A huge hint might be if your gas mileage drops, you can hear audibly your car popping or mis-firing, you don't pass smog, or you lose power considerably, and lots more.
tune it up
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You need to tell us what sort of tune up you are talking about ... HOME , CAR .... etc.
How do tune up 2005 Mercury Mariner
idle screw
go to a dealer and get a tune up
A tune-up for a 2000 Ford Taurus is anywhere from $175, to $230
How much does a tune-up cost on a 2003 Nissan Altima?
300 to 450 depening on the shop and the tune up