i just but an s4 with a TII swap and it dosent have a 02 sensor, the hole for the 02 sensor is plugged...car runs fine with out it...could this cause any problem in the future?

 

You are just going to get some seriously bad gas mileage...

 

It will run fine as long as it stays in open loop mode. If the ECU ever mistakenly decides that the O2 sensor is present, it will run terribly (ben there, done that).

You will also tend to have your catalytic converter working excessively hard to burn up the excess HCs; it will run very hot (mine made the gearshift lever hot to the touch), and over a short period of time it will destroy the catalytic converter. That will cause you to fail your next smog check, or suffere poor running and lack of power from the clogged cat, or both.

Then, you also run the risk of the carbon build-up inside the engine (from always running too cool, due to the rich mixture in open loop mode.) This can lead the engine to seize up (mine did), which will cost you a new engine if your mechanic doesn't know about this problem and how to un-seize it.

O2 sensor is $19.99 at Kragen/OReilly, and $17.99 at Autozone.

So: $20 now, or $500 to $3500 later. I know what I would (and did) choose.

 

Having the O2 sensor will not cost you any performance during acceleration. Under acceleration the ECU goes into open loop anyway, and runs a pre-determined rich mixture. It only goes into closed loop (using the o2 sensor to adjust the mixture) in relatively steady rpm (cruising) operations.

This is actually good for the plugs. Keeping the combustion temps up during cruise reduces fouling of the plugs, and prevents other problems. If your plugs are fouled from running rich in cruise, it will actually cost you performance during acceleration.

 

I recommend you unplug the O2 sensor hole, install a new O2 sensor, and then verify the car is going into closed loop mode at steady speed by conducting the 'green lamp' test. See the FSM for the process of verifying closed loop operation. But instead of getting one of Mazda's (very expensive) digital code checker boxes (which has the green lamp built in), you can easily make your own: you need a LED (doesn't have to be green but that is traditional), a 510 ohm resistor, a few inches of two conductor wire, and two male spade lug crimp connectors. Use the center two pins of the test connector (upper is +12v, lower is 'green lamp' pull-down, ie provides a ground when the lamp is supposed to be on.)

You may have to reset the ECU to get it to realize the new O2 sensor is there. Mine took three or four restarts before the ECU figured it out on its own.

 

If you make the two-conductor wire long enough to route it back into the cabin, you can put it on the dash and check it while driving. That is my preferred method; then you don't have to mess with disconnecting the neutral switch per the FSM procedure, and you can see the system perform while driving.

This will also allow you to identify that a problem exists with one of several other systems; ie. the lamp should be on through all acceleration. If it goes out, your engine is going lean. This allows you to potentially pick up things like vacuum leaks, improperly operating emissions systems (port air, split air), clogged injectors, clogged fuel filter, weak pump, etc.)

 

If you really want to be cool, you can mount the LED in the dash somewhere, and wire it in for good, and then you can monitor it whenever you drive. Some others here have done that, I am planning to go that route eventually as well.

Do a similar trick with the two pins that provide trouble codes to the code checker, and you can have the equivalent of a check engine light, which an S4 does not have. (Don't know about your installation...)

 

You will need a special socket to install the O2 sensor. I recommend the crow-foot style; the socket type is not quite tall enough. Harbor Freight has them for less than $5.

 

6 posts in a row? Just hit the edit button instead, calpatriot.

Back on topic, an 02 sensor is too cheap and easy to replace to ignore it.

 

Might be time to consider a wide band o2 sensor.

 

On my 86, instead of disconnecting the neutral switch, I just put the car in gear. Any gear (talking about checking the closed loop mode in the driveway without actually driving the car). Then the light will blink when the rpms are over 1500 or so rpm showing that the ECU and 02 sensor are *talking* to each other and the engine is indeed in the closed loop mode.

For normal driving, I just run the wires for the LED from pin 1D on the ECU to the LED and find a switched source of batt voltage (there's a plug near the pedals meant for automatic cars that is a source, not used on manual cars otherwise). Like I say, lets you Know your in closed loop and also gives you a clue if your rich or lean at other times. Like pegging the pedal to the floor for rich and letting off the pedal for lean.

 

i have a straight pipe installed
do i still have to install a 02 sensor?

 

You don't have to do anything. Just read the replies above and make your own educated decision.

 

Stating that running open loop constantly will cause enough carbon buildup to eventually seize the engine is the most retarded thing I've read in a while, but that is only my opinion.

I don't run one and I get 17mpg city (better than OEM) with 12.0+ cruise AFRs. No hestiations, no nothing. Do a water/seafoam flush treatment every 5000 miles if you are really paranoid. If its a stock car, the $20 would probably pay for itself and then some with the decreased fuel consumption, though.