About Opel fuel injection systems

L and LE-Jetronic
 Tuning and mods


It is not my intention to explain how a fuel injection works. Through the Internet you'll find a lot of interesting information on these systems. I wish only to explain some devices, their workings, and how to tune them. If there is something I have forgotten to mention, feel free to ask about it on the Message Board.

All Opel CIH engines were equipped with Bosch Fuel Injection Systems.

On the Commodore A and B the injection system was called D Jetronic. Beginning with the Kadett C GTE Opel fitted the newest L Jetronic system.


Basically, the D system calculates the air breathed by the engine through a manifold vacuum sensor. This sensor consists of chamber with a floating piston, as the manifold vacuum changes, an electrical signal is generated. This signal is used by the ECU (brain) as an indication of the load applied to the engine (high manifold vacuum implies high engine load. The ECU uses this signal to calculate the opening time for the injectors. Additionally, you have Throttle potentiometer, informing the ECU about engine transitions(acceleration and deceleration). In fact, this device does not have a resistive track like a true potentiometer; but has a series of contacts which each supply a different signal to the ECU indicating throttle pedal movements. You can easily check that it is working, by listening to the 'click-click' sound of the movement between contacts as the throttle is opened and closed.

In this system the fuel pressure is maintained by the fuel regulator (a sort of bell shaped device ) fitted in the fuel tank return line, from the fuel rail for the injectors. On top of ths device there's a bolt to adjust the fuel pressure. Tightening the bolt increases the fuel pressure, and viceversa . It maintains the pressure in the fuel rail by restricting the amount of fuel returned to the tank. This regulator is not connected to the manifold (plenum).

Ignition timing is controlled using the classic distributor, which contains bob-weights and springs. On the distributor, under the points that drive the coil, there are two contacts, which are used to inform the ECU about the engine RPM (revolutions per minute). These two contacts are present because the ECU open 3 injectors at time, not all six simultaneously. These are the same two contacts, which are used to indicate engine RPM to the ECU, also used to control the opening of the two banks of injectors. Each bank of injectors, has two injectors on 4 cylinder engines and three injectors on 6 cylinder engines.

If you need to adjust the air/fuel mixture, there is a screw covered by a plastic plug on the electronic vacuum device, located on the opposite side from the inlet vacuum hose. By tightning the screw, you are increasing the load on a spring, which allows less movement of the piston inside the chamber. This reduces the signal to the ECU, which reduces the amount of fuel injected, so increasing the air/fuel mixture ratio.


In this system, the air is measured through a vane type airflow sensor. The sensor contains a flap that is connected to a potentiometer. The more the flap is open (more air sucked passed the flap), the greater the electrical signal sent to the ECU (up to 5V).

Not like in the D Jetronic case, the airflow meter causes a restriction on the inlet tract. If you compare the cross-sectional area of a 2.0E airflow meter with the cross-sectional area of the 2.0E throttle body, you will find that the cross-sectional area of the airflow meter is smaller, even if the airflow meter flap is held in the fully opened position. Also, the meter is fitted with an air temperature sensor, and the flap stop (end travel), which both reduce the cross-sectional area of the meter even further. Probably, Bosch kept the area in the meter smaller than the throttle to maintain a high and consistent airflow velocity, so gaining accuracy in airflow measurement.

The distance from the throttle body to the airflow meter plays a part in the engine response when accelerating. The nearer the airflow meter is located to the throttle, the faster the meter shows changes in airflow, which means the engine will respond quicker to throttle movement.

On the OEM (Original Equipment Manafucturer) airbox cover (i.e. as used on the Opel Rekord E), there is a square tube before the inlet to the airflowmeter. DO NOT REMOVE THIS TUBE, it will cause a reduction in engine torque at low-end or over the whole rev range. It is used to straighten the airflow before it reaches the meter, which is very important for correct operation of the airflow sensor.

It is easy to check whether you have the L-Jetronic, or the newer LE-Jetronic system fitted to your car

  1. The L-Jetronic system has an airflowmeter with 7 contacts (pins) instead of 5 as in LE (fuel pump shut-off contacts no longer fitted in the newer system)
  2. The L-Jetronic has two main relays instead of a single one present in LE-Jetronic system. LE's only relay also controls the fuel pump shut-off.
  3. L-Jetronic has low impedance injectors (gray colour, approximately 3 OHMs) instead of the yellow high impedance (13-16 OHMs) injectors used on LE-Jetronic system. To drive the low impedance injectors there are 4 ceramic resistors fitted in the wiring loom of the L-Jetronic system. Never install grey injectors in place of yellow ones or viceversa!

- The L-Jetronic ECU has 35 pins, while the LE-Jetronic has only 25 pins.

Opening the cover of the airflow meter (carefully using a screwdriver) you'll find the carbon (black) resistor track, the wiper (with two points for safety), and contacts for the fuel pump and air temp sensor. After many Km of use (approximately 100.000Km), you can experience a choppy idle caused by a worn resistor track on the meter. When you discover small white holes along the black track, this means that you have open circuit in that point giving erratic readings to the ECU. The first and easy mod you can do is to slightly unbolt the four Philips screws that are retaining the whole circuit, then reposition it in a different location on the carbon track (up or down), so the wiper will find a 'fresh' carbon track.

The meter and sometimes some relays are the only weak points I've found on these systems. The system is normally very reliable!

To keep the fuel pressure stable at the injectors there is a fuel pressure regulator. To maintain the same difference of pressures between the manifold (where the injectors are spraying) and the fuel inlet line. The regulator has an additional connection to the plenum through a hose. At idle, you have a vacuum within the manifold, since the throttle is closed. In this condition, you must have a lower fuel pressure at the injectors. At a higher pressure (or less vacuum) condition, up to full load, you have to increase the fuel pressure. This should explain the matter of that hose. The regulator has two inlets and one outlet, a pipe for the vaacum and it is not adjustable. You can only check that disconnecting the vaacum hose with engine at idle (blocking the manifold pipe!) the fuel pressure increase (ca 0.5 bar) and the mixture become richer.

The Throttle switch is now different from that on 'D' system. It has only 3 positions: idle, mid load and full load (to be exact, a bit earlier than wide open throttle) . Here no 'clicks' can be heard. The idle contact is used by the ECU to recognize the idle condition, and for the fuel cut off (no injectors opening) on deceleration until the engine reaches something like 1200-1400 RPM. Below this RPM range, the ECU starts driving the injectors again. The full load contact informs the ECU that a richer air/fuel mixture is needed (I.e. more fuel), thus the ECU increases the injector opening time by about 9-12% above the time indicated by the airflow meter reading.

The trigger signal for RPM is now taken from the negative pin of the coil.


A tuned engine needs more air and more fuel to release any power gain, obviously...


-Throttle body.

You can fit a throttle body from a Monza 3.0 onto a 2.0 engine manifold, by welding a new alloy plate onto the manifold. The new throttle is mounted on this plate. 2.0 engines have 55mm throttle diameter, whilst the Monza has a 65mm throttle diameter. With easy calculations, you find that the Monza throttle theoretically flows 40% more air than the 55mm throttle body. Only slight mods are required to fit the Monza throttle cable, by matching the parts from the two bodies and cutting a leg of the support of the 2.0 throttle cable. To make a good job you have to check that the inlet of the manifold is not smaller than the throttle outlet.

BMW series 3500 has an even bigger throttle diameter (70mm!), but I think that it is too big for a 4 cylinder cih. Keep in mind that a bigger throttle makes the car more difficult to drive at low revs. A slight movement of the throttle pedal will cause a larger increase of air, and fuel, to be breathed by the engine, than with a smaller throttle diameter. As a rule, if a given throttle is ok for 6 cyl, it is probably too small for a 4 cyl engine of the same displacement and same state of tune. Why is this? After all, a 3.0 engine has 180 HP while a 2.0 engine has only 110 HP. It happens that a 4 cyl engine has longer delay between engine pulsations which are coming from the cyliders. A 6 cyls has more pulses per amount of time. More, a 6 cyl manifold contains more air volume to fill the cylinders.

A>B for engine with same displacement but differents number of cylinders (i.e. 2.5cc on 4 cyl and 2.5 on six cyl and same hp target). The engine with 4 cylinders needs a throath bigger than the six cylinder engine!

