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I have a 2002 Ford Focus that has been intermittently misfiring at idle and sometimes stalling out. I’ve taken it to two mechanics - one replaced the ignition coil, which did not help & the other (the dealer) could not catch the misfire long enough to measure what might be wrong so they told me to drive it until it gets worse. The problem seems to decrease when I pop it into neutral. Any ideas?

Next time the problem occurs, have the diagnostic code(s) read from the car’s onboard computer and provide them to the mechanic. Since this is an intermittent problem you need to get the codes read quickly after the problem surfaces, otherwise the computer will clear the error codes after a few cycles. Many automobile parts stores will read these codes for you free. Also post the code(s) on this web site to have some professional independed assessments.

The intermittent rough idle and stalling problem sounds a lot like a vacuum leak. Stick your nose under the hood and listen for some obvious hissssssing sounds when the engine is at idle. Also look at all the connections between the air filter housing and the throttle body. Make sure all these connections are air-tight. Any stray air that gets sucked into the throttle body or intake manifold via loose/broken vacuum lines, gasket leaks, or leaking fuel injector manifold gaskets will result in the problems you have identified. If you do find a suspect leak, spray some carburetor or throttle body cleaner around the suspected leak. This will cause a temporary “fix” in the leak and the engine will idle properly for a short period of time. Of course there are numerous other potential fuel / electrical causes, but start with the ones that that you may be able to find yourself. Good luck!

How many miles are on it?
What is the full maintenance history of the car in terms of tune up-type items (plugs, wires, filters, etc)?
Is the check engine light on and have any codes been read from it? What codes? (format: P1234) Does the light ever flash?
Has anyone checked the fuel pressure?

I had the plugs and wires replaced about a year ago & when my last guy was trying to figure it out he replaced the wires again - but that wasn’t the problem. It’s had regular maintenance every 5,ooo miles - it’s currently about 68K. The lights come one when it stalls out but nothing else. I will have the codes checked the next time it happens. Thanks!

After many frustrating trips to dealers & private mechanics - w/no codes to go on- I think it’s finally been fixed. The last guy I took it to replaced the air flow meter/valve and that seems to have done the trick. Thanks all!