Any ideas why a 1998 Mercury Grand Marquis would just suddenly stop running in the middle of the highway going 55 mph and w a half tank of gas in it and now turns over but wont actually start again?

There's a l.c.m located by the gas pedal, just above it over the plastic cover.when this happens, slightly tap the module. If light come on then you need a new l.c.m (lighting control module)...very common concern eith this vehicle, unfortunately not cheap

It may be the battery is going dead. try putting a jump box or jumper cables and try again. good luck

Check your connectors wire pin corroded clean the connector wit solvent and check if you have power in the circuit if yess start your engin it should work great again good luck

sound like your cats are gone bad or your exhaust system is clogged at some point. you may also have your o2 sensors checked.

Are you sure your firing order is correct

A full ignition tune up would be new spark plugs and wires.A cap and rotor if it has those. Do any fuel system cleaning before changing the plugs.

Sure, the more the throttle and the longer its open the more gas is being injected so if the spark is not there it will spotter. I think you have nothing to loose with an ignition tune-up. It'll be more economical than taking it to a mechanic and still ending up with the same problem. You should change the fuel filter also it could be clogged and there is'nt enough fuel delivered at these speeds. You can also try a fuel injector cleaning solution the kind you pour in the gas tank. This might work for you.

Defective sending unit in gas gauge.

you can try just resetting the fuel pump switch located in the car

the switch is originally in the car for in case of an accident, however, if changed, chances are you also have to reset it

in my 98 grand marquis, the switch is on the left side of the trunk