The importance of compression testing when changing injectors

Loss of power, missing on start up, poor running, increased fuel consumption, exhaust smoke…. the problems sounds familiar.  The solution – change your fuel injector.

Most people change injectors when the idle feedback values are outside the normal spec, but not a lot of people understand exactly how those numbers are derived. More often, the change in values and symptoms that many attribute to dirty or worn injectors may be related to issues other than injector deposits.

Looking beyond injector replacements

As the fuel injectors play a very important role in the functioning of a vehicle – they deliver fuel to the car’s engine – the idea of identifying problems with a fuel injector comes natural to many vehicle owners. Most car owners may not realize that they should look more deeply into other components, which can save them large sums of money by not needing to bring their cars to service shops for injector replacements.

EGR contamination

While the diagnostic capability of contemporary engines is vastly improved over early models, owners should pay attention when a fault appears. An increase in the number of engines with head or valve seat values – primarily due to the EGR contamination in the intake manifold – has been found.

Rotational Speed

There is a unique relationship between the engine rotation speed and the injection quantity. In other words, the injector feedback values are generated based on the rotational speed of the engine. The ECU keeps a watch of the rotational speed of the engine, the acceleration profile and the individual cylinders combustion. If there is a difference in acceleration profile, be it over or under, then the ECU will correct the value by slightly adjusting the fuel on the cylinder. Correcting the deviation of the injection quantity based on a correction value used in feedback control of rotation speed has been also employed and that adjustment is what we are looking at with the idle feedback values. The adjustment values are quite useful because in injectors, issues or problems or delays are going to be much more significant when there is a lower rail pressure, and therefore lower hydraulic effort or actuation assistance coming from the hydraulics circuit to the injectors.

Compression test

It is important to perform a compression test every time a tune-up is done as part of preventive maintenance and in any instance when the engine is running roughly or is lacking power. If a cylinder is down on compression, which is a serious issue, and the ECU is in the compression stroke, the rotational speed of the engine will not slow down as much as it should, and therefore does not require as much fuel on the combustion stroke to maintain the same peak speed. The cylinder with a lower compression will measure a minus value, say for example, the leaking valve compression test might be 480 on a good cylinder and might be 333 to 350 for a dead one; it wouldn’t be barely noticeable in the vehicle. But this would record the result of around -1.8/- 2.0 give or take on the injector feedback values.

Conclusion

To sum it up, if you changing injectors because of feedback values, make sure that you are doing a compression test and an intake manifold clean to combat EGR build up. By performing a compression test, internal engine malfunctions, such as bad valves, valve seats, piston rings or excessive carbon build up can be detected before they cause irreparable damage. It benefits the owner to be aware of these problems so they can make an informed decision whether to invest in repairs or sell the vehicle.

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