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I got code PO 305 on a 2002 Mercury Grand Marquis, what is the spark
plug wire part called This will have a direct coil over ignition with a coil over each spark plug. The coil is the electrical part over the spark plug. The coil also has a rubber part that is called the boot or spark plug boot. The boots can be purchased separately but the coils will come with new boots.
is it #5 coil,
Have you checked to see if there is any spark coming out of the number 5 coil?
and should I replace the coil and
spark plug and what should the spark plug be gapped at?
When was the last time the spark plugs where changed? It may be due for all the spark plugs to be changed. The spark plug gap is usually listed on the emissions sticker under the hood. It should be .050 - .056 or very close to that.
Depending on your Grand Marquis's mileage...it sounds like plug wires and plugs...(worn out) and or engine timing/firing order? If plug wires recently changed...the spark plug wires are in the wrong firing order...confirm the firing order from your local parts store by phone...and retrace each one...always start with #1 cylinder...hope this helps.
Check you've reinstalled the wires to the correct cylinders - if you've got this wrong, then you'll have problems.
Off hand, I'm afraid I don't know your cylinder order however - sorry, you'll have to check a manual for that one (unless someone else on here knows it off the top of their head?)
Hope this helps,