How many of us have received some promotion in the mail, newspaper or otherwise about a fuel injection service that we can’t live without? There seems to be no rhyme or reason on the price or recommended mileage intervals. Look in your Subaru factory warranty manual and you’ll find it is not even mentioned in the required maintenance.

First let us look at a fuel injector. A fuel injector is a device (most Subarus have 1 per cylinder) that sprays fuel into the engine’s combustion chamber. This fuel mixes with the air that is drawn into the combustion chamber and is then ignited by the spark plug. The mixture between air and fuel must be an exact ratio or engine power and performance can be greatly affected. Today’s engines are so finely tuned that even the slightest variance from the optimum fuel air mixture can cause low power, rough idle, poor fuel economy and in severe cases, engine damage.

Over time, a buildup of fuel deposits can cause partial blockage of the fuel injector. Also over time there is a build up of carbon on the back of the intake valve and in the combustion chamber. This buildup can cause uneven combustion to occur. The carbon buildup on the back of the valves can act like a sponge soaking up fuel before it makes it to the combustion chamber. This starves the engine for fuel and can cause hesitation on acceleration to occur. The carbon buildup on the top of the piston can cause an increase in compression. This can cause pinging or, on newer vehicles, can cause the computer to retard the timing to compensate for the pinging (thus less power and worse fuel economy). Carbon is also a gritty substance that can accelerate engine wear as it flakes off and is circulated in the engine oil.

Fuel injection cleaning can eliminate many of these problems before the buildup becomes extreme. In my experience, some Subarus experience greater results than others. Subaru SPI (Single injector) engines notice moderate improvement whereas a Legacy or Impreza or Forester with MPFI (Multi point fuel injectors) and at least 60000 miles on the odometer will notice a significant improvement in power and off the line acceleration.

On a Subaru it is also important to have the throttle plate cleaned periodically due to carbon buildup. This insures the proper signal from the throttle position sensor.

The service procedure varies greatly in the industry, (unfortunately) but to get the most for your money you should make sure at least the following is included in your service.

• Throttle Housing Cleaning
• Inductive Intake manifold cleaning (sprays detergent down intake manifold while vehicle is running to clean deposits in intake manifold)
• Fuel injector cleaning that involves a running a fuel detergent through your injectors for 15 minutes with the engine off (to clean the injector rails, screens and fuel pressure regulator) followed by 30 minutes while the vehicle is running. (This cleans the injector, carbon on the valves, and carbon on the piston.)
• Reset the ECU. The reason for this is to clear all the learned behavior from the engine control unit computer. Since it has probably been compensating for carbon buildup, you’ll want to reset the computer’s self-learning cycle once the engine is cleaned out.

Fuel Injection Cleaning Systems
Although there are many different brands of F.I cleaning systems out there I have seen good success with MOTORVAC by Sun and also by BG. Although the Motorvac system is probably more thorough, the BG cleaning system gets results also. At our shop we use both a Motorvac machine complimented by some of the BG chemical procedures.

Doing this procedure at least once every 60000 miles should keep the motor feeling fresh and keep the carbon deposits from harming your engine.