Examples: a 56mm throttle plate is good for around 140HP on 4 cyls, 60mm is ok up to 160HP, and a 65 mm throttle is good for up to 190HP. A 65mm throttle on a 6 cyl can produce up to 240HP, while on an 8 cyl engine it is good for up to 300HP. A 75mm throttle body on an 8 cyl engine would be good for over 400HP.


Why not fit a bigger meter to reduce the restriction ? Good, But be aware that it could slow the airflow velocity at low revs, giving less bottom end torque. However, often you can regain these losses by increasing the ignition advance, but this does not work on all types of engine.

For engines equipped with L jetronic (7 pins meter) you can install an airflow meter from an old BMW series 3500cc. It has a very big throat and , quite importantly, its resistance along the carbon track is nearly the same as that used in CIH engines. The resistance is given by the flat green resistors that are linked in parallel along the steps on the carbon track (A point on the pic). If you're an able electronic technician, you can susbsitute (with differents values) the resistors depending on the engine's needs. Unfortunately, we cannot map the ECU, it is an analog device, so we have to map the airflow meter!

The meter from the 3.0 Monza's has the same diameter as that fitted on 2.0 engines, so it is a waste of time to fit it. I don't know why this is so. Maybe, to give a smooth and constant airflow along the meter for better A/F calibration, and at the same time increasing the airflow velocity to help bottom end torque. In theory, fitting a larger meter on a 3.0 CIH engine would increase the HP significantly. But I have never tried it.


The airbox from the Monza can contain a larger volume of air, so it is always a good idea to use it. Here 'the bigger the better' is the rule. Don't cut (again!) the 'extension' of the meter's inlet inside the airbox cover. That tube helps to straighten the airflow, acting like a sort of trumpet.

At the air inlet to the airbox, there is a sort of cone that has a very small diameter, this is for silencing purposes. You can remove this from the airbox inlet, if you wish. As far as the 'minimize air restriction' rule can be applied, this mod does no harm, but remember that you have to make a sort of trumpet (as on the original piece) to help flow. Then, fit an extension to the trumpet, to suck cold air from the grille. The colder the air, the denser the air, which means the greater the HP produced.

If I remember correctly, Jamex produces a foam filter which completely replaces the bottom of the airbox, giving a full breathing surface. This item is manufactured and sold as a mod for the Golf GTI II (K Jetronic), but, from my memory, the air filter used on the Golf and those used on Rekord, Manta have identical dimensions. So you may be able to fit one of these beautiful foam filters.

On the K&N site, (Sorry no link present here until Cicco tells it, webmasters comment) there is a formula to calculate the required filter surface for a given application.

Fuel system

I will start by telling you that if you've converted a carburetted car in a fuel injection system and the outlet from the fuel tank is still the same (8mm diameter), then your fuel pump will have difficulty sucking fuel from the tank. The pump has a 12mm inlet diameter and the fuel tank must be fitted with a 12mm outlet, to give best operation. This will also help to maintain a stable fuel pressure to the injectors.

You can increase the fuel flow by fitting larger injectors, by increasing the injector opening time, or by increasing the fuel pressure.


The 2.0 injectors fitted on L Jetronic (grey colour) are 0280 150 105 Bosch number. This injector flows 187 cc per minute at 3 bar fuel pressure, it is called 'Style 5' or hose type (no C clips on fuel rail) and it has a low impedance (about 3 OHMs).

Using the follow formula:

FUEL FLOW (cc per minute)=HP(single cyl) x 4.6 (for NA engines), or HPx5.6 (for turbo/supercharged engines)

you'll find that these injectors are good for up to 160 HP (at continous flow!). 150-160 has been practically the limit.

If you need more fuel you can fit 0280 150 152 injectors from an Alfa 75 Turbo. Four of them are ok up to 200 HP at the same fuel pressure. The fitting of the injectors is unchanged.

If you need even more fuel there is the 0280 150 151 injectors from BMW 3.0 L (6 cyls), Jaguar 4.2 L (6 cyls), Volvo B200/B230, Citroen CX 2.3. Four of them are ok up to 260 HP, at same fuel pressure and are easy to fit.

