07 Suburban 1500 5.3 P0300, running very poorly

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Help a euro tech out with one of these junk cars! Ok heres the backstory. Wednesday, of course, 2:30 pm, my landlord at the shop drops his car off. "Do what you can, I really need it for the weekend" Right. Running very poorly and knocking. All that shows up in the scanner is P0300 for random cylinder misfire. When I watch the misfire counter, its skyrocketing on cylinder 2,5, and 7, with a smaller amount of misfires on 1 and 3 showing maybe 5 after a few minutes of watching. The car has a very long crank, but will catch and idle poorly. I moved the plugs to difffernt cylinders, and the coils to different cylinder than the plugs to chase it, but the misfires stay on mostly 2 5 and 7. It sunds like a sort of chugging noise through the intake. Ive seen tons of TSB on carbon buildup on the valve stems, but Im not sure if those symptoms match up to what the bulletin says. Anybody who sees these regularly see anything similar? Thanks
 
Random multiple misfires..... Gonna need to find the list for diagnostics for that code on that model of truck and start checking them off (I dont have access to it)
 
I can almost guarantee the AFM lifters have failed. The heads will need to come off.

Does it have the updated valve cover yet? There's also carbon issues due to excessive oil burning. Pull the plugs on the driver's side bank and compare. Are 5 and 7 oily or fouled?
 
This exact thing happened to me with a 97 Chevy truck with the small V-8. All said and done, the injectors were found to be filled with some type of crud that would dissolve in acetone but not gasoline. It looked and felt like a fine plastic sand. The guys who did the work were totally baffled by how such a material got into the injectors seeing as it had to come through the fuel filter. Apparently, it was lightly soluble in gasoline but would start to precipitate out in the injectors. The cure.....clean the injectors and replace those that would not respond to cleaning. If I remember, three of the injectors had to be replaced on the engine.
 
The more I read into the AFM lifters, the more it makes sense. So hes probably going to need a motor rebuild, or Ill buy the car and flip it. Ill spend more imt eon the lifter diag. What do the updated valve covers look like? I doubt hes ever had those done, do they have better PCV passages?
 
Originally Posted By: Audios
What do the updated valve covers look like? I doubt hes ever had those done, do they have better PCV passages?


Just one valve cover is replaced, driver's side. Yes, bigger passage.
 
Originally Posted By: Audios
The more I read into the AFM lifters, the more it makes sense. So hes probably going to need a motor rebuild, or Ill buy the car and flip it. Ill spend more imt eon the lifter diag. What do the updated valve covers look like? I doubt hes ever had those done, do they have better PCV passages?


Most of the covers were done recently, so it's usually a lot more shiny than the right bank. If you do buy it to flip it you're better off just deleting the AFM. You can do the heads pretty easily in the truck.
 
I dont think a plugged fuel filter or weak pressure would be giving me misfires on specific cylinders all over the engine. Ill check again Monday, but I know the pressure was above 40 when we did a quick one on weds.
 
On that engine, it is easy to pull a valve cover, let the ignition coils hang off to the side, start the engine and look for rocker arms that aren't moving.
 
For your rebuild you can buy parts that will make the motor a non-AFM one.


I would agree the AFM lifters are probably toast.


They sell the AFM delete kit here-
https://www.texas-speed.com/
 
Last edited:
Originally Posted By: CKN
They sell the AFM delete kit here-
https://www.texas-speed.com/


That's what I was going to ask about. You answered my question before I got a chance to ask it !
smile.gif
 
As an update, he went with Jasper motor. Customer took i tot an engine friend, upon disassembly, heavy scoring was found, and 2 of the lifters were missing pieces. 100k warranty so hopefully it holds up.
 
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