Just like you would do any other car, change and gap spark plugs and torque only to 10 ft. lbs. and only do the work on a cold engine, you don't want to strip the threads on the aluminum heads. Change the PCV valve, check the spark plug wires for cracks etc. and replace as needed. Look down the spark plug tube and if you see oil then the O rings are going bad in the valve covers. Look over the two coil packs and check for cracks etc. and replace as needed. This is for the 1st generation Marks, the second generation Marks (97-98) are a little different, they have coils on each of the spark plug wires. You didn't mention what Mark you have.
every 100,000 miles
go to markviii.org. they have a answer for almost anything with the lincoln mark viii
around 360 bucks
I get mine done at the dealer for $354
New plug new wires OEM 2 new coil packs Spark some clean on them master flow sensor
You can't unless it's electric, otherwise just pull up on the switch on the side of the seat.
From $400 to $500.
Needs a Tune up
The cost to tune up a 1998 Lincoln Navigator depends on the needs of the vehicle, the cost of supplies, and the cost of labor. There is not a standard price for the tune up. A mechanic can give an estimate for the job.
usually manual says when it should be done...
After you get the tuner hooked up, you play a string and if the needle goes to the left of the mark the string is flat and you tune the string up. If it goes to the right of the center mark it is sharp and the string needs to be tuned down. ☺
Could be time for a tune up.