2003 Silverado 1500. 5.3. Getting code P0300 and P0306 mostly P0306. Missing at idle on cyl 5 and 6. Worse when warm. Missing above idle at 6 only. Only occasionally missing on 6 at above 1500 rpm. Compression test 6= 150lbs. 4=180 lbs., 5= 190lbs. Did the following: Major tuneup including wires, plugs, filters etc.. Cleaned throttle body. Cleaned and tested maf. Swapped maf with known good no change. Swapped coils, plugs, wires and injectors to other cylinders. Same code. Replaced cps and performed cps relearn. Checked for bad ground. Pulling the little hair I have left out. Suggestions appreciated. Thanks.

 

perform a cylinder leak down test and see if you see any bubbles in the coolant, or if you hear air comming from the intake or oil fill. how many miles are there?

 

150k. Will try this week.thanks.

 

A code P0300/0306 may mean that one or more of the following has happened:

Faulty spark plugs or wires
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector(s)
Burned exhaust valve
Faulty catalytic converter(s)
Stuck/blocked EGR valve / passages
Faulty camshaft position sensor
Defective computer

Possible Solutions
If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

Random misfires that jump around from one cylinder to another also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open

 

try looking at this post...
https://www.truckforums.com/forum/sh...ad.php?t=16007
I was having similar problems with clyinder 1 (code p0301) and turns out the plug was bad out of the box... go figure...maybe this is a cheap fix for ya?