Fuel Injection General Topics Page
Fuel Injection Problems Page
My apologies if this is a bit noddy, bit it's intended for people who (like me) got left behind when cars acquired injection systems and who need to be told the basics. I've based it on a description of the Bosch L-Jetronic system (which I gather is very similar to the TVR/Range Rover 14 CUX system) in the Haynes book on injector systems, on Lucas diagnostic notes for the TVR 14-CUX, conversations with a couple of people and by poking about under the bonnet.
The system on the TVR uses multi-point injection. It is essentially just a high-pressure fuel pipe carrying an electrically-operated injector nozzle for each cylinder. This looks after metering the amount of fuel that goes into the engine. The amount of air is controlled by your foot via the throttle pedal. The injector nozzles open to allow the pressurised fuel to spray into the inlet manifold very close to each inlet port, where it mixes with incoming air and is drawn into the combustion chamber. The amount of fuel added to the airflow during each engine revolution is determined by the duration of the injector pulses, and this is controlled by an Engine Control Unit (ECU). The injector pulses are not timed to coincide with the opening of each inlet valve - all the injectors fire at the same time, twice for each engine revolution. The ECU takes readings from sensors located around the engine, and uses a lookup table built into a microchip to calculate how long each pulse should last. Because an ignition rev-limiter would damage the catalytic converters (by filling them with unburnt fuel), rev-limiting is achieved by programming the ECU with an upper limit on the frequency of the injector pulses.
Where the duct joins the plenum, there is an alloy box containing a Disk Valve which is attached to the throttle cable. Opening the thottle rotates the disk and creates a larger aperture for the air to flow through. There is a Throttle Position Sensor plugged into the side of the Throttle Valve (it works like the volume control on a radio), to tell the ECU how far the disk has rotated and therefore how much welly you are applying. The ECU also makes a note of how fast the sensor position is changing and in which direction. If the throttle is opening, the ECU lengthens the injector pulses to enrich the fuel/air mixture (for better acceleration). If the throttle is closing it shortens the pulse to weaken the mixture (because you're slowing down and need less power). You can often feel the latter effect after being on overrun for a few seconds, when the engine braking suddenly increases a little. At full throttle the ECU also enriches the mixture for maximum power, and at minimum throttle it weakens it or switches in an idle speed control depending on the circumstances.
The injectors can be seen nestling in the V of the engine, attached to the manifold at the base and to a metal pipe (the Fuel Rail) at the top. A high pressure (60 psi) Fuel Pump delivers petrol to the Fuel Rail. A Pressure Regulator (just visible below the Plenum towards the back of the engine) maintains the pressure in the Fuel Rail at a constant 35-40 psi, by bleeding fuel into a pipe that circulates it back to the petrol tank. As a further refinement there is an air tube attached to the Plenum that changes the Pressure Regulator's spring weight as the manifold vacuum alters (rather like the vacuum advance on the distributor), so that the Fuel Rail pressure is always constant relative to that in the manifold. The fuel pump operates only when the engine is running or the starter motor is engaged (and the car is not upside down - there is a cutoff switch to cater for this!).
For cold starting there is a Starting Valve in the centre of the manifold (hidden under the Plenum). This is a spray nozzle that operates for about 30 seconds if the starter motor is engaged when the engine temperature is below about 15C.
At the back of the plenum is another little gizmo called a Stepper Motor. This sits at the end of a rubber tube that bypasses the throttle valve and bleeds air into the plenum when the throttle is shut or almost shut. The stepper motor responds to a Roadspeed Sensor which is sited on the speedo cable in the engine bay on early cars and at the back of the gearbox on later ones (obviating the need for a speedo cable since it feeds the speedo as well). On over-run or at low speeds the steeper motor lets in just enough air to prevent an excessive vacuum in the manifold. The motor is also activated by the coolant temperature sensor to give a fast idle speed when the engine is cold. Erratic idle speed control can be a result of a faulty Roadspeed Sensor as well as a faulty or dirty Stepper Motor.
There are a few more sensors dotted around which also feed information to the ECU. The fuel rail has a Fuel Temperature Sensor attached to it (once fuel starts flowing through the injectors it gets cold like the body of an aerosol can, but on start-up the fuel can be quite hot and the ECU needs to know that and compensate for it). There is also a Coolant Temperature Sensor attached to the front of the engine block, which tells the ECU if the engine is cold so that it can richen the mixture until it warms up. Engine speed is monitored through the ignition coil. Each catalytic converter has a Lambda Sensor attached to it, which measures the amount of unburt fuel and oxygen in the exhaust gases. The Lambda Sensors are electrically heated so that they work properly while the cats are still cold.
