A misfire under load complaint can usually be broken down into 2 categories. The "soft" misfire, often called a surge, is commonly associated with fuel related issues. The "sharp" misfire, on the other hand, frequently relates to ignition problems.
In diagnosing the misfire, some information is needed about the maintenance of the vehicle. Are the spark plugs due for replacement? Has the fuel filter been serviced?
Next we need some detailed information on the conditions under which the misfire occurs. Included in this information we need to know the temperature and the weather conditions. Useful too would be knowledge of the transmission gear selected, the amount of throttle being applied and certainly the speed of the vehicle.
So let's set up a scenario for our "misfire under load" complaint. You're driving on the highway at approximately fifty five miles per hour. It is a nice sunny eighty degree day. You have just started climbing a moderate grade. The vehicle loses a little speed so you apply more throttle. The transmission downshifts and the engine revs up to about three thousand rpm. Within a couple of seconds you feel a sudden lurch or bucking. You look at the dashboard instrument panel and see first a flashing, then a steady "check engine" light.
In our scenario we have more clues which your mechanic will want to know about. Did you note the "sudden lurch or bucking"? And what about the "check engine" light? Remember to let your mechanic know that not only is the check engine light on, but before it came on steady it was flashing. Now using these clues we can perform more specific tests to locate the root of the problem. First we need to find out what "code" is being stored in the vehicle's computer. The check engine light indicates our vehicle computer has detected a problem. When the problem is detected and the check engine light comes on, the computer stores this information as a alpha numeric code.
We now plug in our scan tool to retrieve the code. For the sake of this scenario we will say code P0303 is stored in the computer's memory. To break down what this code means we can tell that the "P" refers to Power train. The zero three zero three indicates the problem is a misfire affecting cylinder number three.
Next we perform a visual inspection under the hood. Is there any sign of physical damage to the wiring or hoses? Are any broken or disconnected parts noted? We now should start and drive, if possible, the vehicle. Is the problem present at this time?
Let's say the misfire is present but no under hood damage can be seen. We need to do some deeper investigation on the engine's number three cylinder. If possible we should remove the spark plug. As we remove the spark plug it is important to note the physical condition of the coil and spark plug wire. And there we see it. The spark plug wire has a burned area where the spark has been jumping to the engine cover, never making it to the spark plug.
We replace the wire, road test the vehicle to confirm the repair and finally clear the code from the computer.