hi i am trying to gather info as i am doing a tpi swap
what is open an closed loop ?
thanks chad

Open loop is what the computer uses until the motor has warmed up. Basically a fixed set of parameters to run the motor; air/fuel mixture and timing. After warm (closed loop) up then the ECU uses information from the sensors to manage the motor.

does your vss effect open /closed loop ?

VSS is a signal from the tranny that the ECU uses to manage the motor. By the ECU knowing what speed you are going it adjusts air/fuel accordingly. In automatics the ECU will use this to determine when to lock up the torque convertor. It has no effect on when the ECU goes from open to close loop. That I'm aware of this done only by engine temp.

VSS is a signal from the tranny that the ECU uses to manage the motor. By the ECU knowing what speed you are going it adjusts air/fuel accordingly. In automatics the ECU will use this to determine when to lock up the torque convertor. It has no effect on when the ECU goes from open to close loop. That I'm aware of this done only by engine temp.


ok let me ask this , i am trying to put a 1987 350 tpi (maf) in my elcamino . the computer i am using came from the same car the motor did and the car was a automatic , but i am going to be using a 4 or 5 speed trans . what will happen if i do or dont use the vss ?
thanks chad

The ECU will not know the speed you are going and will not be able to give you the best air/fuel ratio. You could wind up too lean or rich. Hard to say, I'm not real familiar with the TPI ECUs and their limitations/capabilities.

The VSS can be worked around. I have a MAP TPI system in my 72 and no VSS. I think that it can be programmed out of the chip. Bought mine as a kit, so don't know for certain how its done. Check with one of the "chip burners".

I know this is an old topic... but the explanation of open loop left out too much to let slide.

Closed loop means that the EFI computer is using the output of the O2 sensor to adjust the air/fuel ratio. This feedback closes the control loop since it validates that the right inputs are being used.

Open loop means that the EFI computer is not using the output of the O2 sensor to adjust the air/fuel ratio. In this case your results are only as good as your programmed tuning parameters.

Think about closed versus open loop like how you regulate you speed: you push the loud pedal some amount and then get feedback from the speedometer to make sure that cop over there is not going to notice you. The speedometer feedback makes this a closed loop control system. Without the speedometer you run open loop and you speed control is not as accurate.

Your EFI computer will switch between both modes depending on several conditions, the least of which is engine temperature. The cheapest, and thus most common, O2 sensors are "narrow band" and thus are only really very good at targeting the lean 14.7 optimal a/f ratio. So the EFI tends to try and run closed loop during cruise modes when presumably economy is most desired. It will switch to open loop for idle to keep from over-controlling and also for full power when you want to target the better power and cooler operation of 12.5 a/f ratio.