climb or road conditions present.
All of the early oxy sensor FI systems were far cruder in their measurements and adjustments ... and the systems prior to using heated oxy sensors were far more critical to fuel economy issues than later systems. It wasn't unusual to see early single wire Bosch oxy sensors fail within 50,000 miles ... and I used to replace them as a matter of routine service when tuning any car with one at that service mileage. K and L Bosch systems were rather touchy in this regard, needing correct system fuel pressure, a proper functioning airflow sensor, etc. I saw a lot of cars that were untunable until a good FI system cleaner had been run through them for at least a half tank of fuel ....
While we are on early FI which is what the 'roc has - the original recommendation was to change the O2 sensor at 30K miles - with improved fuel purity now compared to the early 80's, should I expect it to go somewhat further, maybe to 50K? Of course I'll just keep logging my fuel mileage, and when it starts going down I'll change it...My point on the air filter is that with a carburator of course any clogging of the air filter acts as a choke and your MPG goes bad pronto, while a FI system, so long as it remains in closed loop, will be running on a stoichemetric mixture even if the air filter is not fresh - not saying it has no effect, and of course for you guys at altitude it becomes critical sooner as your air density is already below standard.I guess I am old school enough that I tend to pull the air filter to check it, and if it's dirty I am enough of a pack rat that I have spares on hand in Garage Mahal, so, it gets changed. The question is more theoretical than practical, at least for me.Getting back to the OP's situation, while he didn't complain about the car's running, could a partially clogged cat be the culprit? Or would the car run rough, or otherwise have drivability symptoms, if the cat was clogged enough to hurt economy to the extent he reports?
First thingt I would do is run a intake clean thru the next two tanks and not one of the cheaper ones. You might even need the injectors clean if not using fuels with cleaners at that milege. That can easliy effect the mileage that much especailly if not runing much on the highway at speed.Also check the plug even if fairly new to get a idea how they are burning.
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.
When the oxygen sensor fails the ecu defaults to a very rich fuel setting. On my car if the coolant temp sensor fails it also cause ecu to default to that same rich setting. I'm not sure if there are other sensors that when failing to send a signal to ecu will cause ecu o default to that pig rich fuel setting. On my small car I've seen a sudden 30% drop on gas mileage with those sensors not sending a signal to ecu. In my case it was actually nothing more than the plug to the sensor failing to make a good connection due to bad contacts. I cleaned the contacts with an electrical contact spray and solved the problem. Gas mileage went back up 30% instantly jumping from 18mpg back to 23-1/2 mpg where it should be Places like Autozone read trouble codes for free and I would reccomend investing in a scan tool to read trouble codes that show you exactly what all your sensors are doing. Amazon has a nice scan tool for $130 (innova 3130) and the 130 reviews on it are mostly very high praise.
When the oxygen sensor fails the ecu defaults to a very rich fuel setting. On my car if the coolant temp sensor fails it also cause ecu to default to that same rich setting. I'm not sure if there are other sensors that when failing to send a signal to ecu will cause ecu o default to that pig rich fuel setting. On my small car I've seen a sudden 30% drop on gas mileage with those sensors not sending a signal to ecu. In my case it was actually nothing more than the plug to the sensor failing to make a good connection due to bad contacts. I cleaned the contacts with an electrical contact spray and solved the problem. Gas mileage went back up 30% instantly jumping from 18mpg back to 23-1/2 mpg where it should be Places like Autozone read trouble codes for free and I would reccomend investing in a scan tool to read trouble codes that show you exactly what all your sensors are doing. Amazon has a nice scan tool for $130 (innova 3130) and the 130 reviews on it are mostly very high praise.
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?
If the O2 sensor is bad the car runs "open loop" and uses pre-stored values in the ECU to set air/fuel mixture; gas mileage will significantly deteriorate.
Putting a scope on the wires from the O2 sensor (using sewing pins) is the easiest way for a mechanic to check the sensor.
If the O2 sensor is bad the car runs "open loop" and uses pre-stored values in the ECU to set air/fuel mixture; gas mileage will significantly deteriorate.
Putting a scope on the wires from the O2 sensor (using sewing pins) is the easiest way for a mechanic to check the sensor.
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 is responding - no?
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Places like Autozone read trouble codes for free and I would reccomend investing in a scan tool to read trouble codes that show you exactly what all your sensors are doing. Amazon has a nice scan tool for $130 (innova 3130) and the 130 reviews on it are mostly very high praise.
