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Home Engine Tech Tip: Diagnosing Fuel Pumps and Injectors

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By Larry Carley
Technical Editor

One of the first questions that should always be answered when diagnosing a fuel-related complaint on a fuel-injected engine is, “What is the fuel pressure?”

All too often, technicians assume fuel pressure is “good” without actually measuring it with a gauge. If the engine runs, they assume the injectors are getting adequate fuel pressure. If the engine cranks but won’t start, and they depress the service valve on the fuel rail and some fuel squirts out, they assume the injectors have pressure. They do, but the question remains, “How much pressure?”

For the engine to start and run smoothly with no stalling, hesitation or misfiring, the injectors have to deliver the proper amount of fuel with every squirt. This is especially important on late-model engines with sequential fuel injection. One bad injector will cause a noticeable misfire and usually set a P030X misfire code (where X represents the cylinder that is misfiring). On older engines where the injectors are all fired simultaneously, the good injectors can often compensate for one or two bad injectors. Even so, for the engine to run right, fuel pressure to the injectors is critical as is the volume of fuel delivered by each injector when it fires.

The fuel calibration curves in the Powertrain Control Module (PCM) are based on OEM dyno testing using a new engine. Fuel pressure is within a specified range for that engine, and the injectors are all clean and new. The adaptive fuel control strategies that are built into a PCM that allow it to adjust short-term and long-term fuel trim to compensate for variances in fuel pressure and fuel delivery can maintain the correct air/fuel ratio — but only within certain limits.

If an injector becomes clogged with fuel varnish deposits and fails to deliver its normal dose of fuel when it’s energized, or fuel pressure to the injector drops below specifications because of a weak fuel pump, plugged fuel filter or leaky fuel pressure regulator, the PCM may not be able to increase injector duration enough to offset the difference. This can leave the air/fuel mixture too lean, causing the cylinder to misfire.

IT’S ALL ABOUT FUEL PRESSURE
According to Jim Linder of Linder Technical Services in Indianapolis (a provider of fuel injection training), fuel pressure is probably the most critical factor in the fuel injection system. Linder says that only a 1 to 3 psi change in fuel pressure can often cause noticeable driveability problems.

He says the first thing technicians should always check when confronted with a driveability or emissions problem is fuel pressure. Look up the fuel pressure specifications for the vehicle, hook up a gauge and check the pressure with the key on, engine off, then again with the engine running. If pressure is not within specifications, there’s a problem that will require further diagnosis.

On certain Jaguar engines, for example, the factory spec calls for 37 psi of fuel pressure. If you see 36 psi or 38 psi, you need to replace the fuel pressure regulator.

It’s also important to check the fuel pressure with the key on, engine off (KOEO), then again with the engine idling. The idle pressure should generally be 6 to 10 psi less than the KOEO pressure reading. This confirms the pressure regulator is working.

FUEL VOLUME IS JUST AS IMPORTANT
The volume of fuel delivered by the fuel pump to the injectors is also critical. Some pumps may develop adequate fuel pressure when the engine is at idle or running at low speed, but the pump isn’t spinning fast enough to keep up with the engine’s fuel requirements at higher speeds. This causes the fuel mixture to lean out and the engine to misfire or lose power.

The old rule of thumb that says a “good” fuel pump will flow about a pint of fuel in 15 seconds (half a gallon per minute) still holds true, but some engines need more than this. So the fuel delivery specifications also need to be looked up to see if the pump is delivering an adequate supply of fuel to the engine.

A fuel flow meter is the most accurate means of measuring fuel delivery. The floating ball on the meter shows the fuel flow in gallons per minute (gpm). A “good” reading would typically be 0.3 to 0.7 gpm.

The flow meter can be hooked up to the supply line that runs to the fuel rail to measure flow. But Linder says a better method for checking fuel flow and pump capacity is to hook up the flow meter to the return line that runs from the fuel pressure regulator back to the fuel tank. Then check the return flow at idle, 2,500 rpm and 5,000 rpm.

The volume of fuel flowing through the return line will drop as engine speed increases because more fuel is flowing through the injectors. Even so, the return flow for a good fuel pump with adequate pumping capacity at 5,000 rpm should still be about half the volume it had at idle (say 0.23 gpm versus 0.46 gpm). If the return flow at 5,000 rpm drops to 10% or less of the idle return flow rate, the fuel pump probably does not have enough reserve capacity to keep up with the engine when the engine is under load. The weak pump will starve the engine for fuel, causing it to misfire and lose power.

