Ok, I just installed a set of Speed Engineering shorty's on my 03 Sierra 4.8L. Right after installation my engine light came on and I had a rough idle. Come to find out I broke the coolant temp sender and I had 2 plug wires that were jumping fire #5 and #8. I replaced the coolant sender and all plug wires. of course the P0118 went away but now there is a P0300 for a random missfire.

I hooked up a tech II and see that #4 and 5 are the problem cylinders. I have checked and double checked that I got all of the wires on good and can not see any fire jumping.

The last time I had a randome missfire it was my fuel pressure regulator. Anyone think that might be the problem again or should I be looking somewhere else?

 

you might have a cracked spark plug too

 

If misfire is bad enough on one cylinder, it could affect the next cylinder in the firing order! Cracked plug would be enough to cause a residual misfire.

 

If you suspect fuel issue, hook up a pressure guage and see what yiu get running. And you have a tech 2 so you should also be able to do an injector leak down test.

 

Thanks guys!

Cracked plug on #5