my car, mercury couger has 2 hard shakes per second at idle and at stop lights and in park, in 1st gear i can feel it running rough as it goes faster its seems to be running good, runs like crap at really slow speed and idle. any clue what the problem is, its had a head replacement about 10,000 miles ago, i just repaced the spark plugs,(someone told me that might be it) but nope, can anyone help, the car brings home the bacon.oh yeah, i do smell antifreeze sometimes after driving.

do u have any aftermarket parts / mods on it?!

sounds like a missfire to me could be spark or fuel issue most probably spark, i would check/change all your leads,also could be an injector problem, but you say you have had your head done so it could be somthing related to that if they havent done the work to a good standard so it's pretty hard to say but i am leaning more towards a spark issue

also i am not familiar with that model car so i cant help you with specifics

well i did just change out the plugs to bosch double spark and its still the same,no there is not no modifications on it car its just a normal everyday vehicle.

also i just let out some oil, it was overfilled by a quart mabey a lil more, oh yeah smelles like antifreeze after a long drive,

what about your leads, they could be arcing out which will cause a missfire like you have described

I am with Bex. Check your spark plug wires then I would go back to your coil and module.. sounds like just a break in spark

If it sounds like antifreeze after a long drive i would check if there is a leak coming from the car first and if there isnt, leave the car running and put your hand by the exaust. If your hand gets wet its a head gasket starting to go.

Good luck.