You would think with all this metering and measuring going on that the emissions would be as fresh as a fairy's fart, but no, there is still the dread possibility of raw petrol vapour escaping into the atmosphere. This is a Bad Thing, so another lot of pipework runs across the engine bay to the offside wing, and carries any surplus fuel vapour from the mainfold to a Purge Valve and a Carbon Filter.
As you can imagine, there are an awful lot of electrical connections involved, inlcuding a power supply to the ECU from the battery. If any of these comes loose, or the battery voltage is low, the ECU won't work properly.
There was a query about this on the mailing list a couple of months ago, and I finally got round to asking Mark Adams (the club's fuel injection specialist) what the answer was. This is a somewhat simplified version of what he told me, as I can't remember complicated things.
Not all injection sytems have road speed sensors, although they are becoming more common. The Range Rover system has one, and on Range Rovers it serves two purposes. One is to limit the car's top speed to the speed rating of the tyres - useful on a Range Rover but not something your average TVR owner would appreciate, so that function is disabled. The other purpose is to stop the idle control operating when the car is still moving. So now the question is, "what's an idle control for?". It's there because getting an engine to run well at high speed is fairly simple, but getting it to run smoothly at tickover is a lot more difficult. At a few hundred rpm the engine is producing very little power, in fact just enough to keep all the gubbins going round. Small changes like the engine getting hotter or an electrical load making the alternator work harder, are significant at these low power levels and can affect the idle speed. One solution is to increase the idle speed so that these factors become proportionally less significant, but that uses a lot more fuel which is not what the world wants you to be doing in city traffic. The solution takes the form of an idle control system - a separate throttle with its own air supply that opens and closes to keep the engine speed steady. The air supply consists of a rubber tube that by-passes the normal throttle valve (the one you control with the throttle pedal) and instead feeds air through its own valve which the ECU controls with a stepper motor. The stepper motor moves in response to changes in engine speed, which the ECU monitors through signals coming from the ignition coil. Obviously you don't want the idle control dithering about when you're trying to drive, so it only operates when the normal throttle is fully closed. That sounds fine, but imagine the situation when you approach a road junction and lift off the pedal. The throttle is now closed, but the engine is still revving because the inertia of the car keeps it turning. In fact under heavy engine braking the fuel supply can be cut off completely without any risk of stalling. If the idle control tried to take over at this point it would be incapable of pulling the engine speed down, even with the stepper motor valve fully closed. Two things would then happen. Firstly, the ECU would be getting (to its simple mind) two pieces of information that could not both be true - high rpm and a fully closed throttle. It would assume there must be a fault in the throttle position sensor, and switch into a 'fault condition' mode that allowed it to muddle along without it. Secondly, when the car did eventually stop, both the normal and idle air supplies would be fully closed and the ECU would be unable to catch up in time to stop the engine stalling. These conditions can be prevented if the ECU knows whether the car is still moving when the throttle is closed, and that's where the road speed sensor comes in. As long as the car is moving at more than 3 mph, the idle control will not activate, and the ECU will know that the throttle position and rpm readings do not mean there is a fault. The road speed sensor also helps to improve low-speed driveability. During slow gear changes, if the ECU does not know the car is moving it will activate the idle control as soon as you lift off the throttle. As you let the clutch in the motion of the car will increase the engine speed, the idle valve will close in an atttempt to reduce it and you will feel a momentary thump until you push the pedal down again and the idle control cuts out. On cars fitted with catalytic converters, the absence of a signal from the road speed sensor has a slightly different effect. On cat cars the lambda (oxygen) sensors in the cats are ignored when the throttle is closed (the engine is running on open-loop control). The ECU is programmed to operate in a way that will avoid any possibility of damaging the cats, so when it detects what it thinks is a fault in the throttle position sensor it will either increase the idle speed to a safe level or cut the engine completely. All of this means that a faulty road speed sensor can make the car stall at junctions, stall or over-rev when stationary, and lurch during gear changes. On later cars it will also produce incorrect speedo readings, as the sensor provides signals to the speedo as well. There are no home remedies for this, so take the car to a dealer and get it checked. The only thing you might try yourself, if you have a later car with the sensor on the diff, is to clean the glass on the sensor. top