2008 GMC Sierra Misfire

[connord404]

Hello, I looked 2008 GMC Sierra 1500 that I was interested in purchasing with the 5.3L V8 has 174,000 miles. Truck runs rough at idle, rpms are fluctuating. Check engine light is on with codes P0300 and P0301. Can’t troubleshoot the problem. Coils, spark plugs, spark plug wires, new O2 sensors, new catalytic converters. Guy says that rockers all move so it isn’t Active Fuel Management lifters and he says 150 psi of compression across all 8 cylinders. Truck was at dealership, dealership says problem with cylinder numbers 1 and 8 the P0301 explains problem with cylinder 1 but why isn’t code P0308 being thrown? What else could be the problem?

[MaverickZ71]

Check spark plugs on those 2 cylinders.  If oil fouled, but only on those 2 cylinders, AFM is the problem.  May have caused stuck oil control rings on the pistons on those cylinders.  Severe oil usage caused by AFM is probably why the truck needed new catalytic converters--the cats got plugged up.  Also some cam lobes have been known to get messed up sometimes on AFM engines.  

[connord404]

3 hours ago, MaverickZ71 said:

Check spark plugs on those 2 cylinders.  If oil fouled, but only on those 2 cylinders, AFM is the problem.  May have caused stuck oil control rings on the pistons on those cylinders.  Severe oil usage caused by AFM is probably why the truck needed new catalytic converters--the cats got plugged up.  Also some cam lobes have been known to get messed up sometimes on AFM engines.  

If the cam lobes are messed up wouldn’t this cause a rocker to not move? If AFM, what are common parts that fail on the system? Makes me wonder if it’s a bad wire somewhere in the wiring harness that could take months to trace or worst case scenario a bad motor..

[connord404]

Truck also starts hard. Takes about 4 seconds to finally start. 

[txab]

Check fuel pressure and injectors also

[swathdiver]

Don't let anyone sucker you into paying anywhere near full price for this truck if you like it.  Assume it needs an engine and reduce the price for a new one installed by the dealer and if they won't bargain, tell them to call you when it's running right or move on.

 

Think about the time and expense this unknown quantity is going to take to properly diagnose and repair.  I once test drove a Buick that would misfire terribly when you got on it.  The dealer was trying for full price but knew something was wrong with it but not sure what was wrong with it.  I suspected a mass air sensor, gave them a low ball offer, they accepted and drove it home.  First thing I did was pull the plugs and check the gaps, yep, they were off, put in new ones properly gapped and the car was always a beast.

[connord404]

5 hours ago, txab said:

Check fuel pressure and injectors also

Injectors were replaced on cylinders throwing codes.

[connord404]

13 minutes ago, swathdiver said:

Don't let anyone sucker you into paying anywhere near full price for this truck if you like it.  Assume it needs an engine and reduce the price for a new one installed by the dealer and if they won't bargain, tell them to call you when it's running right or move on.

 

Think about the time and expense this unknown quantity is going to take to properly diagnose and repair.  I once test drove a Buick that would misfire terribly when you got on it.  The dealer was trying for full price but knew something was wrong with it but not sure what was wrong with it.  I suspected a mass air sensor, gave them a low ball offer, they accepted and drove it home.  First thing I did was pull the plugs and check the gaps, yep, they were off, put in new ones properly gapped and the car was always a beast.

He is asking very well below book value and truck is rust free. I know the truck is a good deal i just dont want to be spending months tracing the issue .

[connord404]

Update: The guy said he was going to replace ALL coil packs and injectors this week, don’t know if it will fix the problem, but hopefully it does resolve the issue for good.