Toyota Pick-ups/Land Cruiser/4Runner 1989-1996

Idle Speed And Mixture Adjustments



To meet emissions regulations, the mixture adjusting screw on the carbureted 22R engine is preadjusted and plugged by Toyota. Adjusting this screw is not part of routine maintenance. Access to the screw requires removal of the carburetor. Full details of re-setting the air/fuel mixture after a carburetor overhaul are covered in Fuel System . In virtually all cases, the air/fuel mixture screw is NOT the cause of a problem. Even if the engine is tested with an emissions analyzer, incorrect mixture is almost always caused by air leaks in the system or faulty internal carburetor parts (clogged jets, mis-adjusted floats, etc.). We recommend all mixture adjustments be handled by a professional mechanic equipped with the proper emissions test equipment.

On the fuel injected engines, fuel mixture is handled by the computer, based on signals from the oxygen sensor in the exhaust system and many other engine sensors. By having a "picture'' of the engine operating conditions at any given moment, the computer determines the exact amount of fuel to maintain proper combustion and signals the injectors accordingly. Any investigation of the air/fuel mixture on a fuel injected engine requires the use of an HC/CO emissions tester, which is well out of the reach of the home mechanic.


Carbureted Engines

Before attempting to set the idle speed, the following conditions must be met: Air cleaner installed with all ducts, hoses, etc. intact, engine fully warmed and choke plate open, all electric accessories switched OFF, all vacuum lines connected, transmission in NEUTRAL and parking brake set and engine running at normal temperature.

  1. Connect a tachometer to the engine.
  3. Set the curb idle speed by turning the adjusting screw on the side (not the bottom) of the carburetor. Correct idle speed is 700 rpm.
  5. Turn the ignition OFF . Remove the air cleaner. Remove and plug the hoses for both the Hot Air Intake (HAI) and Mixture Control (MC) systems.
  7. Disconnect the hose from the choke opener diaphragm and plug the hose end. This shuts off the choke opener system.
  9. Disconnect the hose from the EGR valve.
  11. Hold the throttle plate (in the bottom of the carburetor) slightly open and at the same time push the choke plate (top of the carb) closed. Hold the choke closed while releasing the throttle plate. This sounds tricky but is easily accomplished with a thin tool.
  13. Start the engine but DO NOT touch the accelerator. The engine is running at high idle, as it would if started cold.
  15. Set the fast idle speed by adjusting the lower screw on the carburetor. Correct idle speeds are: 49 State and Canada, 3000 rpm and California, 2600 rpm.
  17. Turn the engine OFF . Remove the tachometer. Connect the vacuum hoses to the correct locations.
  19. Install the air cleaner assembly.

Fuel Injected Engines

The idle speed on the fuel injected engines are electronically controlled by the Engine Control Module (ECM) computer. The Idle Speed Control (ISC) system electrically oversees the proper idle speed for all operating conditions. This is a good news/bad news arrangement. The bad news is that you can't adjust it; the good news is that it rarely, if ever, needs adjusting. Any roughness or uneven idle is almost always due to other causes such as a clogged injector, air leak or loose, corroded terminals in the wiring harness.