K-Jet In Detail

The basic purpose of a fuel injection system is to provide the amount of fuel an engine needs at any time, mixed with the right amount of air to burn that fuel. In this article, I will cover how the K-Jet system accomplishes this task. I don’t know everything, so READ THE CI GREENBOOKS TOO.
There are three sections to this guide:

The Continuous Injection System
The Lambda-Sond System
The Constant Idle Speed System

Continuous Injection (CI) System

K-Jet systems use a Continuous Injection (CI) system. This is not the same thing as CIS, the Constant Idle Speed system. While all K-Jet systems have CI, only some have CIS. Think of CIS as an add-on to the K-Jet system.

Continuous injection means that the injectors are always running (spraying fuel) when the engine is running. The amount of fuel that gets into the cylinder is controled by the rate of fuel flow. A garden hose operates in a similar way. If you want more water, you turn the handle on the spigot up more.
The CI System is composed of the following:

Fuel Injectors

K-Jet fuel injectors are fairly straightforward. They have a spring-controlled valve that opens at a specific pressure (between 43 and 60psi depending on the equipped injector type.) The higher the fuel pressure, the farther the injector opens, and the more fuel that is squirted out of the injector. Thus, it is called continuous injection, because once the fuel pressure is up, the injectors are open and that’s that.

Fuel Distributor / Air Flow Sensor

The rate of fuel flow is governed by the fuel distributor which is attached to the air flow sensor. When air lifts the sensor plate up, the fuel distributor sends more fuel to the injectors. As the air sensor plate lifts higher, the fuel distributor sends more fuel to be burnt with the increased amount of air. Think of this system as a teeter-totter like you used to play on in grade school.

As the air sensor plate is lifted, it lifts the control plunger in the fuel distributor. This control plunger exposes slots for fuel to move through. There are 4 slots, one for each fuel injector. As the air sensor plate lifts hight, the control plunger moves higher, exposing more of each slot, allowing more fuel to pass to the injectors.

This is all possible because of the fuel delivery system that K-Jet uses. About 10 times the amount of fuel that the car will need is delivered to the fuel distributor. The excess fuel goes back to the gas tank in the fuel return line. So the fuel distributor has two lines connecting to the gas tank: one coming from the tank and one going back. It also has four fuel lines for the four injectors, and a few other lines we will discuss later.

On a Turbo car, the fuel distributor and air flow sensor are located just behind the passenger’s side headlight, above the air filter. They are fairly easy to access. On a normall aspirated car, they are located below the intake manifold and are fairly difficult to access without removing the intake manifold.

Fuel Distributor in Detail

The fuel distributor contains an internal line pressure regulator. This simply controls the fuel pressure in the fuel distributor. If the fuel pressure in the fuel distributor drops below 65.3psi, the line pressure regulator stops fuel from going back to the gas tank in the fuel return line so that the pressure stays above 65.3psi in the fuel distributor.

The pressure regulator valves (there are 4, one for each injector) help maintain a constant pressure difference between the fuel on one side of the control plunger and the other side of the control plunger. Basically, these help keep the pressure constant on the injector side of the fuel distributor, while the line pressure regulator keeps the pressure constant within the fuel distributor.

Cold Start Injector

Another fuel line comes off of the fuel distributor and goes to the cold start injector. It is bolted into the intake manifold with two Allen-key bolts and is sealed with a large rubber O-ring. It is an electronically operated injector (like those in a car with Electronic Fuel Injection). It only sprays fuel during the first few seconds when the car is starting, and it only does this when the engine is cold. It only injects fuel when the starter motor is turbing.

Engines need more fuel than usual when they are cold because combustion of the air/fuel mixture is less efficient at lower temperatures. Thus, to produce the same amount of explosion, the engine needs more fuel at colder temperatures.

Thermal Time Switch

The cold start injector is hooked up electrically to a thermal time switch. This switch is located below fourth injector at the ‘back’ of the head. It is a simple switch that turns the cold start injector on for a limited amount of time, but only temperatures below 95 degrees F (35C). This may seem high, but it is actually quite low for an engine.
When the starter motor operates, electricity flows through the thermal time switch (if the switch is closed) and to the cold start injector, causing the injector to open and fuel to spray out.

