RepairPal is your advocate for car care advice and guidance

Q: P0306 code. misfire on cylinder 6 on 1999 GMC Yukon

started rough idling out of the blue one morning.Fires right up and drives but rough. P0306 code.Engine light flashes when driving but is steady when idling. Changed spark plug from cyl 6 to 4. Still reads misfire on cyl 6. Installed new wires, cap, and rotor. Plugs are burning clean, no signs of fouling. 179,000 miles. Fuel pump was changed 30,000 ago. Could it be the coil? There is spark going to cyl 6 through the wire. Any advice would be appreciated.
Get a Repair Estimate
Guaranteed by certified locations nationwide. Learn more
RepairPal estimates are guaranteed at over 1,700 quality certified locations nationwide. Learn more
Could be a fuel injector or a mechanical problem also.
It's not a good idea to drive it in this condition, you can damage the catalytic converters which are expensive parts.

Thanks for the response. What do you mean by mechanical problem? The compressoin seem good. Can the fuel injector be repaired/cleaned rather than replaced?
You really need to measure the compression to determine if it's good or not.
A worn cam lobe also could cause misfire, since the valves won't open all the way.
Injectors could be cleaned, you can buy a good cleaner like a BG 44K and add it to the fuel tank.
You can do the same with the injector what you did to the spark plugs, switch the #6 with another one and see if misfire follow to that cylinder.

is there an easy way to check the compression at home? or do you need a guage for this?If I buy the injector cleaner I assume I will need to drive it for this to possibly work and given the engine light flashing that could cause a problem with the converter?
Based on what you said the ignition system is working, so if the misfire is caused by a clogged injector there will be no chance for unburned fuel going to the cat. converter, so it might be safe.
There is no way to check the compression without a compression gauge. I believe the best thing would be from this point if you would get the truck into a shop. We are really just guessing without the right tools. Injectors can be pulled and cleaned with the right tool, but it's not something that the average DIY guy has at home.

I checked the compressoin with a guage and it is I have p0306 p0300 and p0171 codes coming up?????
Didn't find what you were looking for?