I have a 2007 Suburban 4x4 5.3L Vin3 with 300k miles on it. Having a misfire issue.

I have owned the vehicle for the past 60k miles. It had this misfire when I bought it, but it was very much driveable when I plugged in my DiabloSport AFM disabler. (it was super cheap, i’ve already gotten my money’s worth out of it. So no regrets with the purchase.) I took long trips, towed the boat, etc. Was fine. But if I ever pulled off the DiabloSport, I get CEL, P0300 or P0307 codes along with limp mode, hard shifts, “service traction control”, etc.

Then I moved from NJ to WI. In NJ we have emissions inspections … and I know the truck won’t pass with a misfire. Also the misfire is worsening, engine seems weaker, MPG is worsening as well. So I switched out all plugs & wires on 1,3,5,7 cylinders. (haven’t gotten to the other side yet.) 1,3,5 looked near perfect - but #7 looked absolutely horrific - all gunked up. Also replaced #7 coil. But nothing changed, and I noticed when running the engine and pulling the spark plug wires off the coils, when I pull #7, I get no difference in performance. So I am pretty sure I’ve found the dead cylinder.

I took off the drivers side valve cover and ran the engine to see if all the rockers were moving (trying to rule out a dead AFM lifter.) All seemed fine.

I pulled the #1 and #7 plugs again, and #7 is fouled. Looks and smells like oil. See pic attached - this is after only a couple hundred miles. #1 is the clean one, #7 is the dirty one in the photo. So … seems #7 is both 1) not firing and 2) getting oil in it.

I haven’t seen a mechanic yet, I’ve been screwed before and i want to do/know as much as possible before shelling out $$ for a misdiagnosis.

So … wondering where to go from here? The drivers side valve cover was original, I could replace it with the updated 2011 version. But I don’t know if that would solve the oil problem in #7. Should I do a compression test? What would it tell me? Or should I swap fuel injectors and see if the problem follows the injector? What about a top clean with some kind of solvent?

Or am I nuts, and should just buy a rebuilt/reman motor since I have 300k on it? The rest of the truck is in good shape - little rust, new tires, new transfer case, new alternator, suspension still good, no issues with tranny.

Any help appreciated!

 

Compression test. All should be within 10% . At 300k you've got your money's worth. If you have to or decide to replace the motor then figure on the trans. also. 300k miles it's well worn out. Your at worth vs cost decisions now.

 

Yeah, I'd dump the entire vehicle. Not too many automatic transmissions make it to 400k even without towing unless it's an Allison.