Pre-Cat Fuel Trim Lean AND Rich?!

[Wayne's World]

Thanks for the reply. I did actually pop in the BMW OE NGK's back in May but that didn't do the trick.

[SlickGT1]

Well too rich means too much fuel is hitting the O2 sensors. That means either you are leaking fuel, or not burning it all.

Since you did coils, plugs, and maf, I think you should get the injectors out, maybe rail.

How is your fuel mileage?

[SlickGT1]

Oh, have you cleaned your throttle body ever? How many miles on the car? Maybe your TB plate is sticking, and the computer compensates by dumping more fuel.

[Wayne's World]

I would agree. You seem to know your stuff. And that's precisely the path I'm headed down. I'm going to pull the IAC this weekend to soak and clean it out in the event not enough bypass air is getting through during deceleration. I also picked up a fuel injector rebuild kit which includes the new o-rings, pintle cap and injector filter. So this weekend the rail is coming off. I've had it pressure tested at the rail and it's 50psi so that's about right.

Fuel mileage is decent. I get 19.2 combined. Last time I reset the consumption values was about 3 weeks ago and it's been about 50% city and 50% highway since. I've always found it to be very accurate after a few weeks of driving.

[SlickGT1]

When you take out the IAC, clean the hell out of the TB. Clean the plate as well. Really only two things left for you to do.

I think you will have to reset the ECU. I believe these codes do not clear on their own.

[Wayne's World]

I have cleaned the TB and IAC but not since I first bought it and replaced the OSV/PCV/Cyclone Separator and did a major 60K tune up albeit at 86k. Car now has 145K on it. So seems as if it's almost time for another 60K tune up.

[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]

As a follow up to this, I replaced the thermostat, water pump and aux fan switch o-ring as well as did a full cooling system flush. This did not resolve the problem. I haven't since invested any more time in trying to figure it out. I figure come time for inspection I'm just going to have my wife do a couple short trips as usual and when the light goes off, drive it over to the inspection facility and it should pass.

I am still concerned however that I'm probably damaging my cats and perhaps not getting the best MPG I could get (although I get 20.1 combined).

[SlickGT1]

Wow you did a lot. Well I can't really think what else it can be. Maybe check if you bulbs are ok. Lol. How is the thermostat on your car. Mine has a sensor on it that gave me an error saying it got stuck open. It actually told me thermostat temp is below coolant temp. Replaced it and all good.

As far as your situation, I think it's some sensor screwing you. Let's back up. That maf you swapped, is it OE BMW?

[Wayne's World]

Originally Posted by SlickGT1

Wow you did a lot. Well I can't really think what else it can be. Maybe check if you bulbs are ok. Lol. How is the thermostat on your car. Mine has a sensor on it that gave me an error saying it got stuck open. It actually told me thermostat temp is below coolant temp. Replaced it and all good.

As far as your situation, I think it's some sensor screwing you. Let's back up. That maf you swapped, is it OE BMW?

The MAF was not OE as in, stamped with the BMW logo, but it was however a Siemens VDO MAF which is stamped on every BMW MAF. I have since swapped back in the original MAF as that didn't resolve the issue.

I agree with the sensor thing. On my last long distance road trip (~300mi) I was monitoring the coolant temp with the unlocked cluster. At certain times it will dip down to almost 20c below and slowly crawl back up. 20c is ALOT IMO especially considering i'm at 80MPH at cruise control. It didn't seem to matter whether I was going uphill or downhill. I couldn't correlate it to increased load or a period of little load. There is an engine block/coolant temp sensor otherwise known as the dual temperature switch that I had planned on replacing until I could never get to the darn thing. You need to cut your spanner in half and even then it would be hell to get the leverage to break it loose from the block. I was definitely going to take it to my indy to have it done. I have the part - just never replaced it. But that's one consideration.