1996 Ford Mustang 3.8 L V6. My car has over 108000 miles one it almost everything still original with the exception of some minor repairs necessary. The problem I['m finding is now and then on start up it will shake, once I get moving though, it stops. Last night however I started it up and the check engine light came on and it was shaking. I got it home and checked all my fluids to find my antifreeze low. I filled it, and turned the car on this morning, no shaking, engine is cool, however my check engine light is still on. As far as I can tell though there's nothing new or different as far as sounds from the engine go, and the engine looks like it's running fine.
The check engine light is on because your car's computer has detected a malfunction with the cars engine management system. The cars performance and gas mileage may or may not be affected depending on what the problem is. There is a problem with emission control system, fuel system or ignition system. Use of a scanner to pull the codes will give you a good direction to go in. If you don't have a code scanner, some of the big autoparts chains will pull the code for you free of charge. : )