HELP: BMW OBD Codes 235, 236

[Wayne's World]

Hi guys - long time! Referencing one of my long threads from about a year ago, it seems my problems have resurfaced. The solution to the problem at that time was an after-cat oxygen sensor.

235 - Pre-Cat Bank 1, Fuel Trim
236 - Pre-Cat Bank 2, Fuel Trim

This time, the symptoms are the same as before:

Idle isn't silky smooth but not anywhere close to rough anyway - I can definitely tell the motor is running by feeling the vibration in the driver's side door panel but otherwise, mostly imperceivable.

MPG doesn't appear affected, averaging 18.5 MPG combined over a period of 1K miles.

Performance doesn't appear affected as it still hauls as much as the 3.0 can.

After two short trips when the motor is cold, the light will be off on the 3rd startup. It's not until I drive for an extended period of time before the light will resurface.

So far I've taken it to my indy and they've done the following:

1. Checked DME code and it is the latest version. There was apparently a service bulletin on this.
2. Done a smoke test on the intake and replaced the air intake boot. Elbow was cracked going into the idle control valve.
3. Replaced the valve cover gasket as it was leaking.
4. Replaced my NGK Iridiums with the NGK originals. Iridiums read as a normal condition leaning towards rich-running. Torqued to 23ft/lbs.
5. Replaced my MAF with a Siemens/VDO one.
6. Replaced the air filter
7. Cleaned throttle body and ICV.
8. Removed and inspected the DISA valve/Manifold adjuster unit.

Oil Separator, spark plugs, fuel filter and pre-cat O2s were replaced roughly 2 years/40K miles ago. In this time I've done a comprehensive 60K service. Since 40K miles ago, I've really done nothing other than the above to treat this issue which cropped up in November. I suspect I ran too strong a dose of fuel system cleaner and this could have screwed up my O2 sensors prematurely (Berryman's B12 Chemtool - 2 cans in 1 tank) and perhaps this in conjunction with the winter-blend/ethanol gas might have just been too much?

I really want to think it's the O2s but hate to fault the O2s for reporting a problem.

My only real inclination is that because of the short trips when the motor is cold, the O2s run a self-heat cycle and perhaps clean themselves enough to run normally whereas once the engine is warmed up, they return to a 'normal' state and therefore are not running optimally?

Any advice would be greatly appreciated. If anyone knows how to test the O2s that would also be greatly appreciated. I've heard that they run between 1 and 1.5 volts DC but can't recall where I read that.

[1stE53]

Here is a long shot... did you reset the codes after replacing all that? Also, I use an App on my Android called Torque. I am able to watch and LOG all the data from my engine. It will monitor the long term and short term fuel trim on both banks and all sensors. It will also measure the O2 volts as well. I'm still learning how to read all the data but it will post it online or you can export to Excel.

[Wayne's World]

I've actually got Torque installed but I don't have the BT unit yet - might have to splurge on that. I decided to replace the O2 sensors, hopefully i'll get them in the early AM tomorrow before I leave for my trip.

[1stE53]

Originally Posted by Wayne's World

I've actually got Torque installed but I don't have the BT unit yet - might have to splurge on that. I decided to replace the O2 sensors, hopefully i'll get them in the early AM tomorrow before I leave for my trip.

$20 on ebay

[Wayne's World]

Finally broke down and took it to the dealer. They sent the info off to BMW as the tech and foreman couldn't make sense of it. BMW came back and said the data depicts the car taking too long to get to operating temperature thus running rich. They suspected a thermostat leaking internally. I'm going to do a partial cooling system refresh (coolant temp sensor, upper and lower hoses, water pump & thermostat) and see how that plays out.

As for the short trip scenario, the explanation was that while cold, the DME is in agreement that the engine should be running rich and upon those few starts, sees no problem. It isn't however until a longer trip when it determines that it's been running rich for too long and throws up the code.

[Wayne's World]

Well the cooling system flush, thermostat, water pump and hoses did not resolve the issue. The codes cleared on their own with the short trips but as soon as a longer trip (>5mi) happened, it came right back! I'm at a loss. At this point I could just trade it in but I think instead of getting into another car payment, I'm just going to continue driving it! There really is nothing wrong with the way it drives.