Porsche 944 DME fuel-quality switch (FQS): how it works and position chart
The fuel quality switch (FQS) in the Bosch DME allows small stepped adjustments to how the ECU interprets the air/fuel mixture and ignition. It is not a substitute for a true base calibration and does not provide meaningful performance gains. Incorrect use can destroy the engine; treat every change with caution.
Role of the FQS (and what it does not do)
The FQS is a mechanical trim on the DME board. It does not fix vacuum leaks, weak fuel pressure, tired injectors, or incorrect base timing. If the car is healthy and fuel is appropriate, position 0 (factory default) is right for most cases.
Accessing the switch
On the 944, the DME is on the passenger side. The FQS is on the lower side of the DME housing, behind a plug or small metal cover. There is no need to open the ECU lid: the shaft is accessible through this hole.
Typical access (details vary by model year and trim):
- Fold back the carpet on the passenger side (area under the glovebox).
- Remove the panel (wood or other) that covers the DME bay.
- Undo the DME bracket fasteners.
- Pull the bracket / DME forward enough to see the hole.
Tool and triangular socket
The shaft end is a triangular female socket. A small three-sided tool that fits well is ideal; some people mention a trimmed golf tee. Others use a 2.5 mm hex key.
How to count positions
- Turn all the way counter-clockwise until it stops: position 0 (factory default).
- From there, each clockwise detent is 1, then 2, through 7.
If you lose count, return to the counter-clockwise stop and count again.
Which table matches your car?
- 944 eight-valve, between 1982 and 1985½ → first table.
- 944 eight-valve, 1985½ and later → second table.
- 944 Turbo (951) → third table.
The 944 S / S2 and other DME part numbers may not follow these mappings. If in doubt, cross-check the ECU label and workshop documentation.
Porsche 944 eight-valve, between 1982 and 1985½
| Position | Fuel | Ignition |
|---|---|---|
| 0 (factory default) | 0 % | 0° |
| 1 | +4 % | 0° |
| 2 | −4 % | 0° |
| 3 | −6 % | 0° |
| 4 | 0 % | −4.15° |
| 5 | +4 % | −4.15° |
| 6 | −4 % | −4.15° |
| 7 | −6 % | −4.15° |
Porsche 944 eight-valve, 1985½ and later
| Position | Fuel | Ignition |
|---|---|---|
| 0 (factory default) | 0 % | 0° |
| 1 | +3.1 % | 0° |
| 2 | −3.1 % | 0° |
| 3 | +6.3 % | 0° |
| 4 | 0 % | −2.77° |
| 5 | +3.1 % | −2.77° |
| 6 | −3.1 % | −2.77° |
| 7 | +6.3 % | −2.77° |
Porsche 944 Turbo (951)
| Position | Fuel | Ignition |
|---|---|---|
| 0 (factory default) | 0 % | 0° |
| 1 | +3.1 % | 0° |
| 2 | −3.1 % | 0° |
| 3 | +6.3 % | 0° |
| 4 | 0 % | −2.72° |
| 5 | +3.1 % | −2.72° |
| 6 | −3.1 % | −2.72° |
| 7 | +6.3 % | −2.72° |
Positive percentages enrich the mixture; negative percentages lean it. Negative ignition degrees correspond to retard relative to the baseline map.
Safety, emissions, and limits
Changing the FQS alters knock margin, exhaust temperature, and stress on the catalyst. Severe knock can destroy the pistons or bearings. Emissions legality and inspection depend on the country; any position other than factory default may result in a fail.