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Zero=Zero TPS Adjustment


by Rob Lentini

Many BMW R259 “Oilhead” Boxers surge at constant throttle settings and low to moderate speeds. Precise adjustment of the Throttle Position Sensor (TPS), one of the two most important input sensors of the Motronic fuel injection system, is critical in reducing or eliminating surging.

Since 1994, BMW and dealer service personnel have not totally satisfied their customers in solving this problem. At least one factory “service bulletin” has addressed this issue with mixed results.

Performing the following procedure is not BMW approved and may void the warranty if a failure has a “causal” relationship with this non-approved procedure.

Performer should be a reasonably skilled mechanic with knowledge of Motronic fuel injection theory of operation. He/she needs a quality Digital Voltmeter, a mercury or electronic manometer and common tools . All scheduled maintenance should be performed prior to this procedure. Specifically, ignition timing, spark plugs, valve clearances, Motronic coding plug and checking for throttle housing inlet manifold air leaks should be performed per BMW procedures. On rare occasions, the main electrical harness has also been at fault. ALL warranty procedures should be requested, performed and documented on a warranty-covered motorcycle prior to attempting this adjustment. The following should be performed only after there is NO warranty recourse left or warranty in effect. Doing this procedure could be at your own risk. Note: “Oilhead” BMW spark plugs are Bosch FR6DDC of dual “side electrode” configuration. The author has found that standard “over the center electrode” plugs, specifically Autolite 3923, contribute to smoother running and less surge. Simply replacing the OEM Bosch plugs with the Autolite 3923 gapped to .028″ could solve some surging and obviate the need for TPS adjustment. Change from non-BMW plugs at your own risk.


Procedure (reference BMW or Haynes manual for location of components):

1. Generously loosen the free play at the throttle cables to both left and right throttle bodies (note ’96 and newer “Oilheads” have a different throttle cable mechanism–see manuals)

2. Unscrew the left throttle body throttle stop screw (under and inside of the throttle body) counterclockwise (CCW) about three turns or until you can see a gap between the end of the stop screw and the flange it pushes on to open the throttle. You are now assured the left throttle is FULLY closed. This is important! (“Zero” throttle opening).

3. Attach the Digital Voltmeter (DVM) black lead to frame ground and the red lead to the red/white wire going to the TPS connector located on the left cylinder throttle body. The TPS is a black plastic 2″ by 2″ square module with a connector on the bottom. Gently pulling the rubber connector boot down will expose the four wires entering the connector to the TPS. Probing and pushing up with a paper clip is one method of reaching the red/white wire’s contacts. Put the DVM red lead on the paper clip, turn on the ignition and engine cutoff switch, and retract the sidestand. You should see an indication on the DVM near zero. Be SURE you are on the DC Volts selection.

4. Loosen the two Allen head screws securing the TPS. Adjust the TPS by slowly turning it within its slots until the observed voltage decreases to about .006 DC volts. It will probably go no lower, which is OK. DO NOT turn it any more as the wiper of the pot has already reached the end of electrical travel. Stop turning the TPS just when the voltage stops decreasing and then turn it in the opposite direction to increase the DVM by .004. Lock it down there. Going slightly up by .004 volts will ensure your are not going “below” zero volts and this will typically result in a “zero” value of .010 volts. Be careful of DVMs which “autorange” and can confuse readings! (“Zero” TPS voltage .006 +.004 volts = .010)

5. With “Zero = Zero” set (zero throttle plate opening and zero voltage), turn the left throttle body stop screw clockwise (CW) until the voltage starts to rise. Continue turning until you reach .370 – .400 volts, the voltage the BMW analyzer looks for the TPS at idle to cause a digital signal to “flop” or change symbology. Lock down the stop screw with the jam nut if so equipped (early units are an interference fit). Blip the throttle several times to check accuracy and repeatability of the voltage to be within .370 – .400 volts. Readjust the stopscrew (not TPS Allen screws!) if needed. Following this, reset the left throttle cable free play to .5mm. Now you are forever done with the TPS adjustment.

6. Connect the mercury or electronic manometer to both throttle bodies. Prior to starting the engine, use a common screwdriver to lightly close both rear-facing brass air bypass screws on the throttle bodies, then open them to:

R1100 RS/RT: 1 ½ turns from closed

R1100 GS/R: 2 ¼ turns from closed

R850R: 2 ¼ turns from closed

Start and warm up the engine to get 5 bars on the RID oil temperature if so equipped. Idle will be rough due to the need to synchronize left to right throttle bodies. Use a big rear-facing fan from the front of the bike to control cooling, if required.

7. After warm up, LEAVE THE LEFT STOP SCREW ALONE! (The one used to set .370- .400 volts). Adjust the right throttle body stop screw to get perfectly equal mercury in both columns or electronic indication.

8. Adjust the right throttle body cable for equal mercury readings at just off idle, around 1500 RPM. Use the throttle to increase RPM, not the “choke” start lever when synching.

9. Finally, adjust both brass bypass screws individually or together in conjunction with the manometer to yield a proper idle speed of 1000-1100 rpm and equal mercury or vacuum readings. Closing the bypass screws will decrease RPM; converse for opening them. On rare occasions where proper idle speed cannot be set with the bypass screws, return to step 5 and incrementally increase or decrease the stop screw voltage from the .370 – .400 setting then repeat subsequent steps. Raising the stop screw voltage will increase idle RPM and decreasing it will lower idle RPM.

10. Inspect your work and test ride, perhaps re-performing synch using ONLY the bypass screws and right throttle cable to “fine tune.”

The TPS was set for zero volts at zero throttle plate opening (+.004 volts to assure not going “low”). Then the left throttle body TPS was set using the stop screw to .370 – .400 volts followed by an accurate synch of the right throttle body.

This is a simple procedure and it has worked on countless “Oilhead” BMWs. You will notice good throttle response, best possible fuel consumption, little if any surging and good top end. Properly performed, emissions are unaffected if not improved.