O2 Sensor Issues

I am measuring with a multimeter. I just replaced the MAF, which was looking bad, and the idle seemed smoother, but the mileage dropped drastically (190 miles on 12 gallons). I suspect the o2 sensor is the issue, but with the wire grounded, I think I may have to trace the wire and replace it.

You should not get any voltage until it has reached working temperature.

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Erock, Not sure if the 02 sensor wire on an 89 SOHC KA engine is supposed to be grounded. According to my FSM if the voltage is running towards 1V you are running rich while at 0V you are running lean. Are you sure it's the 02 sensor that's causing the poor gas mileage? Have you tried putting the ECU in diagnostic mode & checking to see if the ECU is kicking a code?

scroll down to the left till you get to 02 sensor then click the link. Thelink will tell you how to put your 240 into diagnostic mode to test your 02 sensor. There is also another link to check for any diagnostic trouble codes that your car may be putting out causing the poor gas mileage. I know for a fact that a poorly grounded strap wire between the firewall & engine block can cause rough idle problems along with a poorly connected wiring harness leading to the engine coolant temperature sensor. These two problems often cause poor gas mileage. However, most 89-90 240sx owners I know say they average about 22 mpg - they ALL have lead foots though!! However if you are still unsure, you may want to consider posting your question to the

240sx-chat list at 's a free chat group/mailing list of all 240SX owners in the United States andCanada. It was started years ago by a 240sx owner who wanted to connect with other 240sx owners. You might want to ask the group if there is a local chapter in your neighborhood. The local groups often have "tech" days where they work on their cars. these 240sx enthusiast can't answer your question, I doubt anyone else can.Good luck!! ID 95 240SX SE ...member SoCal240SX.org

Erock wrote:

ID,

The o2 sensor...appearantly there is a zip tie on the wire holding it up between the head and the firewall, and it was cintched so hard that the wire and the shilding were touching, causing it to be grounded. After replacing this part of the cable, the o2 sensor was reading properly, but the idle was skipping around still.

I checked the grounds, and the little strap going from the engine to the firewall was really really dirty. I bought some 8 gauge wire and piggy backed it. I also cleaned the 3 grounds next to the temperature sensor, and the cable from the engine block to the battery. I replaced the spark plugs and they were very sooty (as it was running very rich). I also replaced the air filter with a k&n, wires, cap and rotor. We'll see how it goes from here. The idle seems to be smoothed out, but I haven't ran it much. Thanks for the help! I'm moving to Rancho Bernardo soon (I currently live in ny), so maybe I can hookup with the socal club sometime. Thanks,

Eric

Hrm.... still got 21.5 mpg on the last tank, and this is almost all highway (my commute is 35 miles each way) and the engine still has a funny idle. It idles at about 750 rpm, and every so often it makes a funny sound (tssssk, misfire?/knock?) and kicks up to like 1500 for a while, and then the idle falls drastically to ~400 almost stalling, and then levels off again. So far I have:

*replaced the o2 sensor *replaced the maf from another 240sx (no difference) *replaced the cap,rotor,plugs (old ones were VERY sooty), wires *cleaned AIV and then removed it (didnt make a difference either way) *cleaned the throttlebody *cleaned the IAA valve *established a second ground for the IAA and cleaned existing grounds *added a second ground from the body to the engine block and cleaned existing *replaced air filter box with k&n

anybody have any idea what could be the problem?

Check for millivolts (should be 0.00v) on ground wire to MAF. Even

0.1v up from 0v due to poor earth will give an increased voltage output and can result in a rich mixture. Splice new ground very close to MAF connector (the cable is shielded, check shield is connected to ground).

Check blow by hose for leaks. Check intake between MAF and manifold for leaks. Check carbon canister signal (small one - should not be able to suck though this one) and purge (bigger) line for leaks. Check ports for purge signal and purge line in throttle body are clear. Check timing. Check compression. Check engine temp sensor. Check ECU for codes, check loom, check resistance 2.5Kohm cold, 300ohm hot, disconnect and if improved replace, clear code due to disconnection. Use diagnostic mode to check idle switch operation is correctly detected by ECU. Check switch adjustment. Power to switch, check switch is 0ohms when closed. If switch is OK and not detected by ECU check loom for breaks.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!