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Automotive Industry

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Motor, Apr 2004 by Marinucci, Dan

Fuel injector performance evaluation needn't be a high-stakes guessing game. Dan explains how pressure drop test equipment produces repeatable, reliable test results.

Last month, I said that analyzing a secondary ignition spark line could lead you to a restricted injector. At the end of that column, I reminded you that a properly performed pressure drop test yields the final word on potentially restricted injectors. This time I'll explain that procedure and discuss the latest equipment available for it.

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I've heard many complaints that pressure drop testing is too time-consuming. This perception may change, fur several reasons. For one thing, injector access is getting worse instead of better. You can't replace some injectors without removing the intake plenum first. A prime example is the rear injectors on the Toyota Avalon shown in the photo below. For another, increasing numbers of service-age vehicles are equipped with returnless fuel systems. A returnless system doesn't route fuel through the fuel rail and then back to the tank anymore. Therefore, we're beginning to find that injectors located at the far end of the fuel rail tend to load up with dirt.

Last but not least, just wishin and hopin that replacing injectors will solve a customers complaint is an extremely expensive gamble on many vehicles. Not long ago, I saw a careless technician gamble six injectors at 250 bucks (plus labor) each on a Toyota that actually had a failing coil. Ouch!

Okay, pressure-dropping injectors may be unfamiliar to some of you, but it s definitely not anything new. I can remember doing it as far back as the late 1980s. Like today, the most common method back then involved an injector "buzz" box and an analog fuel pressure gauge. A buzz box-a hand-held injector pulsing device-is available from such companies as KentMoore, Matco Tools, OTC, Snap-on, etc. Meanwhile, the recommended fuel pressure gauge for this task is one with an accuracy of ±1%. You cannot-and should not-assume that all gauges meet or exceed this spec.

You connect the gauge to the injector rail and the buzz box to the first injector, then pressurize the rail by briefly running the engine or just the fuel pump itself. Then pulse the injector with the buzz box and record the resulting drop in fuel pressure. Pressurize the rail again and pulse the next injector. Repeat this procedure for all injectors. The rule of thumb is that all readings should be within 1.5 to 2.0 psi of each other. Little or no pressure change indicates a restricted injector.

Many technicians are still performing this basic procedure today. If an injector shows little or no pressure drop, they play it safe by repeating the test on the suspect injector. To get the most consistent results, work as quickly as practically possible. This minimizes potential inaccuracies due to changes in fuel temperature. What's more, pressure-dropping a relatively cool fuel system minimizes the risk of fuel bubbles occurring and skewing the test results.

Automated Pressure Drop Testing

Automated testing improves accuracy and consistency by minimizing or eliminating human error. I think this approach is more timely than ever, as well as easier than ever, to do. Experience contributes a lot to the validity of any test procedure, automatic or manual. The On-Car Injector Flow Bench is the latest tester from Waekon/Hickok. It boasts more features and convenience than previous designs, so lets look closer.

The Flow Bench's control unit (see photo on page 10), which has an LCD screen and soft-touch keypad, controls everything during the test, including the injectors and the fuel pump circuit. This equipment gives you three ways to connect the control unit to the injectors. The first is with a harness with individual connectors that attach to the injectors, accommodating up to 10 cylinders. The second is with a similar, direct-connection harness that does only four injectors at a time. If you're working on V12s, you'll have to test six injectors at a time.

The third possible hookup takes you directly to the injector wires at the ECM/PCM. If the vehicles computer is easy to reach, you can unplug the harness containing the injector wires. Waekon/Hickok provides three different adapters for this approach-a cable with smaller round terminals, one with larger round terminals and one with small spade terminals. These slide into the female terminals of the vehicle computer harness. On one hand, this approach takes some time and requires a wiring schematic; on the other, it does test the ground side of the injector wiring.

The operator's guide for the Flow Bench also shows several ways to control the fuel pump-through a direct connection to the pump itself, through an ECM/PCM harness, through a signal to a pump relay, through a diagnostic connector, etc.

Now you have to connect the fuel management unit (the smaller of the two tester components shown in the photo) to the fuel system. To do this, tee the unit into the fuel supply hose at the injector rail. The fuel management unit has its own return or drain hose, on the right side. Safely route this hose into an approved fuel container. (That's a gas can, son.)

Automotive Industry

Foreign Service

Motor, Apr 2004 by Marinucci, Dan

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