Why does my car misfire?

Question

I have a 2001 jeep grand cherokee laredo and i did a tune-up like about a year ago, well i got a check engine light, got it checked out and says #2 and #7 cylinders have misfire? could it be the coil? thats what the mechanic said it might be wasnt sure tho....

Answer 1

Not the coil if only the two cylinders are having problems, first insure the plug wires are tight if so replace the spark plug wires, and pull those two plugs see if they are intact, and gapped correctly, if not change them, if still not right replace the distributer cap might be a problem there. Total cost less that $50.

Answer 2

The shop technician's scanner tells him the "long and short term fuel trim ratios" which indicate whether the engine's running lean or rich. A misfire could be caused by either rich or lean air/fuel ratio imbalances. So your going to need a very competent technician who knows how to use the scanner to diagnose the misfire type and what it's due to. Many, many engine conditons can cause a misfire, so the indication of a misfire is very general and doesn't lead directly to the underlying problem. A common type of misfire is due to an ignition misfire which would probably be checked first on the diagnostic tree of possible problems the technician follows. A typical occurrence on modern fuel injected engines is that the high voltage secondary ignition spark path to the spark plug breaks down in some way so that the spark can't get all the way through the spark plug properly. Then the high voltage spark has to find some other way to reach "ground" as it normally does through the spark plug. So it ends up going backwards through the ignition coils and even the ignition module and burns them out on the way! Before you install new ignition coils and ignition modules, you have to be sure that the spark can get all the way through good spark plugs to ground.

Answer 3

Most of the time it is the wires breaking down over time. If it is one that has the coil connected to the sparkplug and no wire, yes it can be the coils.
here is how to find out.
Since the coils are all the same say the #1 and 3 are giving you the reading, Change the coil to say 2 And 4 and see if it changes the code. If it does, change the coils. If not suspect something else like say a bad plug.
Hope this helps you some.

Answer 4

It could be...
You recently changed the spark plugs, but how long were they in there before? Keeping spark plugs in too long can cause damage to the coils... which would cause a misfire.
Why did the mechanic think it was the coil? What did he do when you brought the vehicle in? You say you got it checked out, but what? Just the light? It probably would have been best to have him finish checking it out to know exactly what the problem was.