Electronic fuel injectors spray a vast quantity of fuel through a very small orifice. The injector pintle typically opens just 0.05mm off its seat and it's critical they are free of blockage. A partially blocked injector can cause poor idle, crook drivability, low power and - in extreme cases - a piston-melting lean-out at high engine load. And you don't want that...
The most effective way to ensure your car's injectors are clean and working correctly is to remove them from the vehicle and have them bench tested and ultrasonically cleaned. And here's how it's done...
Spray Pattern Testing
The spray pattern test is used to identify a poor injector discharge, which can cause poor light load engine operation through not spraying an evenly atomised mix of fuel into the intake.
A typical test rig comprises a variable electric pulse generator (to operate the injector), a fuel tank with high-pressure pump, filter and an adjustable fuel pressure regulator. The injector's fuel feed is connected to the fuelling system, while the variable pulse generator is used to control the injector opening duration.
The process involves aiming an individual injector into a container, activating the pulse generator and fuel pump and visually examining the fuel spray pattern. There are no specific standards to judge an injector, but the aim is to have even fuel distribution throughout the spray area. A partially blocked injector will show certain areas dense with fuel - these are known as fuel streaks. Or worse, in extreme cases of neglect, the injector may dribble like a garden hose.
The variable pulse generator is sometimes adjusted to fire the injector for different duty cycles, simulating the conditions the injector may operate under at say, 5000rpm and full load. This variation ensures the spray pattern is constant across different operating ranges.
In some workshops, fuel distribution is determined by moving a high intensity light beam through the injector spray. The amount of light that passes through to the other side gives a guide to fuel density.
If the spray pattern test reveals uneven fuel distribution, the injector is ultrasonic cleaned and its spray pattern is then retested.
Fuel Flow Testing
There are several ways to test injector flow. One method uses a test machine and there's a more involved technique where fuel flow is measured by hand. Injector flow tests are performed at a duty cycle of 85 percent - similar to when an engine is running at maximum power.
The test machine is usually the preferred method of flow testing for workshops doing routine work since it's quicker and easier. All that's needed is injector connection to the fuel system and pulse generator, and plugging the injector nozzle into the test machine. The test machine displays the quantity of fuel flow in real-time, but it is limited to a fairly small flow volume - the measuring scale goes up to only 180cc per minute. Our demo MY94 Subaru WRX injectors, however, flowed too much for the test machine to cope with so we had to take the other approach...
The hands-on method of injector flowing is a lot more time consuming. Each individual injector is hooked up to the aforesaid variable pulse generator and fuel supply system, but - this time - the injector is aimed into a measuring beaker. Operating at 85 percent duty cycle, the injector is fired into the beaker for a hand-timed 15-seconds and, afterward, the fuel quantity is measured. This process is performed three times per injector so that an accurate average figure can be attained - there may, however, be a slight inaccuracy due the rising fuel mist that doesn't get caught by the beaker.
The flow of our WRX injectors came up almost perfectly even - injector numbers 1 and 4 flowed 83.3 millilitres, injector number 3 came in at 83.6 and injector number 2 was an even 83.0. That's less than a one percent spread.
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Side Feed Hassles...
Unlike most conventional injectors that have a fuel feed through the top, Subaru EJ20 turbo injectors are fed fuel from around their circumference - these type of injectors are known as side feed.
The dilemma associated with side feed injectors is they're more difficult to bench test - you can't simply push a fuel hose over the top. Instead, the adjoining fuel rail (which surrounds the side feed ports) is required so that the test rig hoses can be pushed on. Often, the fuel rail also incorporates the car's fuel pressure regulator, which negates the use of the adjustable unit used in the test rig. As seen here, the Subaru flat-four's fuel rail is quite intricate - and it's a bugger to remove from the engine...
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Injector Cleaning
If your injectors are blocked the best way to clean them is in an ultrasonic bath. This device removes any injector build up by process of sonic vibrations and an alkaline solution. Injectors usually sit in the ultrasonic cleaner for around an hour. Similar technology is used to clean expensive and ornate jewellery. (And to smash kidney stones and other foreign objects in the human body!)
Note that our side feed injectors first had to be removed from their rail. Once cleaned, they were then retested for their fuel spray pattern and flow...
Results
The spray pattern test showed perhaps a very slight visual improvement, while - likewise - fuel flow was improved oh-so slightly. The average fuel flow for each injector rose from 83 to 84 millilitres per hand-timed 15-seconds - an improvement of just over one percent.
These almost imperceptible gains in this instance beg one general question - is injector cleaning worthwhile?
A Waste of Time?
If your car starts and runs, the likelihood of its injectors being clogged is quite slim - though there are instances where there's an increased possibility. High performance cars can benefit from having the injectors hand-selected so that each cylinder's injector flows just the same as the next, ensuring near-as-dammit fuel mixtures for each cylinder. With only one oxygen sensor (or per bank) and group-fire injection there's little chance of the engine management system being able to 'tune' the duty cycle of an individual injector to ensure a proper air/fuel ratio in its cylinder (although some current cars are able to do this by 'sniffing' individual cylinders' exhausts.)
We're told that - as a whole - the quality and cleanliness of fuel in Australia is very good. Most fuels incorporate a detergent additive that helps keep injectors clean and, furthermore, original equipment fuel filters are very capable - as long as they are changed at the recommended intervals. Clogged filters may not simply block fuel flow, but allow fine contaminants through, forming a sludge in fuel lines and eventually blocking and/or wearing the injector pintle.
The potential for injector blockage is higher for cars that have sediment inside the fuel tank and/or use a cheapie fuel filter. And we've all heard of 'bad' batches of fuel (sometimes in the city, sometimes in rural areas) that do injector damage. Cars fitted with LP gas are also prone to blocked injectors, since many people seldom run the car on conventional fuel. The result is the fuel goes stale and gums within the fuel lines, pumps and injectors. If you run a dual-fuel car, it's wise to flush and maintain the complete petrol system by filling the petrol tank and running the car on it once every few months.
The use of injector cleaner fuel additives may help in some instances, but we're told their effectiveness is severely limited by the chemicals that a fuel pump can handle. However, the marketers of these products claim that using a bottle of in-tank injector cleaner every few thousand kilometres - for instance, around the same frequency that your oil is changed - can lead to far better driveability by helping prevent gunk build up in the fuel lines and injectors.
Our recommendation is to take the plunge for an ultrasonic clean at 100,000 kilometre intervals, or before you undertake any significant power-up modifications to your car. Cars used significantly for stop/start driving may benefit from cleaning more often. Do you know what driving your 'new' second-hand car was subject to before you bought it?
Although these WRX injectors revealed a barely worthwhile improvement, we're told up 10 percent extra fuel flow has been seen after cleaning a significantly clogged injector. A clogged injector can sometimes be a great hindrance to driveability, especially at light loads.
How much is an injector test and clean?
Garden-variety top feed injectors found in many cars can be done for $25 each, but the troublesome side feed cousins - such as these - usually cost a little more.
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Good For Experimenting...
While our WRX's injectors were out, we thought it'd be the ideal opportunity to see how upping the fuel pressure affected fuel flow and spray pattern.
We by-passed the factory fuel pressure regulator - which maintained 3 Bar rail pressure when under atmospheric pressure - and used the test rig's adjustable regulator. With rail pressure increased to 4 Bar, the injector spray pattern widened considerably and fuel flow went up from 84 to 100 millilitres per hand-timed 15 seconds - a nineteen percent gain (with the regulator under only atmospheric pressure, of course).
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