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Fixing Engine Misfire Codes

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Engine misfire codes are another common reason for check engine lights to come on. This is one of the more serious diagnostic codes that a vehicle computer can set. Drivers with a cylinder misfire will often complain about a lack of power, sputtering or rough engine idle.

This might include a strong surging or bucking when trying to maintain steady highway speeds. Rotten gas mileage and failed emission testing are a couple side effects just to name a few. Note that multiple codes might be set in your computers memory due to the rich air fuel mixture caused by a cylinder not firing properly.

In my opinion you should always tackle the engine miss code first. A very important point to make about this failure code. If you continue to drive around with this malfunction you could eventually do engine damage or even ruin the catalytic converter by dumping raw fuel into it.

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 on OBD 2 equipped vehicles.

PO300 stands for random engine misfire. If an individual cylinder has been detected as not firing properly 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. The problem is much easier to track down then if you set the random misfire code.

Common Problems Causing Engine Misfire

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. Also a vacuum leak where too much unmeasured air is entering the engine can cause a lean mixture miss.

Do it Yourself Automobile Repair Information

But more often then not you will find a very basic failure of the ignition system that is related to the spark plugs or wires responsible for combustion detonation. Here is a real world example of this. I had a problem on my friends car where it set a code P0300 for a multiple random misfire.

I approached this Nissan check engine light diagnosis in the same manner I normally do. First thing was to clear the code and do a road-test. It ran poorly and the light came back on within about 2 miles. Next I checked for technical service bulletins.

I found only one related to engine misfire codes. It turned out to be exactly what was going on with this car. The automotive 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 of the check engine light problem was an intermittent misfire due to water intrusion around the spark plugs.

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 have a vent to let moisture out. Moisture is created from the temperature differential of a heating and cooling engine against the outside air.

spark plug wire and boot

I pulled the number one spark plug wire and saw that the tube was filled with water. 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.

Properly 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 plugs and ignition 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. This diagnostic code can set as a history code and viewed by the inspection station.

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