I’ve got a misfire problem with my Focus. It started about a month ago and did not trip a code until last week. It stumbles on acceleration between 2,000 and 4,000 rpms. It is only a slight stumble and although noticeable, it does not impede driveability much. Last week I was able to get it to finally trip an OBD code. P302 - Cylinder 2 misfire. I changed the plugs and wires with no effect. I thought I had it narrowed down to the coil or injector until my wife noticed a new wrinkle.
Under normal driving conditions the misfire is annoying, but my wife found that if she turns on the A/C, it is undriveable and will not accelerate past 30 mph without some patience and a neck brace for the bucking. My first thought was the A/C compressor was freezing up. I turned off the A/C and left the blower on and it would not stop missing. I turn off the blower and it smooths right out to the annoyance I described first.
Right now I drive the car without A/C and blower whenever I accelerate. If I’m going a steady speed, all is well and I can run the blower and A/C.
I thought I had it narrowed down and I was going to buy the $80 coil tomorrow, but now I just don’t know. Can anyone help?
Before purchasing any parts, purchase an aerosol can of MAF sensor cleaner and give the MAF sensor a good cleaning. Misfire codes and poor engine performance can be attributed to a dirty MAF sensor.
Diagnosing a bad ignition coil is a simple matter of checking the OHMs (resistance value) of the primary and secondary of the ignition coil, and comparing reading to the specified value. Of a matter of fact the same can be said about testing the spark plug wires. Throwing parts at the problem can get expensive. It might be a good idea to run a compression test to rule out any engine problems.
Just because you get good resistance readings on the coil does not mean its good. The ohm meter is only using 1.5 to 9v depending on model to test for conductivity. The coil operates at much higher voltages and could be suffering from an insulation breakdown.
You can test coil to ground with a megohm meter, but that does not test for any breakdown between the primary and secondary or any turn to turn shorts. A TTR (transformer turns ratio meter) can detect any turn to turn dead shorts. Not many shops have meggers and TTRs laying around. Now if you have a buddy that works at the local utility, you might be in luck.
True using a ohmeter may not necessary always confirm a good coil, however if the coil or spark plug wires does not ohm out to the specified value it will confirm a bad one.