The multiport fuel injection system Toyota has used from the early 1980s to the 1990s on its various engine families is based on the Bosch L-Jetronic system. The system has evolved over the years and has earned a reputation for being relatively trouble-free. Even so, older high chilometroage cars and trucks can develop problems common to all fuel injection systems: pumps wear out, regulators fail, injectors become dirty or worn, cold start injectors can leak, and intake systems can become restricted due to accumulated carbon and fuel varnish. HOW TOYOTA FUEL INJECTION MEASURES AIRFLOW To regulate the air/fuel mixture, the engine computer needs to know how much air is being sucked into the engine. On the older Toyota EFI systems, air flow is measured mechanically with a flap-style airflow meter. A flap inside the meter rotates when incoming air pushes against it. Connected to the flap is a arm that rubs across a resistor grid (potentiometer). This changes the airflow meter's output voltage in proportion to airflow. The greater the air flow, the higher the resistance created by the potentiometer. So the meter's output voltage drops as airflow increases. Older style Toyota flap-style merchanical airflow meter. Over time, the potentiometer's contacts inside the airflow meter can wear causing erratic or inconsistent readings. Shorts or opens in the circuitry will also disrupt the voltage signal, depriving the engine computer of this vital bit of information. The result can be poor cold drivability, hesitation or poor performance. The TCCS (Toyota Computer Control System) should set a code 2, 31 or 32 if the airflow meter signal is missing or out of range, but it may not always detect an intermittent problems. To find this kind of fault, an oscilloscope can help you analyze the airflow meter's output voltage as a waveform. If you don't see a nice linear change in the output voltage as the flap moves from idle to wide open throttle, it means the potentiometer is skipping and the airflow meter needs to be replaced. Another way to check the operation of the airflow meter as well as the entire feedback circuit through the computer is to use a scope to compare injector dwell (on time) to the airflow signal. If you have a good airflow signal but injector dwell fails to increase as airflow goes up, there is a control problem in the computer. The flap type air flow meters should also be inspected by pushing the flap with your finger. There should be no binding when the flap is pushed open, and spring pressure should return it to its closed position. A buildup of varnish or dirt may cause binding. Be sure to inspect the air filter if you find any dirt in the unit. A temperature sensor located in the intake plumbing is used to measure air temperature so the computer can calculate how much air is actually entering the engine. Cold air is denser than warm air, and requires a slightly richer fuel mixture. The air temperature sensor changes resistance, so if the signal goes flat or disappears it too can upset the air/fuel mixture and cause drivability problems. Codes that would indicate a fault in the air temperature sensor circuit include 8, 23 and 24. You can use an ohmmeter to check the sensor's output. If the reading is out of specifications or fails to change as the temperature increases, the sensor is bad and needs to be replaced. Newer style Toyota mass airflow sensor. 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 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, …

Fonte: AA1Car.com