Engine misfire codes
Engine misfire codes are another common reason for the check engine light to come on. This is one of the more serious diagnostic codes that your vehicle computer may set. When you have a cylinder misfire, you will receive poor drivability and rotten gas mileage. Also note that multiple codes may be set in your computers memory due to the rich air fuel mixture caused by a cylinder not firing properly. Always tackle the engine miss code first.
Damage to the cylinder head and valve train can also occur if the miss is caused by a lean condition such as a vacuum leak. Engine misfire codes are in the P0300 range. PO300 stands for random misfire. If an individual cylinder has been detected as not firing a P0301 would set for cylinder #1 and a P0308 would set for cylinder number eight. If you have set a code for a misfire on a particular cylinder consider yourself lucky. This is because the problem is much easier to track down then if you set the dreaded random misfire code.
Reasons for engine misfire codes
There can be several different malfunctions that will set a misfire code. You can have a leaking injector or even a clogged fuel injector, causing an imbalance in a cylinder's air fuel mixture. You can also have a vacuum leak were too much air is mixed in with the fuel charge. But in most cases you will find a basic failure of the ignition system that is related to the spark plugs or wires responsible for combustion detonation. I had a recent problem on my girlfriends Nissan Ultima where it set a code PO300 for a random misfire. I approach this Nissan check engine light diagnosis in the same manner I normally do. I checked for technical service bulletins and found one related to engine misfire codes. The technical service bulletin stated that the affected model years for the Nissan ultima are 1995 through 1998 equipped with the four-cylinder engine. The cause for the Nissan check engine light to illuminate was an intermittent misfire due to water intrusion around the spark plug tubes.
Nissan check engine light
On Nissan's four-cylinder engine, the spark plugs fit down inside of tunnels in the valve cover. The spark plug wires have tubes and rubber seals that prevent moisture from getting in.
The spark plug tubes and rubber seals also have a vent to let moisture out. This moisture is created from the temperature differential of a heating and cooling engine against the outside air temperature.
The little check valve vent in the rubber boot that seals against the valve cover was stuck closed and allowing water to collect around the spark plug and wire connection. I used compressed air to blow out the water, and I replaced the spark plug wires. These original equipment spark plug wires were about 10 years old and had close to 100,000 miles on them. Nissan makes a fantastic product, but nothing in the automotive business will last forever.
Diagnosing check engine lights
The lesson to be learned from my experience with this Nissan check engine light is that no matter how complicated the vehicle becomes in most cases the failures are located in the most basic systems on the vehicle. Spark plug wires have been causing engine misfires ever since the invention of the modern high-energy ignition system. When you set an engine misfire code remember to start with the basics and check things like spark plugs and spark plug wires. You can also check for service bulletins related to your set diagnostic code. Chances are if you are having a problem many mechanics before you have also discovered the cause of this very same problem. An engine misfire code cannot only do engine damage if it is left undiagnosed and repaired. But you will also fail an emissions system test due to a rich mixture. If you live in California or Arizona, you will also fail the smog check because this diagnostic code is set as a history code and will be read by the inspection station.
 Custom Search This Nissan check engine light and engine misfire codes page is just one of my many articles from my check engine light help section. To learn more about the common problems that may cause your service engine soon light to illuminate visit my page with more helpful hints on how to repair
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I pulled the number one spark plug wire and saw that the tube was filled with water. 
