My O2 sensor hates me. Help me fight back.

[Soobster]

First of let me say that you guys rock, and vrg3, you rock the hardest. Thank you all for rocking!

I recently moved to San Francisco. The first time I drove my Soobaru around town, the "check engine light" came on. The check engine light had never illuminated previously, even when my crankshaft pulley was falling off (but that is another long story). My wagon was also running really poorly and making a tragic sounding exhaust noise.

I checked the codes and got a 32 and a 42 then I decided to change the O2 sensor. I replaced the sensor with a brand new Subaru part from the dealer and that made the exhaust noise go away, but I still get the check engine light every now and then. Always throws a 32 and a 42 and the check engine light only comes on when I am in San Francisco, never outside of the city so far regardless of the weather or the terrain I'm driving over/through etc...

Note: when the check engine light comes on now, it is not associated with any negative engine performance or really anything noticable besides the red light on my dash.

I am planning on making/using vrg3's scantoll program and connector (once again, you rock) but I will not be able to use it until I recover my old laptop from storage.

Some things that might be relevant to the problem are:

1.) When I bought the car the previous owners (read jackasses) had punched holes in the exhaust combiner/catalytic converter where the O2 sensor is located. I had the holes patched by an exhaust shop (and I passed smog with flying colors) but I'm wondering if the jackasses messed up the guts of my exhaust which is negatively affecting my tempermental oxygen sensor.

2.) Exhaust Leak. There is a chance that I still have a hole in my exhaust system somewhere.

3.) Intake leak. Chilton manual talks about intake leaks causing a 32 code to be thrown. All my rubber hoses are hooked up and clamped but some are getting a little tired. Plus I have no idea how to tell if I have an intake leak.

4.) Large barometric pressure fluctuations. I have the sneaking suspicion that this problem is arising due to the nature of the San Francisco bay area and the rapidly changing weather and elevations.

5.) I never took the time to reset my ECU after installing my new O2 sensor. I am going out to do this now, so we will see what happens tomorrow.

So what do you think? What would you do?

I am personally looking forward to running the scantool but until then any advice is always appriciated.

......You rock rock, you just sit there and that is what we need right now.......

[legacy92ej22t]

Well, you will still see the codes until you clear the ECU. Try clearing the ecu and then when you get a CEL recheck the codes. The O2 sensor code may not even pop back up.

To check for an intake leak, spray the engine bay with starting fluid (ether) and listen for changes in idle. If it's running rough and when you spray a certain area it smooths out or the RPM raises slightly, then that's the area of the leak.

[Soobster]

Resetting the ECU seems to have done the trick. I haven't had a check engine light for the last wo days and gas milage is noticeably better.

I guess I've learned my lesson to not get lazy and make sure to reset the ecu after changing out sensor parts.


Also, just because I'm curious.

If the atmosphere is more dense (more O2 per cubic inch) does this cause the ecu to run a richer mixture? How does barometric pressure work into this equasion (for a naturally aspirated engine)?

Does the relationship above affect gas milage? Will my car get better gas milage in Denver as opposed to Death Valley? Assume other factors such as driving habits, hills, accessory usage etc are constant between the two scenarios.

[vrg3]

Huh, that's a little odd that resetting the ECU eliminated the CEL. I guess that's good though.

The engine does not run richer or leaner in a denser or thinner atmosphere; the mass airflow sensor measures the mass, not the volume, of air ingested. So even without knowledge of barometric pressure, the ECU would still achieve appropriate air/fuel ratios.

The ECU does have a barometric pressure sensor, though. I'm not sure what exactly it does with its signal.

You do generally get worse fuel economy if the air is thinner. This is not because of the air/fuel ratio but rather just because the thinner air burns less fuel, producing less power, while all the friction in the system (rings against cylinder walls, crankshaft drag through the oil, tires against the road, etc) remains about the same.