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Q: Misfire at cylinder #6 and EVAP system not ready

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Hi my gmc sierra have new coil new spark plugs good compression 150 psi new all ignition systems and i can't find the problem on this misfire also new ecm and other is evap system not ready New purge valve new vent valve new fuel cap and i am tired to try to find the problem and my scanner mark the freeze frame is for P0306 and with other scanner the only code is P0300 But the truck run fine at start in the morning not misfire not nothing after 50 seconds. start with the misfire and i press gas pedal at 1500rpm and misfire go away not vacuum leaks and i don't know if this misfire make all this problem all monitors are ok only evap monitor system not ready thanks guys i hope you can help me with this problem ahh other thing the stabilitrak come on when purge valve start to make vaccuum and after press gas pedal the stabilitrak light go away

My car has 255000 miles.
My car has an automatic transmission.

A: Hello. The misfire that you are getting on ...

Hello. The misfire that you are getting on that cylinder may be caused by a dirty or failing fuel injector. If compression is fine and if there is good ignition then it may be a fuel issue. If the engine tends to run fine, then it may just be a bad crankshaft position sensor that is reading incorrectly. I usually start by checking the signal from the crank sensor and doing a balance test of the engine to see if all cylinders are working correctly. If not, then I can narrow down the issue. I have seen a lot of problems with lifters going bad and not opening the intake valve enough on this engine as well as excessive carbon build up causing this. If you are going to have this cylinder misfire repaired, consider YourMechanic, as a certified mechanic can come to your home or office to diagnose the misfire and follow through with proper repairs.

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