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The idle adjustment on the throttle body has almost no effect on the idle speed of the engine. To test this further I blocked off the auxiliary air supply from the warm-up circuit after the engine had warmed up. The car immediately died after the restart. I had to back out the idle adjustment screw almost all the way to keep the car running at idle. Backing out the idle adjustment screw all the way also produced a loud suction noise at the throttle body. Any idea on what is going on here?
This is not much of an answer, but you might consult the manual because there are lot of things at play given your very general description. Also, this forum contains several posts concerning Djet diagnostics. Describing how or when this condition first developed might also aid in any long distance diagnosis.
Assuming your ignition system is properly functioning, (e.g., ignition timing is not seriously retarded
or advanced) have you tried adjusting the potentiometer on the ECU? In short, this can affect idle mixture, assuming all other systems are in working order. (See Slow running adjustments - below.)
Next, is the electrical connection to the intake manifold sensor (on the forward part of the "log") intact? If disconnected/broken, this will likely richen the mixture. All of the temp sensors, (near the auxiliary air valve) and their connections can affect engine operation, so it is always a good idea to check wiring condition and connections.
You mention "blocking off" the auxiliary air valve, but what about the cold start injector? Again, assuming that your ignition system is properly functioning, is it possible that your cold start valve/injector is not switching off and still supplying the engine with fuel after the engine is at normal operating temperature? This would richen the mixture to the point where additional air is necessary to maintain an air/fuel balance upon which the engine can function. Since the cold start injector is
always pressurized by the fuel pump, leaking is not uncommon. Unplugging the electrical connection may not terminate any leak if the injector is stuck open or even
slightly open.
Other (non-ignition-related) possibilities include a maladjusted fuel pressure regulator and a failing pressure sending unit.
HTH :wink: