I understand a blinking CEL means a misfire. I took it in to the mechanic today and he changed the air filter and added a fuel additive. He said my sparkplugs were good. This can be so many things. The car is an 02 3.8 L Special Edition with only 22,500 miles on it so I have no idea what to do.

 

A P0300 code was pulled with a scanner, no specific misfire codes were pulled.

-Fully stock car no tunes, not turbocharged.

 

If I recall, P0300 means "multiple mis-fires". A good scan tool, it can actually report a live stream of misfire counts and from which cylinders while the car is running. That data would help narrow things down a bit. Such as if two specific cylinders are high in mis-fires, it *could* be a coil pack. My next step would be to trade the coil with another one on the car and re-check if the misfires follow the coil. If it does, odds are pretty strong it is a bad coil. But if it doesn't, it *might* be an Ignition Control Module (ICM). Some auto parts stores can do a test on the ICM.

Also, don't forget the air filter, double check that is not plugged up with dirt and debris (if so, pop in a new one).

Could also check if any of the wires are worn and potentially arcing (but I am doubting that since this is linked with higher speeds). Could also double check the gap on the spark plugs and ensure that is correct. And how old are the plugs? They might "look" OK, but if they have over 100k miles, they should be swapped out just for general maintenance.

Some food for thought.

 

Thats unfortunately very true with that code. The next step is to use a scan tool to read the sensors in real time and figure out what's going on. IE what cylinders are misfiring, if theres any pattern to them, what the other sensors are reading when it happens, etc. Without that diagnostic data you'll just be shotgunning parts in the hopes you replace the right thing (ie air filter and dumping additive in).