Fuel Injection — Idle Speed, CO and Fuel-Pressure Adjustment (8-valve)

This is the factory testing-and-adjusting procedure for the 8-valve fuel injection: the idle-speed and CO setpoints, the fuel-pressure check, and the idle/CO adjustment itself. It complements the DME self-diagnosis — that guide finds a fault; this one sets the adjustable values once the system is healthy.

Note: Setpoints differ by market and by model year — these are market variants, not unit conversions. Cars built for the USA, Canada, Japan (and other catalytic-converter markets) run a different idle speed and a tighter CO window, and their CO is measured ahead of the catalytic converter with the oxygen-sensor plug disconnected. Use the metric/imperial toggle to switch the units (pressure, torque, temperature); the market figures stay as quoted.

Three injection generations are covered, with their own setpoints:

• 944 (early L-Jetronic).
• 944 and 924S, from 1985.5.
• 944 2.7-litre (engine M44.11/12), from 1989 — system fuel pressure was raised from 2.5 bar36 psi to 3.8 bar55 psi.

Before you start

The car must be in good order before any adjustment is meaningful:

• The engine must be in perfect running condition.
• Switch off all electrical equipment for the duration of the adjustment. The radiator fan and the air-conditioning compressor must be off while adjusting.
• Work as quickly as possible — a hot intake gives a false CO reading.
• Ambient temperature 15 °C59 °F to 35 °C95 °F.
• Bring the engine to operating temperature and check it with the oil-temperature tester (Special Tool 9122), not by guesswork.

Tools. Oil-temperature tester 9122 (with sensor 9122/1, or 9122/2 on later cars); a CO/exhaust-gas tester with the appropriate test line (US 8023 / SUN 120-239, or exhaust sensor US 4492 on later USA cars); a fuel-pressure gauge with hose (P 378, or VW 1318); and a locally made test lead.

Specifications

944 (early L-Jetronic)

Fuel-system test	Specification
Electric fuel pump — delivery rate	at least 850 cm³ per 30 seconds
Fuel pressure (engine stopped, pump bridged)	2.3 bar33 psi to 2.7 bar39 psi
Control value at idle	approx. 2 bar29 psi
Leak test — minimum pressure after 20 minutes	at least 1 bar15 psi

Market	Idle speed	CO
Europe	800 (+50) rpm	0.5–1.0 %
USA / Canada / Japan	900 (±50) rpm	0.6 ±0.2 % *
Australia + Sweden	800 (+50) rpm	0.5–1.0 %

* Measured ahead of the catalytic converter, with the oxygen-sensor plug disconnected.

Set the idle speed to the upper limit for cars with automatic transmission or power steering.

944 and 924S, from 1985.5

Fuel-system test	Specification
Electric fuel pump — delivery rate	at least 850 cm³ per 30 seconds (750 cm³ per 30 seconds with the speed-reduced pump, green colour code, Bosch No. 0 580 464 028)
Fuel pressure (engine stopped, pump bridged)	2.3 bar33 psi to 2.7 bar39 psi
Idle-speed control value	approx. 2 bar29 psi
Leak test — minimum pressure after 20 minutes	at least 1 bar15 psi

Setting	Without catalytic converter	USA / catalytic-converter cars
Idle speed	840 ±20 rpm	840 ±20 rpm
CO	0.5–1.5 %	0.6 ±0.2 % *

* Measured ahead of the catalytic converter, with the Lambda (oxygen) sensor plug disconnected.

944 2.7-litre, engine M44.11/12 (from 1989)

Fuel-system test	Specification
Electric fuel pump — delivery rate	at least 850 cm³ per 30 seconds
Fuel pressure (engine stationary, DME relay bypassed)	3.6 bar52 psi to 4.0 bar58 psi
Control value at idle	approx. 3.3 bar48 psi
Leak test — minimum pressure after 20 minutes	at least 2.0 bar29 psi

Setting	Without catalytic converter	USA / catalytic-converter cars
Idle speed	840 ±40 rpm	840 ±40 rpm
CO	0.5–1.5 %	0.4–0.8 % *
HC	≤ 300 ppm	≤ 300 ppm *

* Measured upstream of the catalytic converter, with the Lambda-probe plug disconnected.

Note: On the 2.7-litre, system fuel pressure was increased from 2.5 bar36 psi to 3.8 bar55 psi, but the test connection is unchanged — check fuel pressure exactly as below. Tightening torque for the cap nut is 12 N·m9 ft·lb.

Checking fuel pressure

1. Unlock and fold up the cover. Unscrew and remove the capped nut on the test connection of the fuel-distribution line.

   Caution: Take care that the sealing ball does not fall out when removing the capped nut.

2. Connect the pressure gauge to the test connection with the hose from pressure tester P 378 (or VW 1318).

3. Pull off the fuel-pump relay on the central electric board. On 1985.5-and-later models the fuel-pump relay is in the central electric board at the left front, in the blower/fresh-air chamber.

4. Bridge terminals 30 and 87b with a piece of wire — the fuel pump must now run.

5. Read the control values:

   • engine stopped (relay bridged): 2.3 bar33 psi to 2.7 bar39 psi;
   • engine running at idle: approx. 2 bar29 psi.

6. Tightening torque for the capped nut: 22 N·m16 ft·lb (12 N·m9 ft·lb on the 2.7-litre M44.11/12).

Adjusting idle speed and CO

Idle speed and CO are set together: idle speed at the throttle housing, CO at the air-flow sensor. Read the requirements under Before you start first — they are part of the procedure, not optional.

Note on the idle-speed control. On models from 1985.5 and the 924S, the idle-speed filling (auxiliary-air) control must be stopped while you set the idle speed, or it fights the adjustment. Stop it by bridging the round female plugs B and C on the diagnosis plug with a locally made lead; restore it (remove the lead) when finished. The engine idle speed can be checked and adjusted with a VAG tester (e.g. VAG 1367) instead of a separate tachometer.

Without catalytic converter

1. Run the engine to operating temperature — oil temperature 70 °C158 °F to 90 °C194 °F (early cars: approx. 80 °C176 °F) — checked with oil-temperature tester 9122.
2. Connect a separate tachometer per its instructions.
3. Stop the idle-speed filling control (bridge plugs B and C on the diagnosis plug), then turn the control (bypass) screw on the throttle housing until the specified idle speed is reached.
4. Check the CO level. Turning the air-flow-sensor mixture screw clockwise = richer, anticlockwise = leaner. If the CO is outside the specified window, correct it on the air-flow sensor (see Correcting the CO setting).
5. Restore the idle-speed filling control (remove the lead) and re-check both adjusted values.

USA and catalytic-converter cars

These cars are set with the oxygen sensor disconnected and the CO read ahead of the catalytic converter.

1. Slide the exhaust test line or exhaust sensor (US 8023 / SUN 120-239, or US 4492 on later cars) onto the CO test pipe / catalytic-converter test connection.
2. Run the engine to operating temperature — oil temperature 70 °C158 °F to 90 °C194 °F (early cars: approx. 80 °C176 °F).
3. Pull off the rubber cap and disconnect the oxygen-sensor plug, then connect the CO tester per its instructions.
4. Set the idle speed: stop the idle-speed filling control (bridge plugs B and C), turn the control (bypass) screw on the throttle housing to the specified speed, then restore the control.
5. Check the CO level. If it is outside the window, correct it on the air-flow sensor (see Correcting the CO setting). Turning the mixture screw clockwise = richer, anticlockwise = leaner.
6. After finishing, reconnect the oxygen-sensor plug and plug the CO test pipe in the engine compartment. Coat the threads of the cap nut on the catalytic-converter test connection with Bosch VS 140 16 Ft or Optimoly HT grease and tighten to 30 N·m22 ft·lb. Re-check the adjusted values.

Correcting the CO setting on the air-flow sensor

On catalytic-converter cars the mixture-regulating screw sits behind a sealed plug that must be removed for access.

1. Remove the air-flow sensor.
2. Take the plug out of the bore that gives access to the mixture-regulating screw: drill the plug with a 2 mm drill (against the steel insert), then pull it out with a left-hand twist drill (No. 2).
3. Reinstall the air-flow sensor. Adjust the idle speed and correct the CO with a screwdriver on the mixture-regulating screw — clockwise = richer.
4. When finished, press a new plug flush into the bore.

Caution: Never reuse a drilled plug.