The performance of the fuel pump can also be checked with an inductive pickup and digital storage oscilloscope. Attaching the pickup to the fuel pump power supply wire at the fuel pump relay can show you pump speed and the condition of the commutator. If the pattern has missing humps, it would tell you the commutator is worn.

A fuel pump will run slower than normal if the fuel filter is restricting flow, if there is a voltage drop in the pump power supply circuit or if the pump itself is worn.

One thing you should never do is run a dry electric fuel pump to test it. The fuel pump requires fuel for lubrication, and running it dry may damage it. That’s why running out of gas is a common cause of fuel pump failure.

GET THE RIGHT REPLACEMENT PUMP
Many part suppliers list flow rates for their fuel pumps. But the rates don’t necessarily correspond to the actual fuel flow rates on a vehicle because the pumps are rated by pumping fuel into a container. There is no fuel filter or fuel pressure regulator in the system to create resistance to flow. Consequently, a pump with a catalog rating of 0.6 gpm may only flow 0.5 gpm when installed on a vehicle.

What’s more, some parts suppliers have over-consolidated their fuel pump lines to reduce the number of SKUs needed to provide broad market coverage. Pump capacities can always be higher than specifications, but should never be lower. If you get a pump that is flow rated at 0.4 gpm and you install it in a vehicle that requires 0.5 or 0.6 gpm, the pump may supply enough fuel at idle and low rpm, but may starve the engine at higher loads and speeds. Yet it’s not a “bad” pump — just an under-rated pump for the application.

Another trend to be aware of is that most late-model fuel pumps now use a turbine design. Turbine pumps are quieter, smoother (no pressure pulsations) and draw less current. Some aftermarket fuel pump suppliers now use the newer-style turbine pumps as replacements for the older gerotor and roller cell fuel pumps.

If the fuel pump does have to be replaced, always replace the fuel filter in the fuel line, and the pump filter sock inside the tank. Also, inspect the inside of the tank for debris or rust before installing the new pump. If the tank is dirty, steam clean the inside and dry it before installing the pump. If the inside of the tank is heavily rusted, rust will continue to flake loose and will likely ruin the new pump. Replace the tank with a new one.

FUEL PRESSURE REGULATOR PROBLEMS
The fuel injectors can’t flow normally if they have low fuel pressure due to a bad fuel pressure regulator. If the spring inside the regulator has weakened with age, if the valve or diaphragm that controls return flow is leaking, or the vacuum supply hose to the regulator is leaking, loose or plugged, it will affect fuel pressure in the fuel supply rail.

If fuel pressure is low, disconnect the vacuum hose to the regulator. You should see an increase in pressure if the regulator is not leaking. No change would indicate a bad regulator.

If fuel pressure is less than specifications, the regulator may be leaking. You can pinch or block the return line temporarily to see if pressure goes up. If it does, it means the regulator needs to be replaced. If there is no change, the problem is a weak fuel pump or a restriction in the fuel line such as a plugged fuel filter.

Also, check the vacuum hose to the regulator for the presence of fuel inside the hose (there should be none). Fuel inside the vacuum hose means the diaphragm inside the regulator is leaking and the regulator needs to be replaced.

DIRTY INJECTORS
Another common problem with fuel injectors is the buildup of fuel varnish deposits in the nozzle that restrict fuel flow or disrupt the injector’s spray pattern. On many late-model engines, the shape and direction of the spray pattern is critical for clean combustion and good performance. If the injector nozzle is dirty, the pattern may be distorted or deflected to one side, causing a lean spot in the combustion chamber that can cause misfire, or even preignition or detonation.

It doesn’t take much of a restriction in an injector to lean out the fuel mixture. Only an 8% to 10% restriction in a single fuel injector can be enough to upset the air/fuel mixture and cause a misfire.

Gasoline contains waxy compounds that can leave varnish deposits in the injectors when the fuel evaporates. These deposits tend to form after the engine is shut off. Heat from the engine causes residual fuel in the injector tips to evaporate.

Gasoline is supposed to contain enough detergent to prevent these deposits from sticking and accumulating in the injectors. But guess what? Not all gasoline is the same. Some brands contain much lower levels of detergent than others. Consequently, filling up with the cheapest gas one can find may not be the best idea in the long run — especially for short trip, stop-and-go city driving that causes deposits to form at a much faster rate. To counter this, a growing number of gasoline retailers (Chevron, Conoco, Kwik Trip, Shell, Texaco, 76 and others) now comply with “Top Tier” standards that call for higher levels of detergent to keep injectors clean.

