I have a 2008 4.6 EFI V8 engine with 50,000 miles. Driving down the highway with cruise control on going up a hill, started going down the hill, all of a sudden the engine rev's up to 5,000 rpm. Shut the cruise off, now I have a dead miss. O'rielys Part store computer check reported No. 1 and No. 3 misfire. Replaced Spark Plug Coil at $50.00 ea. Still missing. Replaced all plugs, No 3 had the plug tip burned off. Still a dead miss. Any suggestions out there. I am thinking Burned Valve.

 

Do a compression check. If that is OK, then maybe a bad injector.
Good luck.

 

maybe pull #3 again and have a peek... if you start engine when cold (best if someone else can for you) then use thermal heat gun to measure temp on the exhaust manifolds or with your fingers... they will warm up quickly but you may be able to find the missing cylinder as it will not warm up as fast...

 

Done a compression check 35 psi. Not Good!!!!!! Now what?

 

Get a boroscope to look into the cylinder through the spark plug hole, or just pull the head off. I would bet that the tip of the spark plug broke and damaged a valve or the piston.

 

Bump---- Took both heads off. Only defect was #3 valve had to be replaced. Don't know exactly why right now. Has anyone ever heard of a special tool required to align the two camshafts with the Crank. I was told it could cost up $800.00. Should get the truck running tomorrow. Will post when I have all details. Thanks for the support. Ron

 

End of this ride. #3 Intake valve burned. Replaced Valve. Back running now.