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Strange Issue 2002 5.9 Ram Quad Cab

1144 Views 1 Reply 2 Participants Last post by  ImperialCrown
When starting this engine cold it fires right up and purrs like a kitten...for about a minute to a minute and a half. Then, for about a minute to a minute and a half it misses, hesitates and stumbles under acceleration. Then, it smoooths out and runs fine the rest of the time it is being driven. When started warm, this does not happen. Upon cold start it happens in warm weather, cold weather, dry weather, damp weather. There is no Check Engine Light indicated and there is no other issue with the performance of the engine.

Since the issue occurs with such regularity and always with the same intervals, I am inclined to believe there is a computer command utilized in the cold start sequence that is responsible for this issue. Does anyone know the exact computer sequence utilized by this engine at startup and for the first few minutes of run time?

My first thought was a sensor failure of some sort, but I would expect a CEL if that were the source of the abnormality.

Any thoughts or experiences similar to this?

Thanks in advance for any helpful input.

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· Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
17,323 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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