2001 Mazda Tribute with a 3.0 liter v6 engine Vin 1 and codes P0300 and P0304 stored. The P0304 code is very specific to cylinder #4 so that is where I started. After finding that cylinder #4 was one of the easiest to access I was quite happy. I first removed the coil cover on the front valve cover in order to gain access to the coils and spark plugs.
After the cover was removed I first pulled the #4 coil and inspected for problems. Number 4 is the cylinder to the extreme left of the picture.
When I removed the coil retaining bolt and pulled the coil I inspected for any signs of arcing which I found. The arcing was bad enough that it actually left marks inside the spark plug tube.
You can also see the arc marks on the spark plug boot near the bottom of the picture.
Easy enough but when I pulled the spark plug I knew I had more problems. A new plug and it was not properly gapped. Some one had tuned it up but done a poor job of it. Time to replace all of the spark plugs and boots and inspect for any damage along the way. The rear spark plugs require that the intake plenum be removed for access. I chose to only lift it out of the way as needed. The first thing that had to be done was to remove the egr valve from the plenum. I removed the two bolts and positioned it slightly out of the way. Next I removed the hold down bolts in the center of the plenum and lifted. Check for hoses, wiring and linkage that may restrict movement.
After lifting the plenum up I placed clean shop rags in the intake ports to prevent anything from falling in the engine. This is extremely important because if you drop something in the intake and cannot retrieve it, the engine will have to be disassembled or the engine will have catastrophic damage.
With the rags in place I could reach under the plenum and remove the coils and plugs. I inspected and wiped down the plenum gaskets, installed a new egr valve gasket and reassembled.
This is what I found in one of the back cylinders. The spark plug had not been properly tightened and combustion gases were leaking past the seal and causing what you see in the following picture. This is why I try to use a torque wrench on every spark plug I install. I also re click several times to make sure that the gasket is compressed properly.
Thankfully I had enough experience to know that I could have a problem with the coil on this cylinder and I reinstalled it in one of the front cylinders that are easy to access. Guess what I had a misfire in the new cylinder that I placed that coil in. Now it was easy because I had put it in an accessible cylinder. Tricks of the trade.