hi everyone,,, new guy here
my 2007 grand marquis (4.6L) is getting a misfire code cylinder 5 and 6. I would like to replace spark plugs to begin.
I thought best to use a motorcraft plug,
Question, would the SP405 be best choice ??, its fine wire platinum,, or would SP493 standard platinum be preferred?? or is there a better choice. Appreciate any advice you can give on plug type and my misfire problem. Thank you, Dave

 

Hello, Dave -- and welcome to the Mercury Forums.
After many years of Grand Marquis ownership, there have been dozens of discussions regarding Spark Plugs on this and other forums.
Many owners choose to stick with "Original Equipment" plugs, which is always a good idea -- but doesn't take into account that spark plug technology does not freeze on the day your car was built. Technology marches on !! Your 2007 is now 14 years old, and "better" plugs are available.
Many people make the mistake of buying by PRICE, which is never a good idea. Sometimes, "you get what you pay for", and sometimes you don't, as there is high-priced junk for sale every day.
Others buy by BRAND, which doesn't allow for the differences in models of the same brand.
I have chosen to buy spark plugs after doing research and experimentation -- leading me to the BOSCH Double Iridium, which have performed excellently for several years.
The early models of the 4.6 Litre (2v, with ignition wires) use the Bosch # 9605, while the Coil-On-Plug engines (like yours) take the Bosch #9606.
I recently pulled a set of #9605s out while doing an intake manifold gasket replacement -- 40,000 miles on the plugs -- all firing perfectly, with only a very slight wearing of the electrodes -- probably could go another 40,000 miles, but I replaced them with a new set of #9605.
Factory gap specs = .052 - .056 -- ALL-WAYS check the gap, regardless of any "pre-gapped" claims made by many plug makers.
I set the #9605 gaps to .054. When installing new Bosch Iridium plugs, DO NOT use any anti-seize -- the plugs are plated to guard against thread galling. I use full synthetic motor oil to lube the plug threads. Tighten plugs with a torque wrench -- 140 INCH-LBS -- to be sure all plugs are running at same temperature, firing equally, and doing equal work. I always CLEAN each plug body with alcohol before installing, to prevent carbon-tracking and misfires.
Before installing plugs, a dab of Dielectric Silicone Grease in each spark plug boot keeps moisture out.
To avoid issues with "bootleg" Bosch plugs, get 'em from RockAuto. -- Amazon sellers often sell the "bootleg" plugs -- AVOID !!
Your misfires MAY be caused by faulty coils. Replace any bad coils with OEM Motorcraft coils, OR a full set of "performance" coils from MSD.
I am running the original FoMoCo coils -- Removed, cleaned, and tested -- now with 167,000 miles, and running strong.

'95 MERC Engine Bay

BOSCH Double Iridium Spark Plug:
'92 - '97 4.6 Litre = Bosch #9605
'98 - '10 4.6 Litre = Bosch #9606