On four-cylinder engines, the #2 and #3 injectors are in the hottest location and tend to clog up faster than the end injectors on cylinders #1 and #4. The same applies to the injectors in the middle cylinders in six- and eight-cylinder engines. The hotter the location, the more vulnerable the injector is to clogging from heat soak.

The cure for dirty injectors is to clean them (on-car or off-car with special injector cleaning machine), or to replace them if cleaning fails to restore a normal flow rate and nozzle pattern.

FUEL INJECTOR ELECTRICAL CHECKS
The solenoid at the top of the injector creates a magnetic field that pulls the injector pintle up when the injector is energized. The magnetic field must be strong enough to overcome the spring pressure and fuel pressure above the pintle, otherwise the injector may not open all the way or not open at all. Shorts, opens or excessive resistance in the injector solenoid can also cause problems.

Typically, when injectors fail, the solenoids often short internally causing a drop in resistance. If the specification calls for 3 ohms, for example, and an injector measures only 1 ohm, it will pull more current than the other injectors. Too much current flow to an injector may cause the PCM injector driver circuit to shut down, killing any other injectors that also share that same driver circuit.

One way to check the injectors is with an ohmmeter (key off). Disconnect the wiring connector from each injector, and measure the resistance between the injector’s terminals. Look up the specifications, rather than guess. Some specs may call for 2 to 3 ohms of resistance (typical for “peak and hold” injectors), while others require 12 to 16 ohms of resistance (“high-resistance” injectors). The specs are fairly narrow, and with good reason.

So if the factory specifications call for 12 to 16 ohms of resistance, and you find several injectors that are only a few ohms higher or lower, the injectors should probably be replaced. And if the injector resistance readings are significantly higher or lower than specifications, there’s no question they need to be replaced.

Another method for finding weak injectors if you don’t have specs is to measure and compare the resistance of all the injectors. If you find one or two that are noticeably lower or different than the others, they probably need to be replaced.

INJECTOR SCOPE CHECKS
If you have an oscilloscope with a low-amp probe, you can also observe the current flow through the injectors with the engine running. You don’t have to unplug anything. Just clamp the amp probe around one of the injector connector wires.
When the PCM energizes the injector, current starts to flow through the circuit. This causes the waveform on the scope to ramp up. When current reaches about 70% of maximum, the injector usually opens, creating a bump in the pattern. When the PCM opens the ground circuit to turn off the injector, the pattern drops back to zero.

On engines that have low-resistance peak-and-hold style injectors, the scope will typically show a pattern with a sharp peak that drops to a plateau until the injector turns off, then it spikes again (two peaks total in the pattern). The peak is typically at 4 amps and the hold (plateau portion of the pattern) is at 1 amp.

On high-resistance injectors, a shorted injector that fails to open won’t produce a bump in the pattern. And if you see a sharp vertical rise in the current pattern, it means the injector is bad.

A shorted injector can sometimes pull down the PCM driver circuit, preventing other injectors from firing depending on how the PCM driver circuits are configured.

On most vehicles, the injectors receive battery voltage when the ignition is on, and the PCM driver circuit provides the ground connection to turn the injectors on and off. Therefore, if you have a dead injector, one of the first things to check would be voltage at the injector terminal. If it’s less than battery voltage, there may be high resistance in the connector or wiring harness. If more than one injector is getting low voltage, the fault may be a bad injector power supply relay.

When the PCM energizes (grounds) the injector circuit, the voltage reading on the supply side should drop to zero as long as the injector is energized. This verifies the PCM ground drive circuit is working and that current is flowing through the injector.

When the PCM opens the injector circuit, it creates a momentary voltage spike, which can be seen on an oscilloscope if you hook up the scope to the injector circuit. When the injector pintle closes, it creates a little bump in the scope pattern, which should be consistent from one pulse to the next. If the scope shows multiple bumps or the pattern is changing, it means the injector pintle is sticking or the injector is dirty.

FLOW MATCHING INJECTORS
If you have an injector cleaning machine with graduated cylinders, you can flow match injectors for optimum performance. New OE injectors may show as much as 4 to 5% from one another. Once flow rates start to vary more than about 5%, you can get noticeable driveability problems on many late-model engines.

For best performance, most experts recommend flow matching all of the injectors to within 2% of each other.

If injectors need to be replaced, you can install new or remanufactured injectors. Reman injectors are used injectors that have been cleaned and tested to make sure they meet OEM specifications. Reman injectors typically cost much less than new injectors, but may not have the longevity of new injectors.

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