SECOND GENERATION TOYOTA AIRFLOW SENSOR
Starting in the mid-1990s, Toyota introduced a second-generation airflow sensor that combines the functions of the airflow meter and air temperature sensor into one unit. The new mass airflow sensor uses a hot wire to measure air mass rather than volume and has no moving parts. A reference voltage is applied to a thin wire inside the sensor that heats it to about 100 degrees C hotter than ambient air temperature. As air flows through the sensor and past the hot wire, it carries away heat and cools the wire. The electrical control circuit for the wire is designed to maintain a constant temperature differential, so the amount of extra voltage that's required to offset the cooling effect and keep the wire hot tells the control box how much air is entering the engine.
With both the early and late style airflow sensors, vacuum leaks can cause drivability problems by allowing unmetered air to enter the engine. Air leaks around the throttle body, injector O-rings, intake manifold gaskets or vacuum hose connections can cause the air/fuel ratio to go lean. So if you find a code 25 (lean air/fuel ratio), start looking for leaks.
Finding an air leak can be a time-consuming exercise in patience. One method is to use a propane bottle and hose to check out suspicious areas. When propane vapor is siphoned in through a leak, the idle will smooth out and the rpm will change. Another trick is to turn off the engine and lightly pressurize (no more than 5 PSI max) the intake manifold with compressed air. Then use a hand bottle to spray soapy water at possible leak points. Bubbles would indicate a leak. Another technique is to use a device that fills the intake manifold with smoke to reveal leaks.
Another often overlooked cause of air leakage is the EGR valve. If the valve sticks open, it will act much like a vacuum leak causing lean misfire at idle and hesitation problems.
TOYOTA FUEL INJECTION CIRCUIT
Fuel flows from a tank-mounted pump through the fuel line to an inline filter usually located in the engine compartment. It then goes to a common fuel rail (which Toyota calls the "fuel delivery pipe") on the engine to supply the injectors. The fuel injectors plug into the rail and are removed as an assembly with the rail. On V6 applications, there's a separate rail for each cylinder bank. Unfortunately, Toyota doesn't include a test valve on the fuel rail for checking fuel pressure. To perform a pressure check, you have to disconnect the cold start injector fuel fitting and attach a pressure gauge.
The pressure regulator is mounted on the end of the fuel rail, and maintains pressure at a constant level as engine load and intake vacuum change. A vacuum hose connected the regulator to the intake manifold so the diaphragm inside can react to changes in intake vacuum. A bypass valve inside the regulator routes excess fuel through a return line back to the fuel tank.
toyota fuel pressure regulator
Toyota uses many different fuel pressure regulators so make sure you get the correct replacement.
System operating pressure varies depending on the application, but is typically from 30 to 37 PSI with the vacuum hose connected to the regulator, and 38 to 44 PSI with the hose disconnected and plugged.
NOTE: If you're replacing a regulator on a turbocharged engine, make sure you get the correct replacement because the regulator on these applications is calibrated differently from those on nonturbo motors.
Also, do not confuse the pressure regulator with a little round plastic gizmo that may be mounted on the end of the fuel rail. This is a pulse damper that helps dampen noise and resonance caused by the pulsing of the injectors.
Starting in 1996, some Toyota EFI systems switchd to a returnless EFI system. The regulator on the returnless EFI systems is located in the fuel tank with the pump.
TOYOTA FUEL INJECTION PRESSURE PROBLEMS
If fuel pressure reads low, or the engine seems to starve for fuel under load, don't overlook the fuel pickup filter inside the fuel tank as a possible cause. In many instances, the system may flow enough fuel at idle to develop normal pressure, but run out of fuel at higher speeds or loads. Rust, dirt and scum inside the tank may be blocking the flow of fuel into the pump. Likewise, accumulated dirt and debris may be clogging the inline filter.
Toyota says the best method for confirming a suspected fuel starvation problem is to road test the vehicle with a fuel pressure gauge safely installed on the engine. If the pressure reading drops when the engine is under load, it means the system isn't maintaining normal pressure. But is it the pump, filter or what?
You can rule out the pressure regulator if the system maintains normal pressure at idle, and the pressure rises when you disconnect the regulator's vacuum hose. No change in pressure would indicate a defective regulator or plugged vacuum line.
A good way to check out the pump, pickup filter and inline filter is to measure fuel delivery volume. Relieve system pressure, then disconnect the fuel supply line at the fuel filter or fuel rail, or disconnect the return hose from the rail. Place the open end of the fuel hose in a measuring cup or graduated cylinder. If you're disconnecting the return hose, you'll have to attach another piece of hose to the fuel rail and use that to route fuel into the container. With the engine off, use jumpers to bypass the pump relay. Energize the pump for 30 seconds and measure the volume of fuel delivered.
