Lots of work for little horsepower: Practical experience with the VP 37 tuning

Hi zusammen

Increasing the injection quantity through modifications to the throttle lever is limited, because the engine control unit checks the ratio of actuator current and throttle signal when the ignition is switched on, and displays flashing error messages if the result is outside the tolerance range.
The tolerance range is so tight that, according to my experience, the achievable additional power output on a 97 AFN is limited to approximately 4 smooth horsepower.
If you're willing to accept heavy partial loads and idle issues, you could potentially gain approximately 15 more horsepower.

Since, during fine-tuning, precision to the 10th mm range, in combination with freedom from vibration and the height of the additional performance, determines the result, it is practically impossible to achieve a usable adjustment without a high-resolution measuring device for inductances (LCR digital multimeter, etc.).
The work should be carried out with a cold engine, as the µH measurement values of the warm HDK sender change during cooling and can lead to incorrect settings.

My recommendations for the process:

1. When the pump plug is disconnected, measure and record the µH values of the reference coil (cable 1 - 2 = approximately 1300 µH) and the measuring coil (cable 2 - 3 = approximately 950 µH).

2. To prevent diesel contamination in the engine compartment, partially drain the pump: Loosen the inlet and outlet lines from the filter, hold the inlet hose in a container, and inject pressure into the outlet hose (e.g., using a large syringe). If approximately 0.1 liter has dripped into the container after about 5-10 minutes, remove the pump cover.
That the µH values of both sensor coils will increase slightly (due to reduced eddy current losses) is normal and has no interfering effect on the adjustment, however, the "new" values should also be noted.

3. Test the range of the motor computer to detect errors: Loosen the screw at the end of the horseshoe-shaped sensor on the belt side and move the reference short-circuit ring so that the measured value increases by approximately 80 µH. Connect the pump cable and turn on the ignition. If the pre-heating indicator does not flash (on my 97 AFN, it only flashes after 30 seconds, so wait), increase it in increments of approximately 10 µH until it flashes. Then reduce the value again, and so as accurately as possible enclose the error margin.
Always open or close the fuel pump cable connection only when the ignition is off!
If the allowable range is only 30 µH or less, then further work is not worthwhile, or you are approaching the threshold of error: re-fix the ring to its original value, and then mount the pump cover and fuel connections.
Otherwise, readjust the reference coil again to the original value for later orientation.

4. To avoid jerky movements, the target size for shifting the measuring ring should be determined according to the rule of thumb.
"0.85 x (Tolerance of the reference ring - 25 µH)"
determine. So, wenn beispielsweise die Vorab-Lampe aufgrund einer Erhöhung auf der Referenzseite um 90 µH ausgeht, sollte auf der Messseite eine Erhöhung von 0,85 x (90µH - 25µH) = 55µH angestrebt werden.
Loosen the fixing screw of the measuring short-circuit ring (at the top of the drive shaft) by approximately 3 turns, while holding the hexagon head with a ring spanner. To remove the cone seat, repeatedly alternate pressing the axle with the ring wrench against both stops.
Rotate the measuring ring in the zero position to the calculated increased value and fix it securely. When tightening the screw, the measured value may change, and exceeding the target value by approximately 5 µH is not yet critical in terms of potential error messages in the future.

5. The increase in the reference µH value should be 15% greater than the actual (!) shift of the measuring range. If the measuring ring's setting is increased by 58µH, the reference ring will be fixed at 58µH + 15% = 67µH, and this (more straightforward) adjustment work should actually be done precisely in terms of µH.
For higher performance instead of perfect smoothness, adjusting the reference ring is sufficient, with a safety margin of about 20 µH distance to the error limit.
Subsequently, install the pump cover and fuel hoses.

Even with a misaligned measuring ring, if you experience rough running, try the following: empty the pump again, open it, and reduce the µH value of the (precisely adjustable) reference coil in increments of approximately 3 - 5 µH; increase it accordingly if there is a power shortage.
If, against all expectations, the glow control is blinking, it is likely that there is a "procedure error" compared to the described method: check and/or correct all sensor values. In case of emergency, please reset to the original values and, if necessary, start the "tuning" process again.