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On Toyota Carbureted Engine: Check AAP Diaphragm for Leakage If Cylinder Rich or MisfiringPosted 7/16/2001By Brad Davis
Vehicle: System: Symptom/Problem: Toyota carbureted systems are fairly simple: There are no duty solenoids in the carb to fail. Most have an external electric bleed control valve. None have an internal control solenoid or duty solenoid. Because of this enleanment type of system, these vehicles need a way to help richen the system when cold or there will be hesitation on acceleration. Here's where the auxiliary accelerator pump comes in: The auxiliary accelerator pump (AAP) diaphragm is found on most carbureted Toyotas except the 1987-1990 variable venturi. The AAP provides an extra shot of fuel to the engine on acceleration only when cold. A thermostatic vacuum switching valve either allows or blocks vacuum from reaching the AAP. Manifold vacuum is applied to the AAP port when cold. The AAP receives its vacuum source from two different locations, depending on the vehicle. Some older models receive vacuum from a centralized vacuum port on the intake, while newer models receive the vacuum signal from a port located in the No. 1 intake runner. If the thermostatic vacuum switching valve fails in the open position: Vacuum is allowed to the AAP when warm and there will be a bog on acceleration from too much fuel entering the engine. If the thermostatic vacuum switching valve fails in the closed position: Vacuum is not allowed to the AAP and there may be a lean stumble from a lack of fuel to the engine. The AAP diaphragm can also rupture, causing fuel to be drawn through the diaphragm. With a centrally located vacuum source, if the thermostatic vacuum switching valve is still good, the vehicle will run rich when cold and be fine when warm. With a vacuum source in the No.1 intake runner, the vehicle will have a dead No. 1 cylinder until the engine warms up - then it will be fine. If the AAP diaphragm leaks over a period of time, it will ruin the thermostatic vacuum switching valve and cause the vehicle to run rich at all times. If you encounter a carbureted Toyota with a rich condition or the No. 1 cylinder is not firing: Check the AAP diaphragm for leakage. The AAP is mounted on the side of the carburetor. Its cover plate is attached with three screws that form a triangle. There is one vacuum port on the plate. To check the AAP: Remove the vacuum hose from the port and apply vacuum to the port. It must hold vacuum; if it does not, the diaphragm is leaking. The AAP diaphragm can be replaced without removing the carburetor from the vehicle. Just remove the three screws, the diaphragm cover plate and the diaphragm - and install the new one. Two common mistakes when checking the AAP: 1) Expecting to see fuel when the vacuum hose is pulled off. Fuel does not usually stay in the line because the vacuum source is direct from the manifold, so it draws the fuel right into the intake. 2) Expecting the vehicle to immediately run better as soon as the line from the AAP is removed and the hose is plugged. Usually it takes a few minutes before the system clears the excess fuel from the vacuum hoses and intake manifold.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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