by Jim Kerr

I get a lot or questions from car owners about their vehicles. Many of these questions are related to “driveability”. Driveability is the automotive industry’s term to identify any problems that have to do with engine performance. It could be a rough idle, stalls, hesitations, poor acceleration, low fuel economy, or even hard starting. As you can see, there are many potential problems related to driveability.

While there can be as many causes of poor driveability as there are types of problems, many of them are related to fuel and fuel injection. In the past, there were two types of fuel injection systems commonly used on vehicles: throttle body and port.

Throttle body systems use centrally located injectors mounted in a housing complete with throttle plates. These systems were pretty trouble free. Throttle body injectors sit up high above the engine and are not subjected to as much heat as injectors mounted directly in the manifolds. They also flow a lot of fuel through each injector because one injector feeds several cylinders. For these two reasons, throttle body mounted injectors have few problems and cleaning them usually made no difference in engine performance.

Port injection, on the other hand, uses one injector for each engine cylinder and the injectors are mounted directly in the intake manifold. The fuel sprays into the intake port and sits behind the intake valve until the valve opens and the fuel in drawn into the cylinder. Because of their mounting, port injectors get “heat soaked” almost every time the engine is run. A heat soak occurs when a hot engine is turned off. Heat from the engine rises, and fuel sitting at the tip of the port injectors evaporates. Left behind on the tip of the injector is a gummy residue from the low ends of the fuel. This residue causes the problems.

Gum on the tips of the injectors changes the fuel spray pattern from the desired cone-shaped mist to an off-centre stream of fuel. When fuel is injected in a stream, it tends to drop out of the air stream or takes longer to evaporate in the cylinder. Either way, the fuel doesn’t burn properly and a driveability problem results.

Cleaning the deposits off the tips of the injectors can be done several ways. One way is to take the vehicle out on the highway and accelerate hard several times. The increased fuel flow tends to wash away the deposits. Most fuel companies add fuel injector cleaning additives to their fuels to help remove these deposits.

Another way to clean the injectors is to add concentrated fuel injector cleaner to the fuel tank. Available at dealerships, corner service stations, and automotive parts stores everywhere, this type of cleaner adds more detergents to the fuel. Several aftermarket companies supply injector cleaner but directions on the container are usually the same: add one container of cleaner to a full tank of fuel. The detergents will help keep the injectors clean, especially if you do a lot of slow speed driving. Do not add more than one container of cleaner to a tank of fuel. The detergent agents in the cleaner are very concentrated and too much can cause rubber parts in the fuel system to fail.

A quicker way to clean injectors is to force injector cleaner directly through the injectors at a concentrated rate. This service can be performed by most repair shops and is usually only done when there is an injector-related driveability problem with the vehicle. To use this method, the technician connects the injector cleaner and its supply tank directly to the fuel lines at the motor. Any fuel return lines to the car’s fuel tank are blocked so the cleaner goes only through the injectors. With this type of cleaning, the concentration of the cleaner is very strong, so injectors are cleaned quickly. It also has the advantage of cleaning some carbon off the backsides of the engine’s intake valves. Carbon on the back of the valves soaks up or blocks fuel delivery into the cylinders, and rough idles or hesitations during acceleration result.

If this method of cleaning injectors doesn’t work, there are two choices: replace the faulty injector or have it removed and cleaned off the vehicle. Off-vehicle cleaning is done on equipment costing tens of thousands of dollars, so very few repair shops have this equipment. Instead, they send out injectors requiring cleaning to a local specialist. The injectors are mounted on the machine and pulsed electrically while cleaning fluid is forced through the injector backwards. An amazing amount of dirt, rust, and water comes out of injectors cleaned using this method, but not all injectors can be cleaned.

After the cleaning process, the injectors are tested on a flow bench to verify correct spray patterns and fuel flow. Any that do not meet the standard must be replaced. I know of some local hot rodders that have the repair shop “match” the injectors. Several injectors are tested until enough are found that have the same fuel flow rates (within 5%). This helps fuel economy and performance. For passenger car applications, the injectors usually flow within a 10% of each other.

Injector cleaning doesn’t need to be done on a regular basis, but if you are experiencing stumbles, stall, hesitations, or other driveability problems with your vehicle, then it may be the gum on the injectors causing the fault.

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