Bad gas mileage - low O2 sensor voltage

[hicas240sx]

hi my car has been getting terrible gas mileage. i checked the voltage at the o2 sensor at idle and its switching between 100 milivolts to 250 millivolts. i can get it to hit about 850 millivolts by adding propane and it can hit 50 millivolts by creating a vacuum leak. am i testing correctly? i dont have a DSO so i cant check it for laziness. any ideas?

thanks.-alex

by the way when i bought it, it already had a dedicated ground (seperate) on the MAF. the voltage drop on that is .03 volts.

[NISTECH]

Aside from the switching you are testing it mostly correctly. It appears your car is running super lean all the time, But you say you have crudy fuel economy, so my thinking here is one of you cyl is not burning the fuel its getting and pumping a huge amount of air through the cyl. Does the car feel like it is running on all 4 cyl's? Assuming its a 4 cyl here. you can do a cycles test by using the ECM but it will need to go above .5 v during the test. All tests on the o2 should be done at about 2000 rpm's . This is due to nissans "clamping" at idle. They do that to give you the best possible idle so it is basically out of closed loop when that happens.

[s13sr20chris]

have you checked for a vac leak? having a vac leak could drive up your fuel trim and cause you to run rich at cruise and during accel.

[NISTECH]

wow talk about getting ahead of myself..lolGood one Chris! I jumped right into the O2 concern.DOH!

[hicas240sx]

i checked the voltage at 2000 rpm and its still the same. i checked for possible exhaust leaks and found that 2 nuts on the exhaust manifold were loose. i tightened them and the voltage is still the same. im going to take off the exhaust manifold and inspect the bottom part of the gaskets. an exhaust leak would cause this condition right? if i dont find one then how do i check for intake leaks? i dont hear anything on the intake side and the car runs pretty smooth.

by the way its a 240sx 2.4Lthanks for the help-alex

[NISTECH]

watch your O2 readinga and spray carb clean where ever there could be a vaccum leak. Watch for your voltage reading to spike.

[Nismo_Freak]

i dont have a DSO so i cant check it for laziness. any ideas?

You can check it for operation, there is an 02 pulse test via the ECU diagnostics. It will flash the CEL in order of the ocillations. Don't recall the number of pulses, it would be in the FSM.

All you need is a FSM, 10mm, and a flat-head screw driver.