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Cold start?

96 dodge intrepid, 3.5. Few yrs back kid flooded it at -10f. I changed wet plugs and it started. Ran ok for 10miN, idled fine .n than I drove it 10 miles and it would not idle. I was 150mi from home and kept going. Car ran so-so. Seemed sluggish at 60mph. Exhaust actually made black/soot stain on bumper. My mileage on hwy was about 1/2 normal. I pulled plugs when I got home and they were black. Took about 1 week of driving to eventually settle down. Never did find any reason for issue. Some said my o2 sensors were buggy. I never changed them and had car for 2 more yrs and it ran great. Any theories on why it would not idle?

Flooding the cylinders with gasoline could have washed the oil from the pistons, oil rings and cylinder walls. Cold starts can be very hard on engines.

Black plugs are an indicator of a rich mix or decreased compression. I’d run a compression and leak down test prior to replacing the O2 sensors. Good luck!

You definitely were running rich.

At the time I would have checked the Coolant Temperature Sensor for calibration. Also would have checked the MAP sensor as that is critical to the mixture calculation. It could have been one or both of the O2 sensors. Did you ever get a Check Engine Light? Do you know if this PCM was OBDII compliant?

It would have been interesting to put the code scanner on the test port and see if there were any existing or pending codes.

Car is gone now. Did not have any codes. No cel light. That motor did not have maf. Had dual throttle bodies and awkward intake routing. I did lots of maintenance over the yrs. plugs, wires, timing belt. Always ran great. Never touched o2 sensors. 3.5 motor was pretty solid.

One theory might be that the cat got flooded with a lot of gasoline, which temporarily plugged it up, resulting in excess back pressure. Poor performance, especially on accel would result. And – I admit this is just a wild guess at this point – the backpressure on the engine from the plugged cat might also affect the accuracy of the ECM’s computation for the fuel/air mixture.
A plugged cat might affect the MAF’s air metering function or the MAP’s manifold vacuum reading. Over time maybe the cat was able to burn all that excess gas inside, so it returned to semi-normal operation with close to normal back pressure, allowing the ECM’s fuel/air computation to return to normal.

If such a thing ever happened again, the way to test this theory is to measure the cat’s back pressure.