I had an experience with a 1984 T-Bird with a 302. The 1986 may have the same system. With that car there was a temperature sensor that affected the idle speed once the car was close to warming up.
With even later models there is a "cold start sequence" which is an electronic way of replacing the ancient choke. The "cold" feature set the fuel mix and the RPM and made starts easier. Now you did not say if the starting problem was when hot or cold or both.
The poor performance when slowing down can be related to the idle rpm being set too low once the thermal switch cuts out. What happens is someone tunes the engine for a rpm that may be correct for the car, but the thermal switch has not dropped the rpm yet. Then when the car really heats up, the idle drops even lower and when you take off your foot from the throttle, the throttle closes too much and the engine cuts out when you are slowing.
Put in a new thermostat. The idle settings can not be set until the thermal switches activate to a run position. Also you may have some vacuum lines crossed which are making the cold and hot cycles reverse. There should be a label with vacuum line routing shown stuck on a part that holds the radiator.
SOURCE: 1986 mercury grand marquis LS overheating
Any car sitting that long is going to have some issues. It could be several things. The thermostat could be stuck closed. The radiator could be stopped up. The fan clutch could be inoperative. The waterpump impellar could be deteriorated to the point that it will not circulate coolant. The worst would be that a head gasket could be bad and causing to overheat and misfire. The first thing to do is replace the thermostat and purge any air from the system. Use water and not coolant to verify the repair. If this solves your issue you can drain the water and fill with proper amount of coolant. I must recommend checking for white cloudy smoke from the tailpipe as it is a sure fire sign of a blown headgasket.
SOURCE: 2007 Mercury Grand Marquis required oil weight
If u have a moderate climate, 10W-30W, if cold climate 5W-30W, do not use any heavier weight of oils.
SOURCE: 95 mercury grand marquis firing order of spark plug wires
go this page for firing order, u have 4.6 L V8
http://www.moddedmustangs.com/reference/firing-orders.php
SOURCE: 2001 mercury grand marquis. Engine stutters or
Start car and use a squirt bottle of water to wet the wires and coil, if you see it spark or run rough one or both are bad, repair the wires first as they are more common.
SOURCE: how can i change fuel filter in my grand marquis
you will need the proper Quick Disconnect tool, Look on the Frame rail for the filter, remove filter from the spring type retainer, remove the safety clips on the lines but not remove them completely, insert the disconnect tool & push into the fitting of the fuel line, push line & tool into eachother then pull filter away from line, if not done properly you will have a big problem putting the line back on
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