Air-Line Controller

Here's the symptom: When you start your mid-'90s or newer Toyota 4-cylinder car, the engine starts okay, but the idle speed is extremely low. You might even have to keep your foot on the accelerator pedal for a minute or so, just to keep it from stalling. The check engine M.I.L. is not illuminated and, when the engine warms up, all seems to be normal, or at least a bit better. Perhaps the idle compensation function—a noticeable idle speed boost when the transmission is put into "D" or "R," or the A/C is turned on—is not spot-on either, but the cold-running problem is getting really irritating. For a relatively quick fix, here's the lowdown on the low-down idle for this common Toyota engine and many others similar to it.

Total Control
Even back in the early days of fuel injection, there was some form of idle control. Manufacturers used to employ a separate system for cold-idle compensation, often not controlled by the computer. They also used separate, electrically actuated valves for A/C and other engine-load compensation. These were often not controlled by the computer either. Hot, unloaded idle was adjusted with a screwdriver.


This system worked well, but manufacturers wanted to simplify things and cut production costs. The idle-control systems on most late-model passenger cars, including the Toyota engine shown here, utilize just one control valve, which is operated exclusively by the computer.


The valve function is similar to a washerless faucet—the difference being that air flow rather than water is the medium that is controlled. Essentially, the valve controls the size of an intake vacuum leak. The bigger the "leak," the more idle compensation is available. There are no adjustments to be made. This system controls all idle functions—hot and cold, loaded and unloaded. It works as well, if not better, than the old system—but is pretty intolerable to live with when wounded.

Guilty Valve
When symptoms like the ones noted above occur, the culprit is usually the valve itself. Specifically, the valve won't let the desired amount of bypass air into the intake, so the resulting idle compensation will be insufficient. Since no adjustments are feasible, and disconnecting the valve and attempting idle compensation by changing the throttle angle will lead to even less tolerable symptoms, the valve must be removed for inspection.