ECU Mode on Cold Start

[VancTalonAWD]

Am I correct that on a cold start, once the motor is actually running, the ECU will be in open loop mode until the O2 sensor heats up. But is the ECU also in open loop while cranking before the motor actually starts?

 

[TALONTRAX]

The ecu is in open loop fuel mode until the front oxy sensor heats up & starts cross counting, so yes the ecu uses open loop fuel maps during cold startups . . .

 

[jrohner]

It also stays in open-loop mode until the coolant is up to a certain temperature.

 

[brownfinger]

When I finaly got my datalogger set-up I was suprised to learn a few things. Bear in mind my car is 1G, I know it's different in most ways. The info that the ECU "learns" in closed loop mode is still being used in open loop mode. Whatever your long term fuel trims are set at, the computer is applying all of that information during open loop operation. The short term O2 feedback trim is what signifies closed-loop operation. The computer uses the short-term info to set the long-term trims. If your LT trims are messed-up due to a vac. leak, bad or mis-adjusted sensor, etc... then the cold start open loop maps will be richer or leaner than default.
To say it another way: Unless you reset the ECU, your cold-start open-loop operation still reflects your screwed-up fuel trims. You have to reset the ecu to get back to the true default fuel maps.
Also, I was suprised by how quick the car enters closed loop mode. Much faster than I expected.