Blinking CEL light.

I recently went on a 280 mile (each way) trip. At about 200 miles in the Check Engine Light started blinking. There was no noticeable difference in the drive ability. Truck was driving fine. I had my scanner with me and checked for codes while I was driving. There was a random multiple misfire code, Cylinder # 2 misfire code and cylinder # 5 misfire code. I tried to clear the code but it would just go out for a second or 2 then just start blinking again. I ended up driving the rest of the 80 miles with the light flashing the entire time.

I got to my destination and the truck stayed parked for 4 days. I saw that cylinder # 2 and #5 share the same coil pack, so I figured that the coil pack went bad causing the misfire in both cylinders that are on opposite banks.
I found an auto parts store before I left and replaced the coil pack that controls cylinder # 2 & #5, cleared the codes and the truck started and sounded and ran fine and the light stayed off. i must have fixed the problem i thought.
I left for the 280 mile trip back home. and the same thing happened. After about 225 miles the CEL started flashing again and gave me cylinder # 2 & # 5 misfire code again, even with the new coil pack. I blank for about 35 miles then stayed on constant. I got home and cleared the code and not it is running fine with no light ......for now.

My question is why am I getting these misfire codes on cylinder 2&5 even after a new coil. AND. How come the truck is running absolutely perfect not missing a beat? No misfire was ever felt and truck has full power.

 

Plugs?

 

already changed them recently

 

If the one plug wire for that coil pack is bad, those codes could be generated. In fact any bad connection in that coil's connector and/or high voltage circuit (coil, plug wire, plug, block, plug, coil = circuit) could be the culprit.
While the coil/plug wire is off, check the plugs, they can loosen up.
If that easy stuff fails there could be a problem with the igniter. Maybe a "heat soak" issue?

 

Just a hint of a thought here, and since others are chiming in on potential problems I will keep my comment to one subject. When that little CEL light starts FLASHING, that is a BAD thing. Owner manuals for every vehicle on the road will tell you if the CEL is flashing you are supposed to stop the engine because you could be causing severe damage to the engine and/or catalytic converter. Just a little FYI there.

 

^^^I know but FYI, I was over 200 miles away from home, truck was all packed with camping gear, It was just me and my 10 year old son going on vacation in the middle of nowhere with only about 80 miles till our destination and it was getting late and close to our max time we were alowed to check in. The truck was driving totally normal so I decided to risk it.

Usually if the CEL light is flashing that means the computer recognizes a fault that is continuously happening at that very moment, that it is flashing. Usually this is seen (or I have at least experienced it when) the car has a misfire and it is missing the light will flash- but usually you can feel the miss and the car will have loss of power and stumble. I know what a miss-fire feels like and I would have felt a little something if I was dropping 2 cylinders.

 

if you check the plugs and they are tight, check your engine ground and check fuel supply. bad injector, clogged fuel line, could be running lean which causes heat build up. check your cat converter too as that could be the first thing damaged by running too lean or too rich.