The injectors fitted on LE-Jetronic system (yellow-amber) are 0280 150 205 Bosch. They flow 170cc per minute each at 2.5 bar fuel pressure, the fitting is 'Style 5', but they have high impedance (13-16OHMs). A high impedance injector means that it reacts faster to the ECU signals. With these injectors the max power target is 148HP.

Keep in mind that a bigger injector will enrich the A/F ratio at all throttle positions, and that if much bigger, it will be hard to tune at low revs.

Some sources say that up to 50% bigger injectors can be used with these LE-Jets so that correct A/F ratio can be achieved by easy tuning. (wm's comment).

Fuel pressure:

You can increase the fuel pressure up to the point that the injector will never open ( caused by excessive pressure on the rear face of the injector valve), or achieve nearly no gain due to the fact that flow through an orifice is not directly proportional to the pressure, but is also dependent on the orifice size!

BTW, there is another formula to calculate the fuel flow after increasing the fuel pressure:

square root (( new fuel pressure / old fuel pressure )* flow at old fuel pressure)

i.e.like at school:
if yellow injectors flow 175cc per minute at standard Original 2.5 fuel pressure, how much they'll flow if the pressure will be increased at 3 bar?
3 : 2.5 = 1.2.
Square root of 1.2 is about 1.1
1.1 x 175cc = 192cc

So the yellow injector cans flow 192cc per minute with a pressure of 3 bar, good for up to about 160 HP. With 3.5 bar, the theoretical HP is 175. Above 3.5 bar the injector shows its limits.

Note that the fuel flow is always calculated with a fully opened injector. To let it cooler you have to close it for a some aumont of time (Dwell time), thus reduce the calculated hp's of about 15-20%.

The fuel pressure regulators on L/LE Jetronic systems are not adjustable. From what I know, the fuel pressure (with vacuum hose disconnected) is 3.0 bar for L-Jetronic systems, and 2.5 bar for the LE-Jetronic system.

As you can see on the pic there's a spring which is pre-tensioned. Increasing this pre-tension will reduce the fuel returning in the tank, so increasing the pressure along the injectors fuel rail. The FSE adjustable fuel pressure regulators (Malpassi, Weber) have just a screw to increase the pre-tension up to the point where you can completely obstruct the return line, so achieving max fuel pump pressure (around 5 bar when the pump is NEW!).

The following diagram gives an idea on how it works.

In reality the lines are 'parallel' up to the point where the force of the spring overcomes the vaacum in the manifold.

Increasing the injector open time

To have a longer opening time you have to fool the ECU, modifying the values coming from the sensors. The ECU has to see an increased engine load, a lower water temp or a lower air temp.

Air sensor

The air sensor fitted on the airmeter is an NTC resistor: as temp increases so its resistance become less.

It has little influence on the opening time. Physics laws say that a colder air is denser than a hotter one, thus to maintain the correct A/F ratio you need more fuel. Disconnecting it means an open circuit (infinite Ohm) for the ECU=a very low air temp=bigger opening time! Short-circuiting the sensor means a closed circuit =hot air=less fuel. Fitting a resistive trimmer in series or in parallel to the air sensor you could find the correct A/F ratio. Be aware that in this way you are increasing or decreasing the A/F ratio throughout the engine range.

Water temp sensor

Like the air sensor, also the water temp sensor it is a NTC device. But since it is related to the engine temp, it has a bigger influence on the opening time of the injectors. A colder engine needs more fuel to run. Here the mods are the same (an additional resistor in series or parallel) like for the air sensor, but the increased-decreased opening time is much more dramatic. As with the Air Temp Sensor, you are increasing or decreasing the A/F ratio throughout the engine range.

Airflow meter

This is the most difficult part to tune. Engines are mapped following their torque curve, not the hp's curve. Normally an engine needs the biggest fuel flow when it is working at its max torque point. Here the VE (volumetric efficiency) is the biggest, you are getting max air cylinders' filling. Past the max torque point, the VE is slowing reduced (depending on engine's configuration) and the engine will needs less fuel than before. Sometimes, tuners (and car's manufacturers) map the engine management to enrich again the A/F ratio at higher load/revs after max VE point. This is done only for a safety reason, in fact a richer ratio with an excess of fuel, helps the cooling effect within the comb chambers. BTW, this happens with engine that have only one point of max Torque. If your engine has two points (or better a valley in the shape of the torque curve, due to exhaust/inlet configuration, you'll need again an enrichment of the ratio.

