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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
17,019 Posts
Welcome to Allpar. When first started cold, the EFI system is in 'open loop'. That means that the O2 sensors haven't warmed up enough to provide an accurate signal to the PCM and the PCM itself runs the show.
The PCM uses a pre-programmed cold 'open loop' schedule to run the engine until the O2 warms up then switches to warm 'closed loop' operation.
Closed loop uses the information from the warmed O2 sensor signal to determine the correct 'rich/lean' fuel mixture (injector pulsewidth time) to run the engine at.
Wide open throttle (WOT) operation reverts the system back to open loop operation for computer control of the engine.
It sounds like a cold/warm transition issue like failing O2 sensors or wrong temperature information provided to the PCM by a temp sensor or wiring problem.
Not everything will set a fault code, especially if the sensor value is still believable to the PCM.
You may want to plug in an OBD II scan tool and watch and compare sensor values with what they actually should be. Look for excessively rich or lean adaptive fuel trim values or a temperature sensor value stuck at 48 deg F, for instance. Positive fuel trim adaptive values indicate that the PCM is adding fuel to cover for a possible lean condition and negative fuel trim adaptive values indicate that the PCM is removing fuel from the engine to cover for a rich condition. Verify that the fuel pressure is correct. There is a screw-on fitting on the fuel rail for a gauge.
Spark plug tips may indicate carbon fouling for rich conditions and get so bad as to cause misfire. The correct Champion spark plug type is important. OEM-spec (Mopar, NGK or Denso) sensors are also important when replacing them.
O2 sensor voltage values should always be in motion, high then low then high, etc. The upstream (before the cat) will switch hi/lo at a rate about twice as fast as the downstream (after the cat) O2 sensor switching rate.
You never want the O2 voltage value to sit still when warm. They can sit at about a 1/2 volt when they are tired or peg at a high (1.0v) or low (0v) signal voltage. When this happens, the engine will run poorly and a fault code should eventually set.
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