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Fuel injection system of J7T

Empirically assigned experiences with the fuel injection system of the J7T engine - employed in J117, and J636, J637 (2.2 gasoline with catalytic converter).

The fuel injection system of the J7T engine is computer controlled and is a multi point injection system with one injector per cylinder.

This diagram shows the fuel and air flow for one cylinder:

Diagram of fuel and air flow of Renault J7T 2.2 liter injection engine

Fuel control system

The computer controls:
  • Fuel injectors
  • Idle speed valve
  • Ignition timing
This is in fact three seperate control blocks:
  • Fuel injection control, which controls the fuel mixture based on various sensor information
  • Idle control
  • Ignition control

Engine room of J117 showing inlet manifold, inlet valve, fuel rail connected to fuel injectors (next to spark plugs), and connector for inlet temperature sensor.

Fuel injection control system

The fuel injection control is a computer controlled regulation system, which controls the fuel injectors. The four fuel injectors open at the same time, displacing a bit of fuel into the four branches of the inlet manifold, which is lead into the cylinder when the valve opens. This is possible because the fuel is under pressure by the fuel pump, and with the proper pressure at the inlet, the injector will lead a certain amount of fuel per milisecond the injector is open, into the manifold.

How much fuel to let in is controlled by four values:

  • The absolute manifold pressure (MAP)
  • The inlet air temperature
  • The lambda (oxygen) sensor in the exhaust
  • Engine revolution count
The MAP sensor, along with the engine revolution count and built in knowledge about the bore and stroke of the engine, tells the computer how much air is passing into the cylinder per second, and the correct amount of fuel can then be calculated to give an ideal fuel/air mixture.

The oxygen sensor tells the computer if the mixture is too lean or too rich - it oscillates between one and the other when the computer is working optimally. It is used to adjust the constants used in the calculation of injector timing to ensure as good-as-possible mixture of air and fuel.

This is the most important part of fuel control with a warm engine. There are two special cases of fuel control: When the engine is cold and a richer mixture must be used to keep the engine running, and when the engine is idling.


The XRd from FASTCHIP in action on a J117. Note that ignition angle is displayed incorrectly on this model. The 'ms' value is injector open time. Note that the mixture is fairly rich here because the engine is still cold (12 deg C).


Inside of ECU from 1992. Note the microcontroller, which is a variant of a 1 MHz 6801 with 4 k ROM, 192 bytes RAM, 29 I/O lines and timer. In the 1987 version from Renix, the component used is an ST EF6801U4PV. Note also the power transistors bracketed to the case of cast aluminum. The pcb is laquered to cope with wet environment.

Idle control system

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Ignition control system

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Sensors and actuators

Idle speed valve

One frequently encountered source of problems with this engine is the idle speed valve. This valve bypasses the main air valve (as controlled by the gas pedal) and is controlled by the computer to supply the engine with air when the gas pedal is not operated. The drawing below shows the function of the regulator:

Idle speed regulator functional diagram.
Inside of a 17 year old idle speed regulator.
Air enters at the rightmost blue arrow and exits at the leftmost blue arrow. The spindle can rotate a quater of a turn. The rotation is controlled by the motor in the valve. The red arrow indicates the turn direction for closing the valve. The inside of a 17 year old idle speed regulator showing a rusty motor.

The regulator is located to the right of the engine right behind the radiator. In the view of the engine room above, it is not visible. It is connected with an airtube connected to the exit of the air filter and another air tube, which is connected to the underside of the fuel injection system.

Symptomps of problems with the idle speed regulator are:

  • Too high idle speed
  • Too low idle speed
  • Unstable idle speed
  • Engine start problems
With a working regulator, the engine climbs quickly to about 1200 rpm after ignition, and returns to 850 rpm after some time (warm engine).

A simple but not definitive way to diagnose it is to disconnect the air tube which is feeding air into the regulator and look with a lamp to see if the valve opens and closes during engine start and load. (The engine can be loaded in idle by turning the steering wheel, thus loading the servo pump.)

The valve consists of two coils connected to a common terminal, thus three terminals in all. The common is pulled to battery +. The computer controls the two other terminals. One coil pulls the valve close, the other pulls it open. I'm not sure how it's controlled, but I suspect it's done by short pulses: A short pulse on the closing coil closes the valve a bit. How much is determined by the length of the pulse. A long pulse closes it completely. If this is the case, it's not so strange that a bit of rust can ruin it completely - dry friction kills such a principle!

Water temperature sensor

Another source of problems in fuel injection systems is the water temperature sensor. It sits on the thermostat just above and in front of the exhaust manifold.

Symptoms of problems with it include:

  • Difficult to start the engine
  • Too high idle speed with warm engine
It's a thermistor, so measuring its resistance should be an easy way to diagnose it.

Lambda (oxygen) sensor

The lambda sensor measures the amount of remaining oxygen in the exhaust gas. It is used to adjust the timing of the injectors to obtain the perfect fuel/air mixture for the engine at the given temperature/MAP/revs. If it doesn't work properly, the measurement of CO in the exhaust will be too high.

There are two different lambda sensors in Espace: One wire sensors and three wire sensors. The latter has a built in heating element so that the sensor reaches it's operating temperature quicker (about 600 deg C).

How to test the lambda sensor:

  • Connect an oscilloscope to the output line of the sensor. Ground the probe to the chassis. If you do not have an oscilloscope, a fast analogue transistor-voltmeter may do the job, however it's critical that input impedance is high.
  • Start the engine.
  • After some minutes the sensor reaches it's operating temperature and starts producing an output between 0V and 1.1V.
  • When the sensor is fully heated, it starts oscillating between 0V and 0.6V - 0.8V. The oscillations will have a period of about 2-3 seconds. This is the computer adjusting the fuel mixture between lean and rich. The sensor is very sensitive, so the theoretical optimum of 0.45V is never achieved.
  • If the sensor shows constant 0V it's defective. If it doesn't reach at least 0.6V, it's probably defective too.
Links:

» www.forparts.com/o-21.htm
» Oxygen Sensor Information




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Page updated: 2006-02-13 19:40:43