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I would think that it would run rich in an open loop. Have you checked your O2 sensors? Your truck should run open loop until the O2 sensors warm up and tell the computer if your truck is running too rich or too lean.
What I meant to say is have you changed out your O2 sensors? They might be bad. If I remember correctly, the computer runs open loop at start up and goes to close loop once the O2 sensors give a reading back to the computer. From here the O2 sensors' readings will tell the computer if you're too rich or too lean. And I don't know what to tell you on running too lean. I think that the O2 sensor is giving a false reading (on account of it being bad) and is also the reason why it's staying in an open loop. How many O2 sensors do you have?
most o2 sensors are heated by an electrical current. the truck will run in open loop until the sensors reach a certain internal temp and send a signal to the computer. as long as sensors are good, the computer will switch to closed loop once it receives the signal. vacuum leaks, worn o2 sensors, bad plugs/wires/cap/rotor can cause ecm to run open loop while it searches for the proper signal. if your rig has over 100k on it and you don't know when the last time they were changed, change them.
rednekper4mince. keepin duct tape and bailing wire in business.