I walked into the auto store the other day and I was completely shocked by how many different fuel additives that promise to optimize efficiency, fuel economy, and even reseal head gaskets! But there was always one that caught my eye. Seafoam. There are countless videos that show how to do it. 1/3 in crankcase, 1/3 in gas, and 1/3 in the vacuum lines. Now one of my friends swears by the stuff, but I just don’t find it “kosher” to pour seafoam into the vacuum line until the car stalls, only to restart it and have white smoke billowing out of the tailpipe (which is supposed to be all the carbon deposits burning out). Now here is my question(s).
Is Seafoam safe to pour in the vacuum line? Is doing so really cleaning the top of the valves etc, and is all that white smoke coming out of the tailpipe really deposits left from the engine burning off, or is it just the seafoam burning off. Will seafoam do any damage or even ADD deposits to my car, and will it improve the quality of engine idle smoothness, power and/or performance?
One more thing. I probably got the wrong ratios wrong on where to add it. No matter, I was just trying to stress that you could add seafoam to any one of those three places. Also, does anyone have proof that it really cleans? Like has anyone used it, then taken apart an engine to see the amount of deposits on some parts compared to a similar engine with average crud buildup?
I have never come across anyone who uses it that has anything bad to say about it. I know several experienced people who swear by it. The smoke billowing out during intake cleaning is junk being burned up.
If you are having some performance problems (idle, power, etc.) don’t forget that issues there are almost very simple and basic things like old spark plugs/wires, air & fuel filters.
I’ve used seafoam before. It really works as far as I can tell. On my old Bronco it definately made it idle smoother and mileaged picked up by about 0.6 MPG. I’ve also used it on Mustang, with less impressive, but still noticeable improvements. It seems that the more mileage you have on the car the bigger the improvement. I’ve heard that Seafoam is hard on the sparkplugs, I don’t know if that’s true or not though. The smoke show alone is worth the $5 IMO 
BG44K and Techron are also recommended.
Twotone
It may be safe, but I wouldn’t bother pouring it into vacuum lines. If you do, you shouldn’t have to stall the engine to do it.
It is not a bad product, but I’d bet that 80% of it is used on cars that don’t need it.
–actually the smoke coming up during intake cleaning is the Seafoam being vaporized and partially combusted. Along with (hopefully) some of the crud.
I’ve heard good things about it and used it too, with fair results. My only worry is what it may be doing to the catalytic converter when you dump all this unburnt solvent vapor and possibly crud into it.