When the Check Engine light is flashing, you're supposed to stop the engine right away. It means too much raw fuel is going into the exhaust system where it will overheat the catalytic converter and damage it. That's an expensive repair. A spark-related misfire is the most common cause of too much fuel in the exhaust.
The first thing I would do is switch the number four spark plug with one of the other ones, erase the fault code, then see if it sets for the same cylinder or the one you put the suspect spark plug in. If it moves to the new cylinder, check the plug's gap first. Be aware too that a lot of engines aren't happy with some brands of spark plugs. In particular, when people think they're upgrading to something exotic like platinum plugs that didn't come originally in that engine, they often end up with misfires and driveability problems.
You might suspect the plug wires too, but keep in mind that each ignition coil fires two spark plugs in series at the same time. A break in one spark plug wire would cause two cylinders to misfire, ... Usually. You also have the variable of which of those two cylinders is coming up on top dead center on the compression stroke. That spark plug is going to require more voltage for the spark to jump the gap. What that COULD mean is both of those cylinders get no spark when number four is supposed to fire, and both of them DO get spark when the mate fires and number four is on the exhaust stroke, (and it doesn't use the spark). Try tightening up the gap a tiny amount on either one of those spark plugs. That will make it require less voltage to create a spark, and will leave more of the available voltage for the other plug.
A weak ignition coil will do the same thing but that typically escalates to both cylinders misfiring very quickly. Internal arcing creates carbon which conducts current and shorts out the voltage for the spark plugs.
Don't overlook arcing under the boot on the spark plug wire. You might hear that as a snapping noise, and you can see it at night. Also be aware the Engine Computer will detect misfires that you can't feel yet. Consider a compression test and a cylinder balance test. The balance test shows whether each cylinder is contributing an equal amount of power. Performing a balance test is as easy as it was many years ago, but today you need a scanner to view live data, in this case, idle speed motor "steps". A worn lobe on the camshaft can cause misfires too, but for now concentrate on the spark plugs because all the other things I mentioned won't result in too much fuel in the exhaust.
Sunday, August 24th, 2014 AT 12:28 AM