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Open/Closed Loop Operation

Open Loop

When the engine is first started the system goes into open loop operation. In open loop, the ECM calculates the Air/Fuel ratio based on inputs from the coolant and MAF sensors. The system remains in open-loop until the following three conditions are met:

Closed Loop

When all of the above conditions are met, the system goes into closed loop operation. In this mode, the ECM calculates the Air/Fuel ratio based, mainly, on input from the oxygen sensor and maintains the Air/Fuel ratio at the desired 14.7 to 1.

Limp Mode or Fuel Back-up Mode

In this mode, the system operates on a fuel back-up logic, programmed into the CAL-PAK portion of the MEM-CAL, to control the engine if the ECM or other specific sensors fail. As indicated by the limp-mode designation, engine performance is substantially less than it should be.

Notwithstanding the claims of the manufacturers of these harnesses, operating in either of these modes may drastically affect the efficiency and fuel mileage of the system. For a TPI, or any other similar electronically controlled system, to function in the closed loop mode, there MUST be an oxygen sensor in the system.

If you are contemplating a rebuild of your Hot Rod, Street Machine, Four wheel drive or grocery getter, or maybe thinking of embarking on an engine swapping project for your Jaguar, here are some advantages and disadvantages of using the Chevrolet TPI system over a conventional carburetted system:

Advantages

Disadvantages


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