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M70 oxygen sensor diagnostics

I have an intermittent check engine light, on my 1991 750iL. I have the Bentley manual, that describes the connector location as beneath the transmission. Is there any easier location to check the voltages to the sensor heaters, and the sensor output?

The check engine light used to come on on the first hill a mile or so from my house on the morning commute, but now it comes on immediately upon startup, and sometimes goes out after highway driving.

I used the manual method of reading out stored engine codes (pumping gas pedal 5 times right after turning on ignition, observing sequence of flashing 'check engine light'), the output is 1444, which I believe means everything OK.

I had the vehicle to a dealership before for the check engine light, and they replaced one of the O2 sensors, upon picking up the car, there was still a check engine light, but with different behavior, in terms of when it turns on.

When I returned the car to the dealership, they claimed the check engine light was due to a slipping transmission. I since had it serviced, and noticed that the harsh down shifts when coasting 40-45 mph went away, I am unsure whether the slipping transmission engine code was reset. But the check engine light never went away.

My gas mileage, is 13mpg (US), in a commute with 50% highway, 50% local roads.

Are there conditions that cause check engine light to come on, without storing a code?

Is there indeed a slipping transmission engine code for a 1991 M70, or is the BMW dealership simply blabbering to cover incompetence?

Any other suggestions for how to troubleshoot this problem?

Jan Fure

Reply to
MGJonesMCMXCII
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The check engine light is emissions -related, not transmission problems. And why the HELL didn't the dealer mechanics tell you that the first time? Bunch of jack-offs! Am I wrong here... anyone? My guess is you need both O2 sensors replaced and both ECUs reset. I say both because the v12 has 2. That's right. Some would say disconnect the battery for a few minutes and you're good to check if the light went out for good. This would be a good time for a trans fluid and filter change, DIY, of course. Can anyone back me up on this?

Bill in Omaha '86 535i

Reply to
Bill

"Bill" wrote in news:kB6_h.287599$ snipped-for-privacy@newsfe14.phx:

I would agree Bill. The first thing I would do is change the other Oxygen sensor as well. You can reset the the ecu's in the way you mentioned or by another proceedure layed out in the repair manual for the car. I am about to change my sensors in my 850 because I am experiencing the same thing as Jan at the moment. There is a fair bit of coverage on this topic in the 8 series forum. Same engine with many of the same problems. Dave

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Reply to
D.Tugwood

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Most of the info I culled from the WWW is somewhat model-interchangeable. That's one reason I like BMW. I need to do a sunroof replacement soon due to a PITA tiny rust spot. I'll be swapping in a glass section from... SAAB! I found several articles and DIY, plus there's a yard here in Omaha with 5 donors. Not a lot of folks know about the 2 ECUs for the v12s. Kudos to all those folks who provided info that has helped me and those I've given the info to. I guess my computer surfing time does pay off.

Bill in Omaha '86 535i

Reply to
Bill

Update: shogun from a BMW forum gave me this useful reply:

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after stomping 6 times, I get a flash code for the left half ofthe engine (DME2/cyls 7-12), from which I got code 2221, which meansoxygen sensor.

This one might be replaced under warranty, as this is the same sensor as I paid BMW to replace for me last year.

Jan

Reply to
MGJonesMCMXCII

Update:

As mentioned in other post, the stored codes can be read out by stomping gas pedal 5 times for DME1, and 6 times for DME2, which represent cylinders 1-6 and 7-12 respectively, only when I learned about 2 manual readouts being needed, did I know for sure I had an O2 sensor code.

Dealership fixed the oxygen sensor code under warranty.

Problem: Oxygen sensor connector loose.

Why: That's the way it was installed by dealer.

Why was it not addressed before? When I first complained to dealer that they did not fix the check engine light, they blamed it on a slipping transmission.

Is this dealership-incompetence, or a deliberate attempt to empty the wallets of 7 series owners?

Jan Fure

Reply to
MGJonesMCMXCII

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I noticed that women, and some men, are treated differently by the dealer/mechanic, esp. BMW owners. We tested this and my wife was the helper. We even used the same car! You know, I ain't sayin' one way or another, I'm just sayin'. Not everyone is like that either. Jan, I'm glad the problem is finally resolved and I wish you didn't have to experience it that way. I do think the dealer has a "ditz-o-meter" they aim at incoming customers to determine how gullible they are to over-priced, mis-diagnosis that fixes nothing correctly.

Bill in Omaha '86 535i

Reply to
Bill

Reply to
Bill

Hi Bill

Been there done that now got the 740i and a new E65 730d SE

Reply to
Oscar

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