In most cars there is one injector per cylinder. These injectors all open and close at the same time when commanded to do so by the ECU. It's important to understand that the injectors do not really spray fuel, they are just valves that open and close. They are needle valves that plug a small hole when closed. They are opened by a tiny electromagnet in each injector that pulls the needle away from its hole enabling fuel to pass into the intake system. When they are open fuel sprays in as a result of fuel pressure, so it's really the pump and fuel pressure regulator that control how much fuel sprays in for a given amount of time when injectors are open. The ECU regulates how much fuel is sprayed in by controlling the amount of time the injectors are open. Your ECU assumes your car has the correct amount of fuel pressure. If you have more or less it will effect your ECU's calculations.
An injector can go bad in a number of ways. They can have external leaks, internal leaks, fail to open, fail to close or they can be clogged with combustion debris. Symptoms of a bad injector or injectors can include difficult or impossible cold or warm starting, irregular idle, lack of power, poor fuel economy, missing, high emissions and external fuel leakage.
If your injector is leaking internally or failing to close fully the car might run OK depending on just how bad the problem is. The way to check for this problem is simple. When you change spark plugs inspect them. If one or two are a lot blacker than the others you probably have a leaking injector on those cylinders.. If the leak is bad or the injector never closes, the car will run poorly at low RPM but improve at high RPM when the engine is sucking in enough air to burn the fuel.
A lot of cars can run pretty well with one injector stuck closed, some can even run with half of their injectors stuck closed although you will know something is wrong. It's a little tougher to tell if an injector is stuck closed by looking at the plugs so let's use another method. We can partially check the injectors by performing a "balance check". This will not tell us if all the injectors are equally poor, but it will tell us if one injector has a serious problem. With the engine running remove the injector plugs one at a time and note the rpm drop. If all of the cylinders drop equally then the injectors are all about the same (assuming the engine's cylinders are all in the same condition). This is not a precise test, and will usually only find gross problems. Of course before you go and remove the injector you should make sure the cylinder in question is getting a spark.
The most common injector problem and the most difficult to detect is the partially clogged or "dirty" injector. Unfortunately there is no practical way to check for this problem without pulling out the injectors so we need to use the process of elimination. If you have checked everything else out and your car still has some running problem then it's probably one or more dirty injectors. Putting injector cleaner in the fuel tank will not clean the injectors. On the contrary, it will usually wash fuel system debris into the injector's filters. Once debris is in those filters, it can't get past. The filter's screen has holes too small to see with the naked eye, almost nothing other than fuel can get through.
Here we see an example of a good injector spraying in fuel.
Here is a picture of a clogged injector. An engine with one or more clogged injector will never develop full power, and will have high emissions and running problems.
If you find one or more bad injectors there is little choice but to buy new ones. Let's get one thing clear, THERE IS NO SUCH THING AS A REBUILT L-JETRONIC INJECTOR! Now I know a lot of people claim they are selling rebuilt injectors, and I don't know what they are doing. However I do know that only a few things can be done to them. They can be cleaned, the tiny filter can be replaced, a new pintle cap (tip) installed and the various external seals and spacers can be replaced.
We offer fuel injector flow checking, and overhauls on the latest type of machine for L-Jetronic hose feed injectors. We have the latest flow checking and injector cleaning equipment.
We can flow test, and/or overhaul your injectors. The overhaul includes ultrasonic cleaning, new inlet filter, pintle cap, and all seals for a very good price. I very thoroughly check the injectors at all the RPM levels and pulse widths your injectors will experience on the car. Most shops only check them at idle. I also warranty the injectors I service for 12 months. I have found that when injectors are serviced by other shops they often start to leak about 6 months after the service. This is because the internal seals are delicate and can be easily damaged by the fluids many shops use to clean and flow check. The correct fluids to test and clean these early injectors gets expensive so many shops don't bother to use them. I do, that's why I can warranty the injectors I service against external leakage, or any other problems.
Pictured below is a set of injectors before and after overhaul. As you can see before overhaul, the injectors were about 45% apart in fuel flow. It's amazing the car even ran. After overhaul they were all within 3% (well within factory spec). Before overhaul with a target air fuel ratio of 12.5:1 at full throttle the individual cylinders would have been between 12.5:1 and 22.7:1. This would have been disastrous on a high compression or forced induction engine.
Most Bosch injectors in a given set will flow within about 3% of each other. For hardcore applications it's possible to get the injectors within 1% of each other. This normally involves selecting a matched set from a number of injectors. We can provide this service, and we recommend it for high compression or high boost applications.