I have a 02 Subaru Outback that my mechanic can not fix. I failed my emmissions test and the codes that came up showed that cylinder 3 and 4 are misfiring. I have had this problem for about 8 months. The check engine light will come on and I will take it in and the misfiring cylinders codes will show up but the mechanic says that everything looks good. He clears out the check engine light and then two days later it is on again. The check engine light comes on after I accelerate very quickly, like trying to get into traffic or passing a car. I don’t understand why he says he can’t find anything when there is obviously a problem. I don’t want to keep brining it in but I don’t know what else to do. What could cause the cylinders to misfire but not show any signs when the car is checked out?
I’m afraid the answer is that you need to try a different mechanic. There are a lot of reasons that a cylinder could misfire, such as bad spark plugs, bad spark plug wires, clogged fuel injectors, leaking head gasket fouling the cylinders, or even a faulty engine control computer. We don’t know what your mechanic did to “check it out” so it’s hard to be more specific other than to say that he hasn’t done a very good job of “checking it out.”
The problem might be with a dirty or defective Mass Air Flow sensor.
Have your mechanic first try cleaning the MAF sensor with an aerosol MAF sensor cleaner. If that shows no improvement, then a scan tool that can monitor real time data should be used to compare the amount of air the MAF sensor is detecting relative to the throttle position as the vehicle is driven. So for example if at 100% open throttle the MAF sensor only detects 75%-80% mass air flow, there’s a problem with the MAF sensor.
You say the mechanic states that “everything looks good”. That doesn’t tell anyone much.
There’s a lot of things that could cause a 3 and 4 misfire but there’s not enough info provided to make much of a guess.
Spark plugs and/or plug wires, coil pack, low compression (engine wear, tight valve lash, head gasket going south), etc. etc.
The first step is to perform a compression test. One always weeds out a mechanical fault first; and given Subaru’s history of head gasket problems and the potential of tight valve lash checking the compression is a must.