how do i check the valve lifters in a 1988 cougar v6
They are probably hydraulic lifters and over a period of time sludge blocks the orifices. To clean them you need to remove the rocker arm and .... if you are lucky they will come up and out with a magnet ... however since your car has seen a few years and perhaps mileage ,,,you may need to borrow a tool to get them out because in addition to sludge some lifters "mushroom" at the top and are very difficult to pull through the opening
If you are about do for a oil change leave out one quart of oil and add Lucas Engine Oil Stabilizer or if just did a oil change remove one quart of oil from engine then add Lucas the tap will go away.
If lifters are tapping I would check the oil pressure with a mechanical gauge first. It would be unusual for them all to start making noise at once due to sludge.
Looking for a Used Cougar in your area?
CarGurus has 199 nationwide Cougar listings and the tools to find you a great deal.
Search Mercury Cougar Questions
Mercury Cougar Experts