This 2005 GMC Sierra came in with a poor run condition and multiple misfire codes stored. Code P0304 was one of the misfire codes stored. While looking at data I noted that misfires on cylinder #4 were markedly higher that most of the other cylinders. Normally when confronted with high misfire counts on this engine style I would suspect a leaking fuel pressure regulator. Fuel would be leaking fuel though it’s diaphragm into the attaching vacuum line that happens to enter the intake just above cylinder #4. Not the case in this situation. Notice how the number four spark plug wire appears to be pulled tight.
A close inspection of the wire and boot revealed a carbon track on the top of the spark plug wire boot. Just above where the wire enters the boot. Click on the next picture to enlarge for a better view.
Although this problem was easily diagnosed with a scan tool, I wanted to show some more simplistic ways of finding a misfire. Water is a great diagnostic tool for finding leaking spark plug wires. Many times I diagnose faulty spark plug wires by asking the customers the right questions. Does it run bad first thing in the morning? Water in the form of dew plays a part in morning misfires. Was it running bad the other day when it was raining? Running poorly on a rainy day but running well a day or two after the rain stops is a clear indicator of leaking spark plug wires. Well let’s introduce some water to this situation and see what happens.
If you will enlarge the next pictures I am pretty sure you will be able to see the misfire in progress. Notice how the appearance of the spark changes as the amount of water decreases. A faint orange glow in the small puddle of water on top of the spark plug boot.
A bright orange spark.
A faintly orange, mostly white spark. When the moisture evaporated away the misfire went with it. Of course that was at idle with no load on the engine also.
Another way to find leaking spark plug wires is to attach one end of a wire to a screwdriver and attach the other end to battery ground. Then move the screwdriver tip along the spark plug wire in question. Look and listen for spark jumping to the screwdriver tip.
The fix for this truck was just a replacement of spark plugs and spark plug wires.