I had to start a few vehicles this past week that spend much of their time sitting around. They were "second" vehicles, used only occasionally for everyday transportation. Some of them started fine but ran a little rough. Others misfired during cranking and ran very rough for several seconds when started, and one would not start at all. What did they all have in common? Dirty fuel injectors.

Fuel injection has done wonders for starting engines easily. Most engines will start almost as soon as the key is turned. This happens because the injectors spray the fuel in a fine mist into the intake manifold runners so that the fuel vapourizes almost immediately. Remember, liquid fuel doesn't burn. It's the vapours that burn, and the finer the spray pattern from the injectors, the quicker it turns to vapour.

When a cylinder misfires, the oxygen in that cylinder is not used for combustion, so it goes out the exhaust pipe on the next exhaust stroke.

The oxygen sensors measure this unused oxygen and the computer reacts as if the engine is running extremely lean. Then the fuel injection computer adds fuel to all the cylinders by turning the injectors on longer. This causes even those cylinders that were running fine to run too rich, making the engine operate worse. One problem leads to more. The original problem, the dirty injectors, must be fixed first.

Our fuel is usually clean and there are filters on the vehicle's fuel system to trap dirt, so how do the injectors get dirty? The filters trap dirt from 10 to 30 microns in size - a micron being only a millionth of an inch, which means pretty small pieces of dirt. Even so, at slow engine speeds there is not a lot of fuel flowing through the injectors and some particles can become lodged at the valve in the spray nozzle tip. Sometimes, a few full-throttle acceleration runs from low speed to highway speed will flush out the dirt deposits.

Sometimes the injectors stick or become dirty because a gummy deposit has built up in the spray nozzle. When the engine is shut off, the fuel evaporates on the tip of the injector. The "high ends" evaporate first, leaving a gum residue behind. Running the engine will sometimes flush these deposits away. Usually however, a little chemical help is needed.

Many fuel companies advertise their fuel-cleaning additives. Some premium fuels do contain more cleaners than regular fuels and help keep injectors clean, but even most regular fuels contains some cleaners.

There are also many aftermarket additives for sale at any auto parts store. I have tried many of them and still don't have any favourites. They all seem to work, but follow the directions carefully. Typically, the additive manufacturers will recommend one container of cleaner to a full tank of fuel. If you add too much chemical cleaner, it may damage some of the rubber parts in the fuel system. More isn't necessarily better.

The best injector cleaning is done at service shops. To check injector condition, a technician connects a fuel pressure gauge, momentarily energizes the fuel pump, and then operates each injector. The drop in pressure for each injector is measured. If they are all close or equal, everything is good. If there is a difference, the injectors need cleaning. This is an "injector flush".

To clean the injectors, the technician will disconnect the fuel injector lines from the rest of the system at the engine and connect another tank, containing concentrated cleaning solution, to the engine. He will then operate the engine for several minutes on the concentrated cleaner solution. This usually cleans the injectors and also cleans carbon off the back side of the intake valves to allow the air-fuel mixture to enter the cylinders better.

Injectors that don't work properly after cleaning need to be replaced.