I just bought this rig (a 2000 Discovery 185,000 miles) and the seller told me the head gasket was bad hense the low selling price. After repairing the head gasket and driving it 40 miles I got a code reader and indicated code P0306 misfire cylinder 6. I then replaced the coils and plug wires it already had new plugs. I am still getting the P0306 code. I have switched spark plugs and no improvement. The spark plug in number 6 is colored different than the rest. The porcelain on the rest of the plugs is pretty much white all the way around. The porcelain on #6 is half white and half black. what do you think is causing this? There is no town driving. All my miles are at 55 to 65MPH and the engine runs at about 180 degrees.

 

Hard to give a good reading without pictures. Does the check engine light flash? If it does i'd stop driving it till you get this sorted. A bad miss can burn out a catalytic converter quick.

To be clear, the head gasket was replaced, correct? Not some head gasket in a can poured in the radiator?

My next check would be to use an old spark plug and make sure that cylinder is getting good spark, but if the plug is showing color that's a fair bet it is.

Up next would be a compression test which will tell you if that cylinder is making the same compression as the rest. Not a bad idea to check all 8, but I'd at least check all four on that side of the motor.

And if that checks out good, then I'd try swapping the #6 fuel injector for another one and see if the miss moves cylinders. If it does then you've got a bad injector.

 

The check engine light does flash occasionally but usually just comes on and stays. The miss is not bad enough to hear it or feel it and I can not feel a loss of power. Yes both head gaskets were changed. I have done a compression check it was 145# after 3 cycles. I should do the other 3 on that side for a comparison. This engine is just not easy to work on. I want to do all the easy stuff first.

 

This engine is much easier to work on than almost everything else of that vintage. Test compression, swap plugs with another cylinder, and then swap injectors if the plug swap does not fix it. 145 is a tad low, squirt some oil in the cylinder and rerun the test.

 

How do you swap injectors without taking the top of the engine off? That takes me about four hours one way.

 

you’ll get much faster at it. You’ll have to remove the intake to swap injectors. It’s not super fast, but it is the cheap way to do it if you can do it yourself.

 

Should be able 20 mins to get the top intake off. Lose the SAI tube connection to the intake, that is not needed. Air intake tube needs removed. DS is the tb connector, idle air connector, just dismount the SAI solenoid from the bracket, vacuum purge connection. PS is PCV, SAI vacuum only. Lift off upper intake. Injectors are right there. Bleed pressure at schrader valve, unbolt injector manifold and it pops right up. Lube the injector o-rings with grease to get them to go in easier.

 

Ok I am going on the clock. I will shoot for an hour. Another question: Is there a history of the fuel injection wiring harness (connectors) going bad?

 

No but there is a history of injector O rings going bad.

 

Well I don"t smell gas. When I swap my questionable injector hope the o-rings work. All I do is wait for parts. I am sure that hydraulic o-rings would not hold up to this new gasoline.