I payed $5 at autozone. It is a can that looks like it is strait outta the 70s.
I used it though the pvc valve on my 99 Gt. Same on a freinds 04 Grand Am Gt (High Miles for a 04).
You add this through your pvc valve or any vaccum that will feed all 8 cylinders. Let it suck in about 1/3 - 1/4 a can. While doing this, you need to make sure you do it very slowly, because the engine is not breathing very well with a liquid going through the intake. Basicly as the engine sounds like it is going to die, remove the liquid so that it gets some air and stays alive. Then turn you car off. The bottle recomends leting it sit for 5 min, but I went 15. What it does is, it goes throughout your engine, and breaksdown extra gump and carbon buildup you engine has (the older/more miles the more you'll have). So during this time where the car is off, it is literally eating away at that crap you dont want.
When you start the car back up (it might be alittle rough at first) you will begin losing all of the seafoam and carbon/gump out of your exhaust. This results in white smoke. You can let it just exit your car while sitting there in idle, but you will have to be reving the car alil. I chose to drive it to keep the smoke away from my house. It will literally look like your smoking your tired the entire trip. You get alot of looks lol.
After all the smoke is gone, it is safe to return back and turn the car off. Let it sit for about a half hour.
The car really comes alive. My exhaust is much louder and more toned, as my engine is running much smoother. Revs are smooth as hell. The first time i reved it, i hit the limiter because i wasnt used to revs flying up that fast. Acceleration is also much much smoother. SOTP feeling is a major thumbs up after.
Not really a mod, but probally the best thing u can do with $5 on a older car.