Control Pressure Regulator

There are two more lines coming off of the fuel distributor. Remember that there were 4 for the injectors, one for the cold start injector, and two for the gas tank. There is an additional pair that travel to the control pressure regulator and back.

The control pressure regulator does two things. First, it supplies a fuel pressure to “control” the control plunger in the fuel distributor from moving up too fast. So when the control plunger is moving up and letting more fuel get to the injectors, the speed at which it moves is limited by the control pressure created by the control pressure regulator. The purpose of this is to keep the movement of the airflow sensor plate and control plunger steady under fast accelleration. That is to say, if you stomp on the accellerator pedal, it keeps the engine from jerking too much.

The second thing that the control pressure regulator does is that it changes the control pressure when the engine is cold to allow more fuel to get to the engine. Basically, it performs the same function as a cold start injector (more fuel when cold) but it does it a different way. The control pressure is lower when the engine is cold. This means that there is less force pushing down on the control plunger, allowing more fuel to get to the injectors than normal.

There are several different types of control pressure regulators. The 1981 and 1982 N/A cars had a control pressure regulator with two vacuum nipples on the passenger’s side of it that hook up to a blue thermal vacuum valve mounted in the side of the engine. This type of CPR is shown in the first four pictures above – this is the “…079” type of CPR if your reading the table below, although the “…082” type of CPR looks basically the same on the outside.

The other type of control pressure regulator is “nippleless” as it has no nipples to hook up vacuum lines. The cars with this type of CPR use a pressure differential switch that is hooked up to the blue thermal vacuum valve. When the engine is cold, the pressure differential switch tells the Lambda-Sond ECU to add more fuel to the air/fuel mixture. This type of CPR is shown in the last picture, which is either an “…004” or a “…014” CPR, but I’m not sure which.

The table below illustrates to the best of my knowledge, the different types of CPR’s you can find on K-Jet Volvo’s. Data is missing regarding 1982 through 1985 240 Turbo’s.

Type of CPR

(by last 3

…004 …014 …021





when cold




when cold




No No ? Yes (2) Yes (2) Yes (2) No
B19E 1977- X
B19ET 1982- X
B21E 1975 X
1976- X
B21ET 1981- X
B21F-5* 1976 X X
1977 (non-USA) X
1977 (USA) X X
1978-1980 X
1981 USA X
1981 Japan X
B21F-9* 1981-1982 X
B21FT 1981-1982 X
1983 X
1984-1985 X
B23E 1979- X

* Note: The B21F-5 has a Bosch, orange-coloured distributor. The B21F-9 (aka B21F-MPG) has a white-coloured, Chrysler distributor. For more information, see the Know Your B21 article.

Auxiliary Air Valve

The throttle plate is what is actually controlled by the accellerator pedal. I call it an accellerator pedal and not a gas pedal because it doesn’t control the flow of gas, it actually controls the flow of air into the engine. Calling it an air pedal would just be weird though. Anyway, when you push down on the accellerator pedal, it opens the throttle plate inside the throttle body allowing more air into the engine.

When you let off the accellerator and the car is idling, the throttle plate is closed. However, the engine must get air somehow to keep running. The auxiliary air valve allows air to bypass the throttle plate. A ‘bi-metallic’ spring presses on the valve when the engine is cold and lets air through, allowing air to reach the engine. As the engine warms up, less and less air moves through the auxiliary air valve due to the nature of the valve. The two wires on the auxiliary air valve are for a heater circuit. Of electricity through this bi-metallic spring causes it to expand, causing the valve to close as the engine warms up.

Lambda-Sond Emissions Control System

The Lambda-Sond system was introduced in 1977 to help control the exhaust emissions by regulating the air fuel mixture in such a way that it ensures optimal ‘cleaning’ of the exhaust by the catalytic converter. Although some early cars had K-Jet without a Lambda-Sond system, most later models were equipped with Lambda-Sond. The system is composed of:

Lambda-Sond Electronic Control Unit

This computer ties the whole system together. The ECU takes information from various sensors and makes decisions based on that information to regulate the amount of fuel and air that is mixed in the engine, as mentioned above. It is located behind the passenger’s side kick panel in the passenger compartment.

Oxygen Sensor

The Lambda-Sond system includes the oxygen sensor which screws into the exhaust manifold or exhaust pipe like a spark plug. It should be replaced every 15,000 miles.

