Fuel Control

• The primary purpose of the 4Runner ECM is to handle fuel injection. Fuel injection, unlike carburetors, is a computer-controlled process. The 4Runner ECM takes readings from the mass airflow sensor, which provides information on air density and temperature, and alters fuel injector pulse and spark plug firing time to maximize combustion.
  
  

Engine Safety

• The ECM also provides engine protection against premature detonation due to poor fuel or low oxygen density. The ECM also receives a signal from the engine coolant and oil temperature sensors to allow it to warn the driver when safe temperature ranges are exceeded.
  
  

Additional Control Modules

• While the engine ECM is the primary computer for the 4Runner, later-model 4Runners will also have secondary control units. For example, on the electronically controlled transfer case for late model four-wheel drive 4Runners, there is a separate four-wheel drive control module that controls selection of four-wheel high and low modes.
  
  

Repair and Replacement

• The ECM sits in the engine bay, and thus over time can be exposed to water and salt which can cause ECM malfunctions. New ECMs are very expensive pieces of hardware, but you can either attempt to swap your ECM with a used one from a parts seller, or have your ECM repaired by a qualified automotive electronics repair shop.