rough idling once in a while

[jcwaters1]

hello all, i was wondering what would cause an engine to idle rough, like to the point of almost stalling? it doesnt do it all the time, and i had my check engine light code read and one of the oxy sensors is bad. but it idles rough sometimes even when the check engine light isnt on. (it comes on and goes off periodically) also, i had the battery disconnected right before i started it up the last time it did it, so i was wondering if it may have just been re-learning the fuel mapping? it hasnt done it since, and that was probably 3 weeks ago.

thanks

4.0L SOHC '98 XLT

 

[2000StreetRod]

Possible intake leak

The SOHC is notorious for leaking intake gaskets. The effect is more pronounced in cold weather when the gaskets shrink and leak more. The PCM can compensate to some degree by adjusting the short term and long term fuel trims that are stored in keep alive memory (KAM). When you disconnect the battery for several or more minutes the KAM loses it's values. The PCM then has to "relearn" the values when the battery is reconnected and the engine restarted. At some point the gaskets may leak so much that the PCM reaches the limits of the fuel trim adjustment and stores a diagnostice trouble code (DTC) and illuminates the CHECK ENGINE light.

See my Lean condition procedure for more information.

 

[thumper477]

I very rarely see bad O2 sensors on fords unless they have been bathed in coolant from a head gasket failure.Most times as stated above O2 codes can be traced back to vacuum leaks.The operating parameters for the OBDII system are very tight and just a small variation from "ideal" will light up the CEL.

 

[jcwaters1]

ok, the guy at ford when i had the codes read said that the computer said it was one of the oxygen sensors was the reason for the trouble code. but this would be something to check out. thanks