Frequency Valve

The frequency valve goes hand in hand with the Lambda-Sond system. It is responsible for regulating the air fuel mixture and is connected to the fuel distributor as shown above. It recieves information from the oxygen sensor.

Constant Idle Speed (CIS) System

The constant idle speed system is an add-on to the K-Jet system. It replaces the auxiliary air valve with an idle air motor. The CIS system was designed to help regulate the idle more effectively than the previous auxiliary air valve system.

From personal experience from owning two 1981 cars – one with CIS and one without – the CIS system made a marked improvement in the idle. It is so good that on a stick shift car, you can let the clutch out without pressing on the accellerator pedal at all, and the CIS system will compensate and let enough air into the engine to let the car take off smoothly without stalling. This feat (that is, getting the car to start going just by letting out the clutch) is impossible on a car without CIS.

The CIS system is found on 1981.5 N/A 240’s, 1982 K-Jet 240’s (1982.5 240’s were LH-Jet 1.0), and 1981 to 1985 240 Turbo’s. It is composed of the following components:

Idle Air Motor

Also called an air control valve or idle air control valve, the idle air motor is responsable for the same functions as an auxiliary air valve – that is, regulating the amount of air that gets to the engine when the throttle plate is closed and the engine is idling. It has three air flow modes. Under decelleration with the throttle closed, it reduces air flow. With the throttle open, it allows high air flow. At idle speed, with the throttle closed, it functions to maintain a steady idle speed.

The idle air motor on a K-Jet car has three wires attached to it and two large vacuum lines (1inch in diameter). On a N/A 240, it is located on the top of the engine, attached to the valve cover between the head and the intake manifold. On a 240 Turbo, it is bolted to the underside of the intake manifold. On an LH-Jet car, it is bolted to the intake manifold support bracket, underneath the intake manifold.

The first idle air motors were a Bosch 500 (the number is on the idle air motor). The 500 is black or bronze-ish. The second series were the Bosch 501, which were bronze colored and slightly smaller than the 500. The third type is the Bosch 520, which is silver and is much smaller than the 500. These three all have a 3-wire connector and are all interchangeable, even though the 501 and 520 were used on LH-Jet cars. The 516 is not interchangable with the 500/501/520 idle air motors.

Here is a nice chart I came across on The Brickboard, thanks to Fitz Fitzgerald:

Type of idle air motor: Cars on which it can be found:
Bosch 500 240 ’81-’82 and 240 Turbo ’81-’83.
Bosch 501 240/740 ’83 through April,’87, also 240/740 Turbo ’84 through April,’87.
Bosch 520 240/700 May,’87 through all of 1988.
Bosch 516 Most late model 240 and 700/900 vehicles with LH 2.4 & 3.1 Fuel Injection (1989+). Note, many late model turbo vehicles still use the 520 (I own one of these such vehicles).

Throttle Microswitch

The idle air motor requires a signal from the throttle microswitch to know what mode to operate in. The throttle microswitch is a small switch that simply tells the idle air motor and the CIS computer whether the throttle is open or closed. It has two wires coming off of it (black and yellow), with the exception of the 1981-1982 N/A cars with the Chrysler ignition system, which have three wires (black, yellow, and orange).

Coolant Temperature Sensor

The coolant temperature sensor provides information to the CIS electronic control unit regarding the temperature of the engine so that the electronic control unit can regulate the behavior of the idle air motor to compensate for a cold motor. While the auxilliary air valve’s behavior was regulated by an internal temperature control (basically by how long it had been on), the idle air motor is regulated by the external coolant temperature sensor. The coolant temperature sensor is mounted in the head, below the first fuel injector, at the ‘front’ of the engine.

CIS Electronic Control Unit

This seperate computer (seperate from the ‘Lambda-Sond’ computer found on all K-Jet cars) takes information from the throttle microswitch and the coolant temperature sensor to regulate the behavior of the idle air motor. ‘Nuff said. It is located under the Lambda-Sond computer, behind the passenger’s side kick-panel.
Wow, that was a lot to write. I hope you enjoyed it. I’m sure I left something out so feel free to post your complaints on the forums. I will modify this guide as I learn more about things. More work definitely needs to be done on the differences between the different control pressure regulators.