2001 Mercury Sable misfiring at stop

My 2001 Mercury Sable wagon with Duratec 24v V6 had a misfiring at stop, and then progressed to missing at all speeds and check engine light came on. This was a week after I switched to a new locked gas cap, having run over the previous locked cap. So, I switched back to the original Ford cap and that did not correct the proble.

Went to my independent mechanic who ran computer analysis for $105 and said that code P0303 indicates cylinder #3 misfire, so he swaps plugs from #3 to #5 and then from #3 to #1 and it still misfires, and says this indicates I need a new ignition coil, which he installs for $210. This was on July 7th and now the same misfiring is starting again but without check engine light which will likely happen next week. Car has 44K miles, recent oil change, air and fuel filters changed at dealer 12 months ago, and original plugs that he says were OK. When I turn off AC at the problem goes away, but that's what happened the last time and it gradually became worse.

I suspect gas cap was/is the culprit, but changing that before did not help.

Should I do an injector cleaner additive and see what happens? Change gas caps back to non-locking original? I have a 12 month warranty on this most recent igition coil. In 2005 Mercury dealer changed coil pack due to code P0301 misfire cylinder #1 at 24K miles in 2005. I do not drive much - mostly short city trips and this is very annoying and expensive to have to deal with and now the car is out of warranty.

Could it be ethanol related? If so, would using mid-grade gas help?

yes you should run sea foam threw your gas it will clean out your injecters.

Avery,

I have the same problem on my 2002 Merc sable with 100,000 miles. I've cleaned my injectors with no improvement.

I do know though that most gasolines now contain up to 10% ethanol everywhere you go. Purchasing a higher Octane fuel still contains ethanol though. Higher octane fuels simply protect against spontaneous combustion of the fuel under pressure (ie. before the spark plug has a chance to ignite the fuel at the optimal time, known as "knocking")
I don't think that's your problem.

If you get the misfiring to stop let me know though.

Guys any solution to this mis-firing? My sons 2002 Sable with the 24 valve DOHC Duratec is doing the same. Usually, mis-fires under light acceleration, but getting worse. Is there like a top three causes, example fuel filter, ignition coil, and plugs. Just want to try and troubleshoot in order.

Thanks,
AvalonJohn