As a rule, a good pump should deliver about one quart of fuel in 30 seconds.
If a pump's output volume and/or pressure is low, the pump motor might be running slow due to internal wear. A typical fuel pump runs at 5,000 to 6,000 rpm and pulls about 3 to 6 amps. But as the armature brushes become worn and the brush springs weaken, increased resistance will reduce the pump's current draw and cause the motor to run slower causing it to deliver less fuel.
The pump motor can be checked using an ohmmeter to measure the motor's internal resistance. As a rule, most pumps should read 2 to 50 ohms if good. If the pump is open (reads infinity) or shows zero resistance (shorted), the motor is bad and the pump needs to be replaced.
Even if the pump motor is okay, fuel delivery problems can be caused by the pump's voltage supply. Low battery voltage, low system operating voltage, a poor ground connection or excessive resistance in the pump's wiring connectors or the relay can all have an adverse effect on the operating speed of the pump. The pump must have normal voltage to run at full speed, so always check the pump's wiring connectors and voltage supply when you encounter a pump with low pressure or volume output.
The pump's supply voltage should be within half a volt of normal battery voltage. If low, check the wiring connectors, relay and ground. A good connection should have less than a tenth of a volt drop (ideally no voltage drop) across it. A voltage drop of more than 0.4 volts can create enough resistance to cause a problem.
RESIDUAL FUEL PRESSURE
If an engine is hard to start when hot, fuel may be boiling in the rail because the system isn't holding residual pressure when the ignition is shut off. To prevent vapor lock and reduce the cranking time when restarting the engine, a check valve inside the fuel pump holds the pressure in the line. Toyota says pressure should remain above 21 psi for five minutes after the engine is turned off. If the system fails to hold pressure, either the check valve or pressure regulator is leaking, or an injector is leaking. Regulator leaks can be ruled out by pinching off the return line. Injector leaks can be checked by removing the fuel injector and rail assembly from the manifold, and pressurizing the rail. No fuel drips? Then it's the pump check valve.
TOYOTA FUEL INJECTORS
Four different types of injectors may be used in Toyota engines: pintle style, hole type (cone valve and ball valve), high resistance and low resistance. Bosch pintle style injectors are used on the older TCCS applications, while Nippondenso hole type injectors are used on newer engines. The hole type injectors spray fuel through holes drilled in a director plate at the injector tip. There are currently three different types including side-feed injectors used on the 3S-GTE and 2TZ-FE engines.
Starting in the mid-1990s, Toyota introduced a second-generation airflow sensor that combines the functions of the airflow meter and air temperature sensor into one unit. The new mass airflow sensor uses a hot wire to measure air mass rather than volume and has no moving parts. A reference voltage is applied to a thin wire inside the sensor that heats it to about 100 degrees C hotter than ambient air temperature. As air flows through the sensor and past the hot wire, it carries away heat and cools the wire. The electrical control circuit for the wire is designed to maintain a constant temperature differential, so the amount of extra voltage that's required to offset the cooling effect and keep the wire hot tells the control box how much air is entering the engine.
With both the early and late style airflow sensors, vacuum leaks can cause drivability problems by allowing unmetered air to enter the engine. Air leaks around the throttle body, injector O-rings, intake manifold gaskets or vacuum hose connections can cause the air/fuel ratio to go lean. So if you find a code 25 (lean air/fuel ratio), start looking for leaks.
Finding an air leak can be a time-consuming exercise in patience. One method is to use a propane bottle and hose to check out suspicious areas. When propane vapor is siphoned in through a leak, the idle will smooth out and the rpm will change. Another trick is to turn off the engine and lightly pressurize (no more than 5 PSI max) the intake manifold with compressed air. Then use a hand bottle to spray soapy water at possible leak points. Bubbles would indicate a leak. Another technique is to use a device that fills the intake manifold with smoke to reveal leaks.
Another often overlooked cause of air leakage is the EGR valve. If the valve sticks open, it will act much like a vacuum leak causing lean misfire at idle and hesitation problems.
TOYOTA FUEL INJECTION CIRCUIT
Fuel flows from a tank-mounted pump through the fuel line to an inline filter usually located in the engine compartment. It then goes to a common fuel rail (which Toyota calls the "fuel delivery pipe") on the engine to supply the injectors. The fuel injectors plug into the rail and are removed as an assembly with the rail. On V6 applications, there's a separate rail for each cylinder bank. Unfortunately, Toyota doesn't include a test valve on the fuel rail for checking fuel pressure. To perform a pressure check, you have to disconnect the cold start injector fuel fitting and attach a pressure gauge.
