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After all this water here in Virginia, I keep getting a bunch of calls about their car doing the same thing. I figured it out was the o2 sensor, after driving through water, or been sitting somewhere with deep water. I have heard of wet o2 sensors doing funny things in the past. What is happening when the o2 sensor is getting wet? It happened to me after the storm, the connectors are all watertight and are done very well, everything seemed sealed. Anybody know whats really going on when its getting wet, and why unplugging it for a day or so, fixes the problem?

I dare you to make less sense.


What is the problem?

Moisture gets into the NON-watertight push-fit connection under the car, and shorts the heater circuit voltage to the ECU pin in the connector. This causes the ECU to think the car is going full-rich or lean (I think rich) and causes the ECU to lean out the mixture to where the car will barely run. Notice that when this happens, you can bring the car to WOT about 2k rpm or so and it pulls just fine under the WOT fuel map. I believe this may be why BMW moved the connector high up in the engine compartment on later models. Liberal amounts of dielectric grease in the connector usually cures the problem after you've dried the connector out. I suppose the moisture could also seep into the actual body of the sensor as well, but I have no evidence to back that up.

Haha I knew this was confusing, I was just wondering why the car runs like absolute nuts when the sensor gets wet. I was curious to what was going on, on a very technical level. Just one of those curiosity questions, no problem.

Kyle in NO wrote:Moisture gets into the NON-watertight push-fit connection under the car, and shorts the heater circuit voltage to the ECU pin in the connector. This causes the ECU to think the car is going full-rich or lean (I think rich) and causes the ECU to lean out the mixture to where the car will barely run. Notice that when this happens, you can bring the car to WOT about 2k rpm or so and it pulls just fine under the WOT fuel map. I believe this may be why BMW moved the connector high up in the engine compartment on later models. Liberal amounts of dielectric grease in the connector usually cures the problem after you've dried the connector out. I suppose the moisture could also seep into the actual body of the sensor as well, but I have no evidence to back that up.

This is exactly what I was looking for, thankyou.

It can also muck up the fuel trim in the ECU so that the car won't run properly except at WOT even after the sensor/connection has been dried out and/or completely disconnected. The solution for this is to disconnect the ECU from power for about 30-60 seconds, then plug it back in. You can either disconnect the ECU from the harness or disconnect the battery.

Jeremy

Jeremy wrote:It can also muck up the fuel trim in the ECU so that the car won't run properly except at WOT even after the sensor/connection has been dried out and/or completely disconnected. The solution for this is to disconnect the ECU from power for about 30-60 seconds, then plug it back in. You can either disconnect the ECU from the harness or disconnect the battery.

Jeremy

When do you disconnect the ECU? After its dried out or after you replace it?

When the car still runs like garbage/not at all after drying out the O2 sensor or replacing it. I had an O2 sensor that liked to freak out if it got too wet, so I'd need to reset the ECU just to get home. Stopped after I replaced the O2 sensor, but it took me a while to figure it out.

ok.. I think I have a bad o2 sensor. I guess once I put a new one in I'll find out if I have to reset mine or not.

If you think you might have a bad O2 sensor, resetting the ECU isn't going to do squat. You'll know when it's required.

I'm certain it's my o2 sensor. When I unplug it, the engine runs ok, but WOT it's fine.

It is a situation that appears to be a VA phenomenon. While driving through heavy rain my car started to run like shit. Rain let up, road dried and it ran fine. Next deluge sidelined the car and unplugging the 02 sensor fixed the ability to drive the car but my car runs much better closed-loop.

There is an issue with the 02 sensor whether the sensor, wiring where most have crimped on a $25 generic instead of the $125 complete with connector, or the connector to the wiring harness. Something gets wet that stops the car...in Virginia based on my experience.

The more I read the more it could be the o2 sensor. p. The ECU was replaced in 2011.