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Correction regarding unplugging MAF & limp mode (archive)

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Posted by Rick Stillwell on April 08, 2001 at 11:52:29:

In Reply to: Yes, if your MAF is the platinum hot wire type posted by guysimpson on April 06, 2001 at 14:26:15:


Just a minor correction:

On my 1990 750iL, unplugging a MAF while the engine is running does cause limp mode, but the affected bank of cylinders (on the opposite side of the engine) DOES continue to run, as evidenced by oxygen sensor output reading normally and rpm reduction by the disconnection of any fuel injector electrical plugs.

On mine, the rpm will increase a little after disconnecting the MAF, and the oxygen sensor will show a slightly enriched mixture, so the motronic (ECM) is making assumptions about the air flow for a given throttle opening rather than measuring it.

However, if you shut off the engine and restart it with an MAF disconnected, I think that side will be disabled.

Contrary to many posts on this forum, limp home mode limits throttle opening and maximum rpm, but does NOT automatically mean the shut down of one bank of cylinders. One side COULD be disabled, for instance if crankshaft speed sensor is not working, but limp mode does not always disable one bank.

This is based on excessive experimentation with my 750iL.

Rick


You don't say what model car you have but in the case of a 750 yes your processes are correct. If you unplug the MAF while the engine is running it will cause the engine to go into Limp mode and run on the opposite bank of cylinders. Hot wire MAF's upon turning off ignition raises the wire temp high enough to burn off any deposits 1000C, hence the momentary red/orage glow. If you suspect a bad MAF, just switch them side to side and see if the problem moves to the other bank.

Email me a fax number and your car model and I can send further information.

Guy Simpson
92 750iL's

Please correct me if I'm wrong
I would remove the Air filter assembly, look into the mass air flow meter, have someone turn the ignition off and see the sensor glow.does that mean it's working fine or is there another test? like if I unplugged it, should the car stop? please give me details, and I am grateful
Jean






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