The pressure regulator is mounted on the end of the fuel rail, and maintains pressure at a constant level as engine load and intake vacuum change. A vacuum hose connected the regulator to the intake manifold so the diaphragm inside can react to changes in intake vacuum. A bypass valve inside the regulator routes excess fuel through a return line back to the fuel tank.
toyota fuel pressure regulator
Toyota uses many different fuel pressure regulators so make sure you get the correct replacement.
System operating pressure varies depending on the application, but is typically from 30 to 37 PSI with the vacuum hose connected to the regulator, and 38 to 44 PSI with the hose disconnected and plugged.
NOTE: If you're replacing a regulator on a turbocharged engine, make sure you get the correct replacement because the regulator on these applications is calibrated differently from those on nonturbo motors.
Also, do not confuse the pressure regulator with a little round plastic gizmo that may be mounted on the end of the fuel rail. This is a pulse damper that helps dampen noise and resonance caused by the pulsing of the injectors.
Starting in 1996, some Toyota EFI systems switchd to a returnless EFI system. The regulator on the returnless EFI systems is located in the fuel tank with the pump.
TOYOTA FUEL INJECTION PRESSURE PROBLEMS
If fuel pressure reads low, or the engine seems to starve for fuel under load, don't overlook the fuel pickup filter inside the fuel tank as a possible cause. In many instances, the system may flow enough fuel at idle to develop normal pressure, but run out of fuel at higher speeds or loads. Rust, dirt and scum inside the tank may be blocking the flow of fuel into the pump. Likewise, accumulated dirt and debris may be clogging the inline filter.
Toyota says the best method for confirming a suspected fuel starvation problem is to road test the vehicle with a fuel pressure gauge safely installed on the engine. If the pressure reading drops when the engine is under load, it means the system isn't maintaining normal pressure. But is it the pump, filter or what?
You can rule out the pressure regulator if the system maintains normal pressure at idle, and the pressure rises when you disconnect the regulator's vacuum hose. No change in pressure would indicate a defective regulator or plugged vacuum line.
A good way to check out the pump, pickup filter and inline filter is to measure fuel delivery volume. Relieve system pressure, then disconnect the fuel supply line at the fuel filter or fuel rail, or disconnect the return hose from the rail. Place the open end of the fuel hose in a measuring cup or graduated cylinder. If you're disconnecting the return hose, you'll have to attach another piece of hose to the fuel rail and use that to route fuel into the container. With the engine off, use jumpers to bypass the pump relay. Energize the pump for 30 seconds and measure the volume of fuel delivered.
As a rule, a good pump should deliver about one quart of fuel in 30 seconds.
If a pump's output volume and/or pressure is low, the pump motor might be running slow due to internal wear. A typical fuel pump runs at 5,000 to 6,000 rpm and pulls about 3 to 6 amps. But as the armature brushes become worn and the brush springs weaken, increased resistance will reduce the pump's current draw and cause the motor to run slower causing it to deliver less fuel.
The pump motor can be checked using an ohmmeter to measure the motor's internal resistance. As a rule, most pumps should read 2 to 50 ohms if good. If the pump is open (reads infinity) or shows zero resistance (shorted), the motor is bad and the pump needs to be replaced.
Even if the pump motor is okay, fuel delivery problems can be caused by the pump's voltage supply. Low battery voltage, low system operating voltage, a poor ground connection or excessive resistance in the pump's wiring connectors or the relay can all have an adverse effect on the operating speed of the pump. The pump must have normal voltage to run at full speed, so always check the pump's wiring connectors and voltage supply when you encounter a pump with low pressure or volume output.
The pump's supply voltage should be within half a volt of normal battery voltage. If low, check the wiring connectors, relay and ground. A good connection should have less than a tenth of a volt drop (ideally no voltage drop) across it. A voltage drop of more than 0.4 volts can create enough resistance to cause a problem.
RESIDUAL FUEL PRESSURE
If an engine is hard to start when hot, fuel may be boiling in the rail because the system isn't holding residual pressure when the ignition is shut off. To prevent vapor lock and reduce the cranking time when restarting the engine, a check valve inside the fuel pump holds the pressure in the line. Toyota says pressure should remain above 21 psi for five minutes after the engine is turned off. If the system fails to hold pressure, either the check valve or pressure regulator is leaking, or an injector is leaking. Regulator leaks can be ruled out by pinching off the return line. Injector leaks can be checked by removing the fuel injector and rail assembly from the manifold, and pressurizing the rail. No fuel drips? Then it's the pump check valve.
TOYOTA FUEL INJECTORS
Four different types of injectors may be used in Toyota engines: pintle style, hole type (cone valve and ball valve), high resistance and low resistance. Bosch pintle style injectors are used on the older TCCS applications, while Nippondenso hole type injectors are used on newer engines. The hole type injectors spray fuel through holes drilled in a director plate at the injector tip. There are currently three different types including side-feed injectors used on the 3S-GTE and 2TZ-FE engines.
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