Hi All,

I recently noticed a rough idle -- with intermittent exhaust backfires -- on my 2001 Firehawk.
At one point I also received a Flashing Check Engine Lamp along with DTC P0302 -- Misfires on Cylinder # 2
It seems as though the engine smooths out when taken above 1,200 RPM. However, I haven't scanned the engine when it is at this elevated Fast-Idle condition.

Drivability, other than idle, to include even WOT, doesn't seem to have any issues.

I have scanned the engine multiple times from a cold start ( Ambient about 60 F ) and noticed that the misfires do not begin until several minutes after the engine has been running.
Each time I have scanned it , the misfires on Cylinder # 2 begin about 7-8 minutes after start. It seems as though the engine goes into closed loop within 2 minutes of initial start as evidenced by the PCM beginning to report STFT values other than zero.

No other Cylinder shows any misfires - ONLY cylinder # 2

I noticed that during the first few minutes of operation, while there are no misfires, the STFT ( Bank 2 ) values hover around 0 to -6. The STFT during this period are ALWAYS NEGATIVE.
After 7-8 minutes, I begin to see reported misfires AND I see that the STFT ( Bank 2 ) values are now ALWAYS POSITIVE.

Sometimes, after 8-10 minutes of misfires, the STFT will begin to go NEGATIVE and the misfires will then stop.

I have a Wideband O2 gauge that seems to validate the Lean ( PCM responding with POSITIVE STFT corrections ) during the misfire events. I realize that a misfire can appear as Lean to the O2 sensors as the unburnt oxygen hits the sensor.

Please look at some captured data I took during one of the Cold-Start data-logging events. This data is representative of several cycles I have scanned from a cold start. i.e. it is somewhat repeatable

Some more information to help with everyone's diagnosis:

1. Fuel rail pressure is measured to be 57 PSI at idle and 55 PSI at WOT
2. LTFT for both Bank 2 ( Cylinder # 2 ) and Bank 1 are at Zero ( 0 )
3. Idle Vacuum is steady ( even when misfires are present ) and is 21.7 In Hg ( I'm @ 3,000' )
4. New Plugs and wires about 20,000 miles ago - this is when Long Tube Headers and a Cat-Back were installed
5. Engine is STOCK except for wrapped Long Tubes and Cat Back - (Exhaust system has NO CATS)
6. Engine has 169,000 miles and uses "about" 1 quart of oil every 1,000 -1500 miles
7. Engine has been running fine until this recent idle issue - nothing has been changed on the engine since the Long Tubes were installed 20,000 miles ago.

What is suggested as my next troubleshooting step ?

Thanks to all for looking at my issue and suggesting a course of action.

Bob

 

Seems like it's heat related. Try switching the plug, then the coil one at a time. If the problem persists after both of those tests, it's probably a loose connection feeding the coil or (I saw you replaced the plug wires), but it would be worth checking at that point.

Nick

 

Thanks for your feedback !

I'll go through a Coil - Plug wire - Spark Plug swap with # 4 ( 1 component at a time ) to see if that helps me narrow down the issue.

Look for me to Post back my findings tomorrow night.


Also - if my calculations are correct ( see attached Word Doc in my first post above ) I am misfiring almost 50 % of the time in Cylinder # 2 during my Cold Start scanning.

Why is this high of a Misfire rate NOT reported by the PCM as a DTC ?

Seems that the Misfire Counters in the PCM have a very high threshold before setting the DTC.

Does anyone know the criteria ( # of Drive cycles - number of Misfires - etc ) before the PCM sets the Misfire DTC ( P030X ) ?

Thanks again for feedback !

Bob

 

Well... I simply swapped Cylinder # 2 Coil and Plug wire with Cylinder # 4 Coil and Plug wire.

The Coil connectors seemed clean and were fully seated when I took them off.

Plug Wires seemed fine with no evidence of carbon arcing etc.

And... after 3 -- 18 minute Cold Start cycles over the course of several hours - NOT A SINGLE MISFIRE !

I then took the vehicle out for about 30 minutes of "spirited" driving along with several stop-light idle cycles.

Absolutely NO Misfires ( YES ! )

Thanks for your suggestion of simply swapping the Coils and Plug wires.

Oh, even though the coil connectors were fully-seated and the connector pins looked clean, I did spray contact cleaner on both the pins and sockets of each coil's 4 pin connectors.

I am a Happy Camper.

Thanks for the help.

Bob

 

Sometimes merely the act of unplugging and re-plugging connectors will clean the connectors enough to solve a poor connection. The contact cleaner didn't hurt things either.

 

You're welcome...