Troubleshooting the fuel injection system

EFI troubleshooting

Before you delve into the sometimes complicated details of the fuel injection system, check that the ignition system (battery voltage, distributor, spark plugs, ignition coil and timing) and the engine itself (compression, adjustment of valves, engine oil pressure) are in proper condition.

If you have a later, Motronic engine management system that is capable of storing an error code whenever it detects a malfunction, start by reading these codes out.

The engine does not start although the starter motor operates.

Check that all hose lines are securely attached and firmly tightened, that the hoses themselves are in good condition, not cracked, kinked or damaged.

Check to see whether the fuel pump operates (it is located above the right rear wheel, its operation can be heard as a buzzing sound if you listen to it while an assistant is cranking the engine), check whether voltage is present at its connector while the engine is being cranked. If the voltage is present but the pump is not working, it has to be replaced. If you found no voltage, the fuse and the relay feeding the fuel pump might be suspect. Check and replace them if necessary.

If the pump does supply fuel, test the pressure in the system. Check the fuel lines and filters as well as the pressure regulator.

Check the CTS. If it is open or became disconnected, the engine will not start.

Jetronic only: If you found the fuel pressure sufficient, check the auxiliary air valve. If it is faulty, replace it. Motronic only: Check the crank angle sensor.

Check the whole air intake system (the manifold and every component and hose attached) for vacuum leaks. You can localize the leaks present by using compressed air and soapy water.

A shortened injector can also cause the engine to refuse to start. Disconnect the injectors one by one until you find the faulty one or check them by measuring their resistance.

Finally, test the AFS. If it became completely faulty or disconnected, the engine will not start.

The engine starts but it dies in a short while.

Test the fuel pressure in the system. If it is sufficient but drops after the end of the starting process, listen to the operation of the fuel pump. If the pump keeps functioning, the pressure regulator is faulty.

Jetronic only: If you found the fuel pressure sufficient, check the auxiliary air valve. If it is faulty, replace it.

Finally, test the AFS.

The engine has cold starting problems.

Check the CTS.

Jetronic only: Check the auxiliary air valve. If it is faulty, replace it.

The engine does not accept fuel smoothly.

Test the throttle switch and adjust it if necessary.

Check the whole air intake system (the manifold and every component and hose attached) for vacuum leaks. You can localize the leaks present by using compressed air and soapy water.

Jetronic only: Check the auxiliary air valve. If it is faulty, replace it.

Finally, test the AFS.

If you successfully found and repaired the failure, don't forget to measure and correct the idle speed and the concentration of CO again.

The engine runs but it misses every now and then.

Engine misses can be caused by bad contacts of the ECU connector. With the ignition switched off, disconnect, clean and reconnect the connector. Check the earth terminals for continuity and good contacts. Similarly, check the wiring to the fuel injector relay.

Check the quantity of fuel delivered by the injectors.

If nothing has cured the problem so far, test the AFS. If the missing spot appears always at the same engine speed (rpm), the cause will probably be a worn AFS track. You can bend the wiper arm carefully so that it touches the track at a slightly different radius.

If the AFS is OK, the ECU itself might be at fault.

Fuel consumption is too high.

Test the fuel pressure in the system. If it is sufficient, check the quantity of fuel delivered by the injectors.

If the consumption is OK on the highway but too high in town, driving short trips, check the functioning of the cold start systems (auxiliary air valve).

Finally, test the AFS and examine the idle speed and mixture settings.

The engine does not reach its maximum power.

Check whether the throttle butterfly is closed. Try to push it farther and observe the engine speed in the meantime. If the speed decreases, reset the adjusting screw.

Test the fuel pressure in the system. If it is sufficient, Check the whole air intake system (the manifold and every component and hose attached) for vacuum leaks. You can localize the leaks present by using compressed air and soapy water.

Check that the air filter is clean and the fuel filter is not clogged.

Finally, test the AFS.

Idle speed problems

Probably the nastiest problems with fuel injection and integrated engine management systems are those of the idle speed. During normal driving the circumstances in the engine are average, dictated mainly by the AFS or MAP sensor (roughly speaking, the position of the throttle pedal), meaning that many other sensor or mechanical errors in the system could go practically unnoticed. On the other hand, the idle speed relies mainly (or in some systems, totally) on the electronic control unit, causing sometimes even the slightest disturbances in the system to be manifested in uneven idle speed.

The first component to check is the throttle switch which informs that ECU about the position of the throttle pedal. If it is out of adjustment or even malfunctioning, the ECU has no way of knowing that it is supposed to work in idle speed mode. Before you start with the actual testing, try to push the throttle butterfly a little bit farther. If the idle speed changes, the switch probably needs readjustment.

To go on, you would need an exhaust gas analyser. Before you start to spend your time chasing faulty components, it is good to be sure that it's not a simple misadjustment that's causing the trouble. You can probably find a garage nearby where they can measure this for you. As a preliminary test, take a look at the spark plugs. Carbon fouling, for instance, means an over-rich mixture.

So, if you cannot set it to the required values of idle speed and CO content, or the idle is still uneven, check the whole air intake system (the manifold and every component and hose attached) for vacuum leaks. You can localize the leaks present by using compressed air and soapy water.

If the engine revolution drops way below idle for a while when you release the throttle pedal suddenly, then climbs back and stays normal (although a cold engine might not come back to normal idle but cut off when the revolution drops that low), you must have a vacuum leak past the airflow meter. During deceleration (throttle closed, engine running) the high vacuum sucks some additional air into the engine through that leak. As this is already past the airflow meter, this additional air is not measured and accounted for by the injection computer. The surplus makes the fuel-air mixture leaner by a considerable amount, starving the engine. However, as the revolution drops, the vacuum weakens, the additional air intake is reduced and the situation normalizes: this is why the engine returns to normal idling after a few moments.

Check the rubber hose carefully between the meter and the throttle butterfly housing for cracks and holes. The crack can be very small and hardly visible, yet cause problems as the vacuum is quite high; the sucking effect opens even a small crack quite wide. Note that two smaller hoses detach from the big hose, one going to the auxiliary air valve beneath the distributor (access is difficult because of the battery, you might want to remove the latter), the second to the engine oil filler cap. Those hoses (and the points where they connect) are suspect, too.

If the crack it not visible immediately, use carburettor cleaner spay. Spray a mist over the suspected part of the hose. If there is a crack, the engine will suck air and the cleaner through it. As the cleaner acts like fuel for the engine, it will speed up for a moment. When you spotted the trouble area, remove the hose and check visually, bending and twisting it until you reveal the crack. You could also remove the suspected hoses, plugging one end and letting compressed air into it. Soapy water will bubble where air escapes.

If you do find a leak, you have to close it. Glue is hardly an option because these hoses are made from hard rubber and are subject to permanent heat and vibration (although you could try using some silicone sealant, after having cleaned the surfaces very carefully. Be sure to use sealant for automotive use, stating 'oxygen sensor safe' if you have a catalytic converter, not bathroom silicone). With the smaller hoses you could use a coolant hose repair kit (a rubber band bandage, eg. from Holts).

If it wasn't an air leak, you have to go back testing. It is important to know whether the idle speed is constantly changing, fluctuating, or just too high or too low. It is also vital to know whether there is any relation between the idle speed problems and the temperature of the engine. If the problems appear only in cold or warm engine, the coolant temperature sensor and the auxiliary air valve should be checked.

On Motronic and Magneti Marelli systems the idle speed is constantly adjusted by the ECU. The Motronic uses an idle speed control valve that opens and closes several times in a second to increase or decrease the current idle speed, while the Magneti Marelli system can adjust it even more smoothly with an idle control stepper motor which not only opens or closes but can fine adjust the amount of air passing through the idle bypass around the throttle butterfly.

The Magneti Marelli stepper motor is situated on the top left of the engine. Pull out the metal cylinder (it's only held by two rubber O-rings), remove the nut and take out the copper cylinder. Clean both cylinder and the housing. Remove the dirt from the four small holes in the top of the cylinder piercing them very carefully with a thin wire. Refit the inner parts completely into the housing and put them back into the cylinder.

Check whether the injectors are in good mechanical condition: with the engine running, detach one injector plug after the other. If the injector is good, the engine speed must drop as its plug is disconnected.

A faulty oxygen sensor can also make the idle speed change or fluctuate up and down constantly.

Test the fuel pressure in the system. If it is sufficient, go on to testing the AFS.

If you successfully found and repaired the failure, don't forget to measure and correct the idle speed and the concentration of CO again.

Irregular idle speed in 16 TGI (Magneti Marelli engine managament system). Revolution rises periodically.

??? Check if the air filter cap closes over the filter edge nicely. Check the hot air hose to the filter for cracks, especially on the exhaust mounting.

Frans Tak

Other failures and problems

The car runs nice for some period (might be a few kilometers or several hours) but then it starts to miss suddenly and finally stops completely. After having rested for some time (a few hours, maybe overnight) everything starts all over again.

When the symptoms are present, go to the end of the car, to the right rear wheel. Listen for the humming noise of the fuel pump. If the pitch of the sound changes periodically, the fuel pump is struggling to supply the fuel pressure needed for the injection system. If you have the necessary equipment, measure the fuel pressure just before the injector fuel rail. If it is all right for a while then it starts to drop, and finally the engine stops, the fuel pump is obviously faulty.

If you live in a country where the petrol you can buy is not so clean, change the fuel filter more frequently than specified (especially on BX 19 models where the factory specifies 80,000 km). Considering the low cost of the filter, it is much more reasonable to change it twice or three times as often. If you have to renew the fuel pump, renew the filter as well.

If my engine happens to stop for some reason, I can't restart it immediately. However, if I wait 10-20 minutes, it will start again.

These are the classic symptoms of dirty injectors. There are injector cleaning fuel additives available from different manufacturers, however, they cannot be always recommended heartily. Some might harm the catalytic converter (although all of them will most certainly say on the label that they are absolutely harmless). Injection specialist workshops generally offer a reliable cleaning service, however, it is much more expensive than a bottle at the gas station.