Quick Terminology Question

Open loop – The computer resorts to a predetermined set of operating parameters for air/fuel ratio, idle speed, spark advance and emission controls using only MAF, BAP, ECT, ACT, PIP & TPS for input. In essence the computer program is like a lookup table or Excel spreadsheet that picks the right value from the table to use.

Closed loop uses all of the above sensors plus the O2 sensors to map the air/fuel, spark advance and emission controls to match a constantly changing environment. There are no preset or default values, the computer calculates the correct values based on the sensor input. Closed loop is the operating mode for maximum gas mileage and minimum emissions.

Black Sun 5.0 said:

So if my engine temp sensor is shot, the car may only be running in open loop? I do also get a check engine light, I've had issues with my O2's (error 41), but the shop tested the sensors and they were switching just fine. I don't know what the heck is wrong with the thing.-

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The ECT helps determine when the computer goes into closed loop. It also provides data that the computer uses to calculate the proper air/fuel mixture while in closed loop mode.

The O2 sensors have a ground wire that is part of the fuel injector wiring harness. The wire comes out of the harness near the throttle body. It can be grounded to any nearby engine bolt.

Here's a book that will get you started with how the Ford electronic engine control or "computer" works.

Ford Fuel Injection & Electronic Engine Control 1988-1993 by James Probst :ISBN 0-8376-0301-3.

It's about $20 from Borders.com see http://www.amazon.com/ . Select boo...very good, and I found it to be very helpful.