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IF YOUR MANUAL SAYS YOU NEED PLATINUM THEN YOU CAN TAKE MOTORCRAFT PLUG PART NUMBER AND BRING IT TO THE PARTS STORE AND THEY CAN CROSS REFERENCE THAT NUMBER. THEN YOU WILL HAVE SOME DIFFERENT BRAND CHOICES AND PRICES.
Your '99 Marquis requires DOUBLE-PLATINUM type sparks plugs.
Ford specifies MotorCraft Double Platinums, but other brands of Double Platinums are available.
I've had the MotorCraft Double Platinums FAIL after only 40,000 miles -- Replaced with DENSO Double Platinums to get me back home.
After returning home, I replaced the DENSOs with the BOSCH Double Iridium, fine-wire, plugs.
The correct BOSCH Double-Iridium for the '99 Marquis is the Bosch #9606, available at best price at RockAuto.com.
Proper gap is .052 - .056 -- I set mine at exactly .053, and let them "burn open"., which should allow maximum plug life while giving maximum performance.
I always stick with the brand and type of plugs which originally come installed with the engine.In your case Motorcraft double platinum. Seen too many issues surface when going with other brands other than OEM.Cheaper, copper core plugs won't last near as long, so I would not go with something cheaper.
Like No Other
In the case of the Mercury Grand Marquis, we must keep in mind that the cars are no longer being made -- and that today's spark plug technology was not available when the cars were built.
Of course, the manufacturer installs the optimum spark plugs for the engine in question -- BUT -- only the Double Platinum plugs were available at that time.
The 95 MERC came with Motorcraft Double-Platinums, which would be the correct replacement brand and type -- however, the new Iridium, fine-wire spark plug technology allows for a major upgrade to spark plug performance and longevity.
I chose the Bosch #9605 (Double Iridium) for The 95 MERC -- Fine-wire center electrode, Iridium firing surfaces. This set has been installed for over 5 years, with no issues of any kind, regardless of driving conditions. Excellent performance and fuel mileage, too.
Sure, the OEM plugs are "good enough" -- but many owners and/or enthusiasts want something better, and the Bosch Double Iridium plugs fill that need perfectly.
YES, they cost more !! Best prices at RockAuto.com.