Cyl 2 and Cyl 3 misfire, not P0420

gdstock · 02-01-2012, 12:29 PM

For the last week to ten days, I have been getting a cyl 2 and cyl 3 misfire. Cyl 3 was pretty random, the cyl 2 showed up as I was climbing hill at highway speed.

So over the weekend, I replaced plugs with NGK dual ground (had single ground installed), checked coils (all 3 ohmed out correctly) and marked coils and reinstalled in different positions to see if problem moved. Plugs and wires had about 8000 miles on them.

I was also getting a constant P0125 - Insufficient Coolant Temp for Closed Loop Fuel Control for a long time. After replacing both temp sensors, my son told me this was actually a O2 sensor problem. So I picked up a Denso O2 sensor to replace the cheap aftermarket O2 sensor I installed about a year ago (just before this code started showing up). I spilced this O2 sensor in as I could not reach connector. Thanks to research here, I found that the connector is nicely concealed beneath shifter boot (wtf?) and was able to plug the new one in correctly.

Plug 3 had broken porcelain at electrode.

Went on business trip Sunday and got back late last night, and did not experience any misfire codes showing up and the P0125 went away.

But on the way back I did get a P0420 Catalyst Syetem Efficiency Below Threshold. I plugged in scanner and plotted the O2 sensors. MOST of the time, upstream was reading higher than downstream, but ther were times when downstream value was very close, and occasionally at or above the value of the upstream sensor.

I banged on cat and it does not have a rattle to it. It does not run like the cat is clogged. NO loss of power, ideles and accellerates good. Truck has 270,000 miles and only one previous owner (my dad) and I do not know if he ever had cat changed. I have owned the truck for 7 years.

Before I drop the cash for a new cat, is this typical symptoms of a failing cat on these trucks? I know on a Dodge Ram, the typcial faiulure is for cat to clog up.

Thanks.

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revenantdead · 02-02-2012, 05:29 AM

check the fuel injectors. if you have one of the injectors stuck open it'll do some weird stuff.

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02-02-2012, 05:29 AM
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