NEED HELP 2006 4.2 misfires,its wierd,please help.P0300 and 301

[TBgonzo]

P0300 and 301 at idle in park,if I raise the idle to 700rpm with HP tuners special feature.It will mis in P or N but not in gear.The engine sounds good,at idle its slightly rough when missing,coil swaps plug swaps,injector swaps,fuel pressure good,compession good,has some misfires on #3 when it will mis.Crank end play tight.Thanks for ideas

 

[gmcman]

Welcome.

How old (mileage) are the plugs?

Have you recently replaced the plugs? If so, did you use only AC 41-103?

 

[Mooseman]

I had (probably still have) a similar issue with no resolution. This thread will take you through the steps I went through to try and diagnose it.

Very Slight Miss at Idle

Yesterday on the TB, at 117k km, saw the CEL come on and code came up for P0304. Fired up Torque and it was missing on cylinder #4 but only at idle or stopped at a light. It's not a big miss since I can't feel it at all and it's just a couple of misses per refresh cycle. While driving...

gmtnation.com

 

[TBgonzo]

Recent plugs swapped around,1 new coil others swapped around.700rpm misfires in park,no misfires in gear,wierd.

 

[TBgonzo]

I had (probably still have) a similar issue with no resolution. This thread will take you through the steps I went through to try and diagnose it.

Very Slight Miss at Idle

Yesterday on the TB, at 117k km, saw the CEL come on and code came up for P0304. Fired up Torque and it was missing on cylinder #4 but only at idle or stopped at a light. It's not a big miss since I can't feel it at all and it's just a couple of misses per refresh cycle. While driving...

gmtnation.com

Mine is very much like yours,

 

[gmcman]

Recent plugs swapped around

How recent? Also, what plugs are in the engine now? Reason I ask, is the 4.2 is picky. Back in the day, almost any plug would work...but not with this motor.

How long has the fuel been in the tank? If a daily driver and replaced weekly or bi-monthly then likely not an issue.

Anytime I get random misfires like this, with known good coils and plugs, my goto can is BG44K. This stuff is not snake oil and works extremely well.

Another test I would run is place an envelope behind the tailpipe so the exhaust flow folds it over while holding one end against the outlet. Does the envelope "snap back" into the tailpipe or stayed folded over? If it snaps back then that points to the exhaust valve not fully seating, sometimes from carbon.

Second would be a vacuum gauge on the TB evap port and ensure the needle is steady.

Are the coils original or aftermarket or OE replacements? I went cheap about 2 years ago and grabbed the 6-pack off the web for about $150 and have since replaced most of them.

 

[TollKeeper]

So a thread that @Mooseman responded to had me curious on this.

Could the Fuel Pressure Regulator be allowing the fuel pressure to dip, and not allow the proper stoichiometry to allow proper combustion, thus causing a random misfire?

 

[Mooseman]

Possible but I checked mine and it was fine.

 

[gmcman]

I remember many threads on Bosch Platinum 4 or splitfire plugs and running issues. That's why I was asking about the plugs.

 

[TollKeeper]

I remember many threads on Bosch Platinum 4 or splitfire plugs and running issues. That's why I was asking about the plugs.

Good point!!! Most car manufactures build engines based on a lot of things.. EVEN SPARK PLUGS!

The 4.2 is also very keen about spark plugs. Even the 5.3, but not to such a degree.