Rough (very) until warm

ZONIAN

MEMBER
DODGE

2005 Magnum 5.7L V-8 75K miles

Symptoms:
Very rough miss, like it's running on 7 or even 6 cyls. Until warm (190-200F), then it runs great. Have CEL & codes P0300 & P0308.

Occurence:
This usually only happens on the first start of the day, or when engine is totally cold.

Variations:
Sometimes it will be in "rough mode" from the minute I start it and stay that way regardless of RPM until it has warmed up.

Sometimes it will start with the touch of the key and idle fine, but if I increase RPM before the engine is warm, it will then go into "rough mode" and stay that way regardless of RPM until it's warmed up.

Troubleshooting:
Since the problem started, I have replaced plugs and EGR valve, and changed oil & filter. I have swapped coil packs and wires between cylinders (still get P0300 & P0308) I have used fuel from multiple stations.
I have checked as well as I can for a vacuum leak.

Any other ideas before I take it to the dealer and spend $$$$?

Paul

1 Reply

RASMATAZ

MECHANIC
75,992 POSTS

Misfires can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.

PO300 is a random misfire code indicates vacuum leak or bad gas.

A misfire in a specific cylinder should lead you to check the spark plug, fuel injector and compression which is P0308

Good Luck! Happy Easter

Using multiple fuel stations is a bad idea-never know what's in that fuel.