petemoe

I noticed that my mileage is going down
the tube. I used to get around 395-405 miles
per tank of gas, now I seem to be getting 370,
tops. How could this be?

I recently wrote about 2 weeks ago, how the
PCV valve change and throttle body cleaning
really helped the idle and kept the revs from
hunting. But the mileage suffered. Any ideas?

PeteMo 95SE

Daniel B. Martin

Your car is using more fuel. Four possibilities exist:
- you are burning it
- you are leaking it
- someone is stealing it
- you switched to gasohol


Burning it...
- fuel injectors are sticking open
- "tired" Oxygen Sensor
- bad fuel pressure regulator
- obstructed fuel return line
- bad Engine Coolant Temperature Sensor
- stuck-open thermostat (engine never goes into closed-loop mode)
- transmission defect (never shifts into fourth gear)

Leaking it...
Search for fuel leaks by sight and odor. However, some fuel leaks are so
tiny that they escape the human senses. One way to find them is to have
your mechanic use his exhaust emissions tester. Switch the tester to HC
(hydrocarbons) and then pass the probe along the entire fuel system,
starting at the fuel tank, forward along the fuel lines, past the filter,
all around the engine, then along the return line and back to the tank.
The tester is far more sensitive than the human eye or nose. If it passes
near a tiny leak the reading will show an abrupt increase.

Stealing it...
Perhaps someone is stealing small quantities of fuel, not for its financial
value but as a prank. Install a locking fuel filler cap.

Gasohol...
- You may have switched from one filling station to another,
and not realized that you were also switching from gasoline to gasohol.
- You may live in one of those regions which is required to sell oxygenated
fuel during the cold weather season.

Now that MTBE is in disfavor, gasohol is the most common oxygenated fuel.
Alcohol has a lower energy density than gasoline.
The increase in consumption you report is on the order of 10%.
Switching to gasohol could do that.

airmattdog

iTrader: (1)

I got my Fuel Filter replaced and I've noticed a slight difference. I guess I might need to get those other things checked out now. I don't think it's the gas I use, as I always use Texaco, Union 76 or Chevron. None of the cheap stuff. I've always used the Premium 91 octane. Should I get the injectors cleaned or replaced? I have a 97 SE with 66,000 miles on it. What would be the best things for me to look for? Thanks.

Matt

chris j vurnis

Hello Daniel:

You mention "tired" O2 sensors. I have a TECHTOM diagnostic unit that can look at the O2 sensor voltage. What should I be looking for to see if the sensor is bad?

Where is the fuel return line? How can I check if its obstructed aside from taking it apart?

How do I tell if I have a bad fuel pressure regulator?

How do I test if the fuel injectors are sticking open?

Thanks

wicked1044

i told you not to mess with it....

Daniel B. Martin

I am not familiar with the TECHTOM device and cannot advise you how to use it.

The signal from a healthy sensor varies from a low threshold (around 0.2 volts) to a high threshold (around 0.8 volts), and it sweeps back and forth between those extremes about 10 times per second. An automotive oscilloscope is the best tool for visualizing a waveform. If the scope trace shows the Oxygen Sensor signal failing to reach either extreme, or oscillating between them at a frequency slower than 10 cycles per second, that is evidence of a "lazy" sensor.

There are some handheld general purpose electronic devices which could also be used for this purpose. Some of the more expensive Fluke, Triplett, and Techtronix meters would do the job. Perhaps TECHTOM is in the same category. I tried to get information from their Web page but it was mostly in Japanese.

Daniel B. Martin

Raise the hood. Locate the fuel filter. Identify the rubber fuel line which conveys fuel from the top of the filter to the fuel rail. There is a clamp on both ends of this fuel line. Note the clamp at the engine end of this fuel delivery line. Just one inch below that clamp there is another clamp on another rubber fuel line. This is the engine end of the fuel return line.

You could
(1) bleed the residual pressure from the fuel rail, just as you would do before replacing the fuel filter
(2) disconnect the fuel return line
(3) connect a length of test hose
(4) apply pressure with your mouth or a Mityvac pump

The factory service manual does not provide a specification for resistance in the fuel line. In my opinion there should be almost no resistance.

Daniel B. Martin

See the Chilton manual, page 5-3, for detailed instructions. This procedure requires installation of a pressure gauge in the rubber fuel line which conveys gasoline from the fuel filter to the fuel rail. I once installed this gauge in a Stanza to diagnose a driveability problem. I liked the gauge so well that I left it in place as a piece of permanent instrumentation. This gauge is useful in diagnosing many fuel-related engine problems. I wish Nissan would provide a fuel pressure gauge as factory equipment.

Daniel B. Martin

The "right" way is to remove all the injectors and have them tested by a shop which specializes in fuel injectors. However, this is how I would do it on my own car.

1) Install the fuel pressure gauge as shown in Chilton, page 5-3.

2) Measure fuel pressure to confirm that the pump is good and the filter is not clogged.

3) Turn off the engine. The fuel pressure should remain high. This is called residual pressure and it helps the car start quickly at the next start-up.

4) If the residual pressure drops to zero within two hours, it is being lost somehow. That loss might be due to a leaking check valve in the fuel tank, or an external leak anywhere along the fuel loop, or an internal leak (one or more of the fuel injectors is not closing properly, and it consequently "pees" that residual pressure into the intake manifold).

5) If residual pressure is being lost you want to know if the injectors are responsible. Turn the ignition ON to build pressure in the fuel rail. Turn the ignition OFF. Use a pair of suitable clamps to pinch off the rubber fuel delivery line and fuel return line. My preferred tool for this kind of job is ViseGrip #29 which has rounded jaws that will not tear the rubber line. See http://www.americantool.com/pi/toolS...=37&size=large
Pinch the delivery line between the filter and gauge, so you can still monitor the fuel rail pressure. You have now isolated the fuel rail. If you still lose residual pressure, it is likely because of leaky fuel injectors.

chris j vurnis

Well, my Techtom unit shows the O2 sensor voltage going between 0.00V up to 0.8V. I see that the sensor voltage sometimes hangs out at the low threshold for several seconds and then sometimes at the high threshold for several seconds... and then sometimes its jumping around....
It doesn't look like its changing at 10Hz like suggested below.


So I guess I should have it checked out by the dealer ???

Daniel B. Martin

If your Maxima is a '95.5-up, it is OBD-2 compliant and has three oxygen sensors. Use your Techtom to take readings from all three. See how they compare.

Don't be too quick to condemn an Oxygen Sensor even if it measures "bad". There are other engine problems which can make an oxygen sensor look bad. See http://www.kemparts.com/tt07/tt07.html for examples.

If the suspect Oxygen Sensor is one of the two front sensors, they are alike. You could exchange them, drive normally for a week, and then take new readings. If the bad sensor became good and the previously good sensor became bad then you would suspect the real problem lies elsewhere.

After due consideration you may conclude the suspect sensor really is bad. At that point, I would not take the car to the dealer for further diagnosis. The dealer will probably charge you one hour's labor for testing. That money is almost as much as the price of a new name-brand Oxygen Sensor. PartsAmerica.com lists a Bosch Oxygen Sensor for $78 + S/H. You would be better off trusting your own judgement and the Techtom readings. Just go ahead and replace the sensor.

See http://www.volksparts.com/o2sensors.htm for more useful information about Oxygen Sensors.