My daughter's 1997 Century Limited with 131,000 miles and the 3.1 Liter V6 has an intermitten miss that occurrs only in a few instances. One instance is going about 55 mph and giving it light to moderate throttle and the other instance is going up a hill at about 35 mph and again giving it just light to moderate throtle. Sometimes the miss is just slight and almost not noticable and other times it can cause the "service engine soon light" to come on and then it will stay on for a day or two before it goes out. When it was on I had the code read and was told that it only reveals that the engine misfired and not much more information beyond that. At times the car can run for a week or two fine and then it stumbles again. I have replaced the spark plugs, wires, fuel filter, coils and the base unit the coils get power from. Any assistance would be greatly appreciated.

Did you use A/C plugs and plug wires? Report back with info.The proper scan tool can tell you what cyl. had the misfire and the number of misfires (A misfire will not set a code if has a count of less than app. 10 but will show in the scan tool window as current )I use
the Tech2 ( GM )

I did use AC plugs but I am not sure about the wires. Does the brand really make that much difference? Sounds like with the proper test tool I can find out which cylinder the problem is showing up on. That is what I really need to know unless it is a fuel problem. What would my results show then? Is the tech2 GM tester a practical tool to buy or is it too expensive for just an individual owner? Appreciate all your advice.

Tech 2 is not for at home use ($3800) .Cheap plug wires will cause a lot of problems .Yours is a slight misfire and is hard to diag. with just a simple scan tool that only reads codes or limited data.You may need to find a local GM tech. to diag. your problem .
You may have a fuel injector that needs cleaning ( not the stuff you put in the tank ) but you need to be able to "see" what cyl. is misfiring and when you have corrected the problem. HTH :beers: