Volvo Coupes/Sedans/Wagons 1970-1989 Repair Guide

Continuous Mechanical Fuel Injection


See Figures 1, 2, 3 and 4

The mechanical Bosch Continuous Injection (CI) fuel system is standard on all gasoline engine 240 and 260 models, 1974 140 models and all 1980 and later models except the B21F-LH and the B23F-LH. It differs from the electric fuel injection (see above) in that injection takes place continuously. Controlled through variation of the fuel flow rate through the injectors, rather than variation of the fuel injection duration, this system has no electronic computer. It is an electromechanical system that will provide suitable air/fuel mixtures to accommodate differing driving conditions.

Click image to see an enlarged view

Fig. Fig. 1: Location of the CI system components-B20 engine shown

Click image to see an enlarged view

Fig. Fig. 2: Exploded view of the continuous fuel injection system (mechanical)-B27F engine. B28F is similar

The complete system consists of the following components: air/fuel control unit (housing both air flow sensor and fuel distributor), electric fuel pump (and fuel pressure accumulator), fuel filter, control pressure regulator, continuous fuel injectors, auxiliary air valve, cold start injector, thermal time switch, main relay, and a fuel pump relay.

The heart of the system is the air/fuel control unit. It consists of an air flow sensor and a fuel distributor. Intake air flows past the air cleaner and through the air venturi raising (four cylinder) or lowering (V6) the counterbalanced air flow sensor plate. The plate is connected to a pivoting lever which moves the control plunger in the fuel distributor in direct proportion to air flow.

Click image to see an enlarged view

Fig. Fig. 3: Exploded view of the continuous fuel injection system (mechanical)-B21F engine

Click image to see an enlarged view

Fig. Fig. 4: CI system component location-240 model shown, others are similar

The fuel distributor, which controls and distributes the amount of fuel to the injectors consists of a line pressure regulator, a control plunger, and pressure regulator valves (one for each injector). The line pressure regulator maintains the fuel distributor inlet pressure at about 65 psi (448 kPa) and will recirculate fuel to the tank if pressure exceeds this value. The control plunger (which is connected to the air flow sensor plate) controls the amount of fuel available to each of the pressure regulator valves. The pressure regulator valves maintain a constant fuel pressure differential (1.4 psi/9.7 kPa)) between the inlet and outlet sides of the control plunger. This is independent of the amount of fuel passing through the valves, which varies according to plunger height.

The injectors themselves are spring loaded and calibrated to open at 47-51 psi (324-352 kPa). They are not electrically operated as on the older electronic fuel injection system.

The control pressure regulator, located on the intake manifold, acts to regulate the fuel/air mixture according to engine temperature. When the engine is cold, the control pressure regulator richens the mixture (4-5 minutes max.). This is accomplished in the following manner; a certain amount of fuel is bled off into a separate control pressure system. The control pressure regulator maintains this fuel at about 52.5 psi (392 kPa). The regulator is connected to the upper side of the fuel distributor control plunger. When the engine temperature is below operating minimum, a bi-metal spring in the regulator senses this and reduces the fuel pressure on top of the plunger.

This allows the plunger to rise further and channel more fuel to the regulator valves and injectors, thereby richening the mixture. When the engine warms, the bimetal spring in the regulator increases the pressure to 52.5 psi (392 kPa), leaning the mixture back to its normal operating ratio. On V6 models, a vacuum feature is added to the regulator, whereby low vacuum situations, such as acceleration, temporarily lower the control pressure and richen the mixture. At idle, full throttle, and steady state conditions, the vacuum is high and the mixture returns to normal.

The auxiliary air valve provides extra air to mix with the richer mixture during warm-up, thus raising the engine speed and improving cold start driveability. The auxiliary air valve, which also has a temperature sensitive bimetal spring, works directly with the control pressure regulator. At cold startup, the valve is fully open. As the engine warms, an electric coil slowly closes the valve (4-5 minutes max.), blocking off the extra air and eliminating the fast idle speed.

The cold start injector, located on the inlet duct, sprays extra fuel into the intake air stream (during starter operation) when the engine coolant temperature is below 95°F (35°C). It has a maximum spraying duration of 12 seconds.

The thermal time switch, located on the cylinder head, actuates the cold start injector. Its bi-metal spring senses coolant temperature and an electric coil limits the cold start injector spray to 12 seconds.

The fuel accumulator, located adjacent to the fuel pump, has a check valve which keeps residual fuel pressure from dropping below 28 psi (193 kPa) when the engine or fuel pump are shut off. Therefore, the system is always pressurized, preventing vapor lock in hot start situations.


Air/Fuel Control Unit

See Figures 5, 6, 7 and 8

The air-flow sensor plate adjustment is critical. The distance between the sensor plate and the plate stop must be 0-0.02 in. (0-0.5mm). The plate must also be centered in the venturi, and must not contact the venturi walls. Loosen the plate center when depressed, it should return to its rest position when released.

Click image to see an enlarged view

Fig. Fig. 5: The intake plenum of the air flow sensor plate is located under the engine intake air supply hose-B21FT engine shown

Click image to see an enlarged view

Fig. Fig. 6: Six-cylinder air/fuel control unit for CI system-"A" is the airflow sensor, "B" is the fuel distributor

To check the air-flow sensor contact switch, turn the ignition key to the ON position-don't start the motor--and depress (V6) or lift (4-cyl.) the sensor plate by hand. The fuel injectors should buzz, and the fuel pump should activate. If the pump operates, but the injectors do not buzz, check the fuel pressure. If the pump does not operate, check for a short in the air-flow sensor connector. Turn the ignition to OFF before performing any diagnostic work.

Click image to see an enlarged view

Fig. Fig. 7: Center the plate on the air/fuel control unit

Click image to see an enlarged view

Fig. Fig. 8: Adjust the distance between the sensor plate and stop at spring "A"-B21F engines

Fuel Pump

See Figure 9

If a defective fuel pump is suspected, perform this test. With the ignition switch on, disconnect the wire connector at the air flow sensor. The pump should work. If not, check fuse No. 7 (1977) or fuses No. 5 and No. 7 (1976 and 1978 and later) and voltage across auxiliary air valve terminals. Live terminals indicate a faulty fuel pump or wiring.

The use of the correct special tools or their equivalent is REQUIRED for this procedure. A fuel pressure gauge with a three position valve and T-fitting is required to isolate the line, rest, and control pressure readings.

Connect a pressure gauge and T-fitting with 3-way valve inline between the center of the fuel distributor (control pressure fuel line) and the control pressure regulator.

Disconnect the coil wire (terminal 15) to prevent burning out the coil windings.

Disconnect the wire connectors at the control pressure regulator and auxiliary air valve. Switch on the ignition and disconnect the wire connector at the air-flow sensor on pre-1978 models, attach test relay as shown on 1978 and later models. The fuel pump should start.

Click image to see an enlarged view

Fig. Fig. 9: Attach a test relay on 1978 and later models

Check the line pressure first. With the T-fitting lever pointing to the fuel distributor, check that the line pressure is 64-75 psi (441-517 kPa). If insufficient, check fuel lines for leakage, fuel pump for delivery capacity (25.3 fluid ounces/750 ml in 30 seconds), or low line pressure adjustment. If too high, check for clogged fuel return line or high line pressure adjustment. Line pressure is adjusted along with rest pressure later in this procedure.

Check the control pressure next. With the T-fitting turned at a right angle to the hoses, check the control pressure. Depending on coolant temperature, the control pressure will be somewhere between 18-55 psi (124-379 kPa), lower for cool temperatures, higher for warm temperatures. If the control pressure is insufficient, try a new pressure regulator. If the pressure is too high, check for a clogged fuel return line, or try a new control pressure regulator. Reconnect the control pressure regulator electrical connector.

After 4-5 minutes, the pressure should equal 44-55 psi (303-379 kPa). If not, disconnect the electrical connector at the control pressure regulator and check with a 12V test light across the terminals. No voltage indicates a defective (open) wire. Voltage indicates a possible faulty regulator. Then check across the terminals with an ohmmeter. Resistance indicates corroded terminals. No resistance indicates a defective control pressure regulator.

On the B28F engine, the control pressure regulator is equipped with two resistor heaters which are wired in parallel in the regulator wiring circuit. One of the heaters is switched off at temperatures above 60°F (15°C). Correct resistance on this regulator is: 32-38 ohms below 55°F (13°C); 16.5-19.5 ohms above 65°F (18°C).

The vacuum function of the control pressure regulator on the V6 engine is checked later in this test.

The auxiliary air valve is checked next. Disconnect the auxiliary air valve hoses. Using a dentist's mirror or similar and a flashlight, check that the valve is partly open at room temperature. Reconnect the wire connector at the valve and, after 4-5 min., the valve should be fully closed. If not, tap on valve and check again. If tapping closes the valve, the valve is OK (engine vibrations will close it in normal operation). If the auxiliary air valve still does not close, disconnect the connector and check the voltage across the wire connector terminals with a 12V test light. No voltage indicates a defective circuit. Next, check across the auxiliary air valve terminals with an ohmmeter. Correct resistance is 40-60 ohms. No resistance indicates a faulty auxiliary air valve.

Check the rest pressure. Connect the wire connector to the air-flow sensor to stop the fuel pump. With the pump stopped, and the pressure gauge T-fitting lever at a right angle to the fuel lines, check that the rest pressure is 24 psi (165 kPa), dropping to no less than 14 psi (96.5 kPa) after 10 minutes. The rest pressure and line pressure are adjusted simultaneously by inserting or removing shims between the regulator plunger and plunger cap on the side of the fuel distributor. Shims are available in 0.1mm and 0.5mm sizes. A 0.1mm shim makes a 0.8 psi (5.5 kPa) difference, and a 0.5mm shim makes a 4.3 psi (29.6 kPa) difference on both rest and line pressure.

If the rest pressure drops noticeably within one minute, check for defective control pressure regulator, leaky line pressure regulator or O-ring, a defective fuel pump check valve, or some external fuel leak.

The vacuum function of the V6 control pressure regulator is checked with the pressure gauge and T-fitting installed, and all electrical connectors installed. On a running, warm engine, with the tee fitting positioned at a right angle to the fuel hoses, fuel pressure should be 50-55 psi (345-379 kPa). When the vacuum hose is disconnected at the regulator, the pressure should drop to 44-50 psi (303-345 kPa). If not, the regulator is defective.

Cold Start Injection

See Figure 10

Remove the cold start injector from the intake manifold and hold it over a container. For a cold engine-coolant temperature at or less than 90°F (32°C)-the injector should spray during starter operation (max. 12 seconds). If not, check the voltage between the terminals of the injector when the starter is on. Voltage indicates a bad cold start injector. No voltage indicates a faulty thermal time switch or wiring.

Click image to see an enlarged view

Fig. Fig. 10: Remove the cold start injector as shown

With the starter off, disconnect the wire connector at the air-flow sensor on pre-1978 models and attach the test relay as shown (1978 and later models) to operate the fuel pump. Check for cold start injector leakage. Maximum allowable leakage is one drop per minute, less is desirable.

Thermal Tim Switch

Remove the cold start injector and place over a beaker. With the engine fully warmed up-coolant temperature over 95°F (35°C)-the injector should not operate. If it does, the thermal time switch is defective. Reminder: on a cold engine, the start injector should not inject fuel for more than 12 seconds during starter cranking. If it does, the thermal time switch is defective.

Continuous Fuel Injection

See Figures 11, 12 and 13

Click image to see an enlarged view

Fig. Fig. 11: When removing the fuel injectors be very careful not to strip the fittings-a line wrench is usually best

Click image to see an enlarged view

Fig. Fig. 12: Always inspect the rubber seals and replace, if damaged or worn in any way

The injectors are simple spring-loaded atomizers, designed to open at 47-51 psi (324-351 kPa) on 1974-78 models and 50-54 psi (345-372 kPa) on 1979 and later models. Critical factors are spray pattern, fuel spray quantity, and leakage after the engine is shut off.

Click image to see an enlarged view

Fig. Fig. 13: Examples of injector spray patterns. "A" is proper, "B" is acceptable, "C," "D," and "E" are not acceptable-the injector must be replaced

To check spray pattern, remove the injectors, one at a time, and hold over a container. Switch the ignition key ON and disconnect the connector at the air-flow sensor to activate the fuel pump. Move the airflow sensor plate. The injector should provide a healthy dose of uniformly atomized fuel at a 15-52° angle in a cone shaped pattern.

To check injection quantity, connect the removed injectors via hoses to 4 (or 6) equal sized containers. Switch on the ignition. Disconnect the connector at the airflow sensor to activate the fuel pump. Run the pump for approximately 30 seconds to pressurize the system. Reconnect the connector to stop the fuel pump. Lift (4-cylinder) or depress (V6) the air-flow sensor plate halfway until one of the beakers fills up. Compare the fuel levels in the containers. If the injection quantity deviates more than 20% between injectors, isolate problem by swapping the lowest and highest quantity injectors and repeating the test. If the same injector still supplies less, clean or replace that injector and fuel supply line. If the other injector is now faulty, the fuel distributor is defective.

The check for injector leak-down (when closed) can now be conducted. Injector leakage beyond slight seepage may be due to the air-flow sensor plate being set to an incorrect height, seizing of fuel distributor plunger, or internal leaks in the fuel distributor. Connect the airflow sensor connector to deactivate the fuel pump and switch off the ignition. Check for injector leakage at rest pressure. Move the sensor plate to open the fuel distributor slots. Maximum permissible leakage is one drop per 15 seconds. If all injectors leak, the problem may be excessive rest pressure.

Throttle Linkage

On all models, make sure the throttle valve(s) are closed when in the idle position and that they open completely when the accelerator is floored. On the B27F, B28F and B280F engines the throttle valves are located behind the front manifold pipe.

Disconnect the link rod between the throttle valve pulley and the throttle valve, then reconnect it: reconnecting the link should not move the pulley at all. If it does, adjust the link rod at its threaded ends. After reconnecting the link, have an assistant floor the accelerator pedal: the pulley should rotate and its arm should touch the full throttle stop. If the arm does not touch the stop, adjust the cable at its plastic adjuster. Release the throttle cable and make sure it returns to the idle position. Recheck the link rod.

To adjust the automatic transmission kickdown cable see Drive Train .


See Figures 14 and 15

This system, introduced in 1981, controls the engine idle speed by allowing a controlled amount of air to bypass the throttle valve. It is used on engines with both mechanical and LH-Jetronic electronic fuel injection systems.

The Volvo CIS system is an idle air bypass valve. It is not to be confused with the Bosch Continuous Injection System (also CIS), or any of its variants found on many Volvo models.

Click image to see an enlarged view

Fig. Fig. 14: CIS system ECU location. For 760 GLE models, look for the ECU on the driver's side kick panel

Click image to see an enlarged view

Fig. Fig. 15: Constant Idle Speed components

There are five main components in the constant CIS system:

The Electronic Control Unit (ECU) processes information from the sensors about engine speed, engine temperature and throttle position. The ECU signals the Air Control Valve to regulate air flow.
The Air Control Valve, an ECU controlled valve that rotates open or closed to allow more or less air to bypass the throttle valve.
A micro-switch at the throttle which signals the ECU as the throttle goes back to idle.
The ignition coil, which provides information to the ECU on engine speed.
The coolant temperature sensor, which allows the ECU to increase the idle speed when the engine is cold and decrease the idle speed as the engine warms up.

On 1982 and later CIS systems, a micro-switch in the air conditioning control unit signals the ECU to set a higher idle speed when the air conditioning is turned on; this improves air conditioner operation and and eliminates some idle vibration.

On all models except the 760 GLE, the ECU is mounted inside the right door on the kick panel; on the 760 GLE, it is located inside the left door. Air control valves are located next to the intake chamber on the B21, B23, and B28 V6; on the B21F Turbo, the valve is located under the front side of the intake manifold.


See Figures 16, 17, 18, 19, 20, 21 and 22

Eliminate all other possible systems faults before investigating faults in the CIS system. Consult the accompanying chart for possible reasons for an incorrect idle speed.

A common source of problems in the CIS system may be the wires and/or connectors.

Click image to see an enlarged view

Fig. Fig. 16: CIS wiring schematic-B28F engines

Click image to see an enlarged view

Fig. Fig. 17: Location of CIS components-B28F engines

Click image to see an enlarged view

Fig. Fig. 18: Location of CIS components-B21F turbo engines

There are two connectors, blue at top and black on bottom, at the ECU, and two other ECU connectors at the center and right hand side of the firewall. The air control valve also has a connector at the valve.

Click image to see an enlarged view

Fig. Fig. 19: Location of CIS components-B21F engines

If the engine idle speed is considerably lower than specified, check the engine vacuum hoses for possible obstructions. The air control valve can be become stuck or obstructed by deposits from the PCV valve and system.

Click image to see an enlarged view

Fig. Fig. 20: CIS wiring schematic-B21F turbo engines

Click image to see an enlarged view

Fig. Fig. 21: CIS wiring schematic-B21F

Click image to see an enlarged view

Fig. Fig. 22: CIS system firewall connectors

A good quality volt-ohmmeter, with a range of 0 to approximately 20 volts is necessary to test the CIS system on all models. It is also helpful to have a test light (the type with a pointed contact on the bulb side and an alligator clamp on the other) as this uses more current then a volt-ohmmeter and might in some cases reveal a bad connection better. A tachometer is necessary as is a supply of jumpers, pieces of wire with connectors or alligator clips on both ends. Jumpers are used to connect two terminals of a circuit. It's extremely handy to buy or make jumpers from various colors of wire. Circuits can then be visually identified quickly.

Although the CIS system voltage and currents are low, DO NOT press in the test point between the terminal contacts; just TOUCH the test point to them. Otherwise, the terminal contacts may be damaged.

Most of the common faults in the CIS system are bad connections in the multi-pin connectors on the firewall, at the ECU module or at the coolant temperature sensor. If the ECU is not getting correct data from the sensors, it cannot respond properly to the engine conditions of the moment.

To prepare for the test, remove the panel that covers the ECU on the kickpanel, and disconnect both connectors at the ECU. Switch the ignition ON, except where noted.


The terminal numbers are marked on the side of the ECU. With the ignition ON, terminal 1 is positive (+) and energized from the ignition switch. Terminal 2 is negative (-) and ground. Connect the voltmeter or test lamp across terminals 1 and 2 in the connector; the voltmeter should read battery voltage and the test lamp should be fully illuminated. If there is no reading, first check an alternate ground, then check fuse No. 13 in the fusebox.


See Figures 23 and 24

Click image to see an enlarged view

Fig. Fig. 23: The CIS throttle micro-switch is located on the throttle linkage

Click image to see an enlarged view

Fig. Fig. 24: Micro-switch test lamp results

Switch the ignition OFF . Connect the ohmmeter test leads across terminal 8 (blue connector) and terminal 1 (black connector). Check the accompanying chart for ohmmeter and test lamp results. If the readings are incorrect, adjust the system according to the instructions below for your particular model.


See Figures 25 and 26

Use the ohmmeter to test coolant temperature sensor resistance. Connect the meter across terminals 9 and 11 of the connectors. Consult the adjacent charts for resistance readings at specific temperature.

Click image to see an enlarged view

Fig. Fig. 25: Test the coolant temperature sensor by checking resistance versus temperature-1981 B21F and 1981-82 B28F engines

Click image to see an enlarged view

Fig. Fig. 26: Test the coolant temperature sensor by checking resistance versus temperature-1982 B21F engines


Connect the tachometer across terminal 12 and any ground point. Start the engine. The tach should show engine speed; if the reading is incorrect or there is no reading at all, check the connectors on the firewall for good contact.


Connect a jumper wire across terminals 4 and 1, and another jumper across terminals 5 and 2. Start the engine. A high idle speed of 1600 to 2400 rpm should be obtained-this indicates that the valve is working properly. If the engine does not develop that high idle speed, the air control valve may be defective. If no fault is found with the air control valve, try a new ECU. Always check all connectors for corrosion and secure fit. Reconnect all connectors and check the overall operations of the CIS system.


The B28F V6 is equipped with two micro-switches actuated by throttle control. The second micro-switch closes a Lambda-sond (the oxygen sensor) circuit at full throttle to provide richer air/fuel mixture at maximum acceleration. Vehicles sold in high altitude areas have this switch disconnected.

  1. To adjust the switch, loosen the micro-switch retaining screws. Turn the switch sideways. The test light should come on, then go out 3 / 32 in. (2.5mm) before the pulley touches the full throttle stop. Reposition the switch as needed and tighten the retaining screws.
  3. To check full throttle enrichment switch operation, disconnect the green wire at the micro-switch. Connect a test light between the micro-switch terminal and the positive battery terminal.
  5. Turn the pulley slowly to the full throttle stop. The test light should light up 5 / 32 - 1 / 32 in. (4mm to 1mm) before the pulley touches the stop. Adjust the switch as necessary and reconnect the wiring.