All these examples are reffered at WOT (wide open throttle). With medium and light loads, your engine will needs less fuel cause less cylinder filling (reduced VE).

If the engine has a fat and large torque curve, the mapping is much more easy. You'll need to spray near the same quantity of fuel all along the torque curve. Also ignition mapping is more easy for the same reason. If your engine has a sudden and steep curve (a 'nervous' engine to drive) fuelling requirements are really differents along the curves. Here both inj and ign maps are a bit hard to set.

After the engine pasts the point (or points) of max VE to reach max hp's point, the fuelling is only a function of the increased rpm's. Yes, you need more fuel to reach max hp's but from now is only a function of engine speed. As speed increases, so fuel increases (injectors pulses are with the crankshaft/distributor linked).

The potentiometer in the airflowmeters fitted on L/LE systems reach its end of the travel depending on the engine and its configuration. I've seen meters going flat out before max VE, at max VE and past max VE. Once the meters reach the end of its travel, the opening time drove by the ECU to the injectors remains a fixed value. In turn, the meter is not able to calculate increased airflow once it reached WOF (wide open flap).

If you vary the VE of the engine increasing its airflow potential , you'll probably need (depending on state of tune) a meter's recalibration or a bigger meter.

How to check if the meter is too small, its max travel, without having a roller-dyno too see it directly? An easy way (without to drill inlet's components to fit vaacum gauges) is using a multimeter in = V setted (10V full scale), putting the black terminal on the left side of the carbon track (low voltage, where the wipe seats at ignition key off position) and the red terminal on the wipe terminal. With engine off and key ign on, open by hand the flap completely (WOF) and measure its voltage. Start the engine, drive the car in 3rd gear, at WOT from 2000 rpm upward to max rpm's, and with the multimeter in hand look where the meter is reaching that measured voltage. If you know the state of tune of your engine, you'll able also to recognize if the meter is too small or not. When you accelerate violently, the flap will soon goes flat open, for a matter of phisics laws and engine config, just for some seconds, then it will stabilize itself with a stable reading. Remember that is true that after max VE point you don't need more fuel, but could happen that (if you've increased a lot the rpm potential than as in origin) the meter passage begins to restrict the airflow, just cause the airflow velocity is faster than before (increased VE and bigger rpm's potential) and is not able to pass throught the meter orifice. Also note that, at some extents, the spread between the max torque value and max hp value is given by the stroke/rod lenght combo. A long rod/stroke combo tends to have narrow distance between hp and torque highest values and viceversa.

Points of mods on the meter:

Point A are the resistances along the carbon track yet mentioned before.
Point B is the mixture screw. It is a bypass of air over the flap. Tightening the screw (closing the bypass) means to have a bigger aperture of the flap, thus a greater signal to the ECU, thus a richer fuel flow. Keep in mind that this screw has effect only for about the first 1/3 travel of the flap, hence you are varying the light loads conditions.
Point C is that device that enstabilishes (?) the correlation between airflow and electric signal to the ECU. Routing the toothed gear in clock sense (?) means that you're increasing the spring of the meter (less fuel sprayed) and viceversa. Varying the position of the gear means to have 9-12% richer/leaner mixture per notch. A very stiff springs is detrimental for engine's performance cause its major effort to keep open the meter. Don't think that a weak spring help you so much during acceleration. There are those two devices (fuel pressure regulator (linked to manifold vaacum!) and TPS) to recognize acceleration, and thus you don't need that sort of acceleration pump effect used on carbs. Moving the notches have a linear effect all along the range of the meter (due to te fact the spring is spiral shaped!), but with the added effect of vary the flap spring stiffness. Point D here you can move only the position of the wiper on the carbon track for more or less fuel without varying the stiffness of the meter's spring.

Throttle Position Switch

Here you cannot do much. Just you could bend the full load contact to achieve before or later the 8-10% enrichment mentioned before. Anyway, if you're intended to set the meter by yourself, keep in mind that results will never be so accurate as before, so emissions and fuel consumptions could really play a big rule.... And in short, after messing with this systems for a long time, now I own a DTA programmable engine management!!!

See: www.dtafast.co.uk