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:

Coolant temperature above 105°F.
Oxygen sensor has varying output voltage (indicating it is hot enough to operate properly).
A specific amount of time (preprogrammed into the MEM-CAL) has elapsed.

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

More power with less fuel-TPI systems utilise very accurate fuel metering.
Better fuel economy and reduced emissions at part throttle.
Better idle quality, especially for cold starting.
Supercharging, Turbocharging and/or Nitrous Oxide Injection setups are a breeze. The system is expandable to control other engine functions.
Factory components available.
Intake manifolds, exhaust headers, cams etc. can be built into the engine without mixture problems.
No fuel slosh problems -- marine installations find this the biggest advantage.

Disadvantages

Cost. Much more expensive than conventional Carb/Manifold setups.
Electronic wizardry can be intimidating. New technology usually takes some time to accept.
Backyard fabrication and installation not recommended or advised.
Initial applications are limited. As trends progress, however, this will change.