My Ford Escape was just serviced at 120K Miles. After about two weeks it started shuddering between 40 and 55 miles an hour. It got worse and after about 4 weeks the Check engine light came on. I took it back to my mechanic and the read out said it had a misfiring piston. What does this mean exactly? How does one fix it, prevent it from happening again and most importantly is it going to cost a lot?

Let me make sure that I am clear about the situation.
You took it back to your mechanic, he read the stored trouble code(s), he told you that you had a misfire in one cylinder, and he did not tell you to leave the car for him to do the necessary repair work?

If the above scenario is what happened, then I want to suggest that you immediately take the car to a different mechanic, as you should have been advised that it is very unwise to continue to drive the car with a misfire condition. In addition to the probability that you will damage the expensive catalytic converter, there is also a chance that you can cause even more expensive damage to a piston or some valves with a continuing misfire condition.

Ergo–the ultimate cost of not doing the necessary repairs promptly is considerably higher than the cost of fixing whatever is necessary right now. This could be as simple as bringing the car up to date with its maintenance or replacing one bad spark plug, or it could be much more complicated to resolve, but this is something that needs to be corrected pronto, unless you are eager for a really big repair bill.

[i] said it had a misfiring piston.[/i] 

I hope that is not exactly what he said. ?

I left it with him to repair, but I was wondering just exactly what a misfiring piston meant, what makes it misfire and what can do to prevent it in the future. Damaging the catalytic converter sounds expensive. What causes this? and why would it happen just a few weeks after a major srvice?

Like VDC I am confused. Is this “mechanic” at some kind of a corporate auto repair chain? Didn’t this mechanic explain it to you, explain the potential remedies, and possible costs?

There’s no way anyone can tell you from a distance what it will cost. It might be a $5 spark plug or wire or a $4000 engine.

The short story on what it means is that the car shoots air and fuel into the engines cylinders (and that’s just what they are - hollow metal cylinders). A piston compresses the air and fuel putting it under high pressure, a spark plug makes a spark, the whole mixture exploses and pushes the piston down in the cylinder. That downward shove is what makes the car go.

A misfire happens when all of that doesn’t work out right. (compression problem, fuel problem, air problem, spark problem)

The most likely reason that it happened soon after service is that they messed something up.

When you hear back from them about it, ask them what the exact error codes were. They look like “P1234.” If they try to give you words instead of a code like that ask them again for the actual error code. Then post it along with their diagnosis up here.

Also include the vehicle year, engine type/size, some info on how well you have kept up with regular maintenance and give some clue as to the type of shop its in - corporate chain? Local, independent? Dealer?

Cigroller has given you a very good explanation of what is meant by a misfire.

In order to prevent it in the future, you need to do the following:

Maintain the car at least as well as the mfr specifies. Maintaining it better than specs is preferable, of course.
Use the grade of gasoline that is specified for this engine, by the manufacturer
Don’t ignore any new noises or symptoms. Get new situations investigated immediately.

Why would it happen a few weeks after a major service?

This could just be a case of coincidence and/or an engine with an accumulating number of miles on it.
And, in conjunction with the car’s accumulated mileage, it could be related to the overall maintenance record of the car over the past X years.

We have no idea of what service was performed at 120k miles, or what services have been performed prior to 120k miles. Even if you did what was specified for the 120k service, if prior maintenance was ignored, the latest service does not wipe out potential damage that was done by lax service prior to 120k.

Or, it could be related to something that was not done properly at the time of servicing.
However, a mistake made when servicing the car would most likely have shown up sooner than 2 weeks later.

Hi Folks
Thanks for the explanation, very helpful. I have been very good at maintaining the Ford since I got it brand new in 2000. It’s a 2001 Ford Escape. I get the oil changed every 3K miles and the scheduled regular maintenance. I do not take it to the dealership as that has proven very costly and they keep wanting to sell me stuff I don’t need.

My mechanic has always been fair and I have been taking my cars to him for the past 15 years and have not had reason to be concerned. I have just had a chat with him and he said the code registered it as a misfire on Cylinder 4, he wiped the code and now is trying to get it to repeat to make sure and the damn CEL is not coming back on. He just said he wants to be sure that that was the cylinder so that he can focus on it specifically and not just start changing out parts as a gamble to fix it. So he has the car for a couple more days and I am praying hard that it’s just an errant spark plug!