Why does my car miss and hesitate during cold weather. 2001 Mercury Grand Marquis. Feels like spark plug and bad wire reaction, but only happens during cold weather.

What do your plugs look like? They may need cleaned, re-gapped, or replaced.

If your spark plugs were bad, than it would be an all the time issue. And never clean or regap spark plugs. They have a particular shelf life for a reason. If they are fouled, or the gap is too large by age, THEN replace them. People overly change perfectly good parts all the time while misdiagnosing problems. How many miles? 4.6L? Any recent services done? Is it constant when its cold or does it improve when the vehicle warms up?

There is no need to waste money on a new plugs if the old one are just gapped incorrectly. Obviously if they are very old it would be better to replace them rather than clean them. If there was an ignition issue, like a weak spark, it could be less noticeable as cylinder temperature increase making it easier to burn the fuel.

Regardless of cylinder temperatures when the vehicle has warmed up, a weak spark would cause a rich condition to occur causing an SES illumination for a multiple DTC fault even if the cylinder temps were over 1500 deg. The only way a weak spark would be negated byincreasing cylinder temperature is if it were hot enough to 'diesel' the engine and then 'a weak spark' would have no bearing. Im also curious as to how all of the spark plugs would magically ungap themselves, mileage ner the matter, and this being the case wouldnt it run like crap regarless of ambient temperature? Also, plugs never get dirty for fun. If the plug has some form of medium needing to be cleaned from it than there is a more serious underlying issue that needs to be addressed such as a fluid leak, rich/lean fuel mixture, igntion fault, etc. Furthermore, if i were to take my vehicle to the shop and id already paid for the labor to remove the plugs to check for a concern, it would be more beneficial to replace them when, a non platinum/iridium plug only runs about two bucks a plug.

I never said the plugs would un-gap themselves but rather that they could have been install without the proper gaps being set. A too large gap combined with deposit build up could be causing a weaker spark making the engine more prone to missing. A lot of people just install plugs straight from the box assuming the gaps are pre-set.

Can cold weather prevent a Mercury grand Marquis from starting or prevent fuel getting through to engine? It does not start when the weather is cold, when the weather warms up it start. Any ideas?

Cold weather shows up all sorts of problems which is why you tune it up and get a new battery (if needed) before winter.