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1991 BMW 850i (E31) V12-4988cc 5.0L SOHC (M70) Fuel Delivery and Air Induction

September 17th, 2009 · No Comments

Situation:
Under certain very specific conditions, a complaint may be encountered of an apparent engine “misfire”, or of a “bucking”. Please note that this is not an engine misfire. This complaint is encountered only when a specific combination of factors simultaneously occur, such factors include throttle position, road speed, the driver’s driving style, and the condition of the road surface. In many cases, only one specific length of roadway may produce this complaint, and even then not consistently.

Cause:
Brief (less than 100 millisecond) fuel injection cut-off, as a result of MSR (engine drag torque control) regulation. This is a normal ASC function that may (under a very narrow set of conditions) occur during a light throttle, minimal load situation.
The ASC and ABS systems are controlled by the same control unit, but act independently of each other. The ABS operation is NOT affected by this ASC-related complaint.

Vehicles Model Year
Affected
750ÌL All 89 & 90 MY
73 5i/iL 91 MY (if equipped with ASC)
53 5i/iA 91 MY (if equipped with ASC)
850ÌA All (automatic transmission) produced prior to 7/91
Early 92 MY 535i/iA & 735i/iL vehicles (produced through 10/91) may also be affected. Verify by control unit number, as above.

Correction:
Before attempting any repairs perform the following:
1. Verify that the vehicle is equipped with ASC. Vehicles equipped with ASC+T, and vehicles not equipped with ASC, are not affected.
2. . Verify that the following four conditions were in effect when the “miss” or “buck” occurred:
A) Vehicle speed between approx. 45 and 60 MPH.
B) Rough road surface (for example, pot holes, railroad tracks, etc.)
C) Light throttle (accelerator pedal depressed approximately 1/8)
D) ASC not switched of….

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