I apologize for bumping this old topic, but I have done quite a bit of research on this, so I want to share my experience.
Seafoam, as with many additives, consists of a solvent in a light oil carrier. Iirc in the case of Seafoam the solvent is naphtha. It works OK for dissolving fuel varnish because that's what it's designed to do. The problem with injecting it into your manifold is that 1) It's not very effective and 2) You end up burning the oil carrier. That's why it smokes so badly. Burning oil from any source isn't good for your engine and it especially isn't good for your catalytic converter if you have one.
I prefer using water for de-carbonization
https://wheelstips.com/best-diesel-injector-cleaner and using a top tier gasoline along with a PAE (polyetheramine) based fuel additive a couple of times a year. That'll keep everything including the injectors clean. Gumout Regane (the yellow stuff in the clear bottle), Redline SI-1, and Techron Concentrate are all PAE based.
I said what the consequences are in a recent thread. It was rebuked so I left it up to the members to decide. You have to remember I deal with this sort of stuff all the time so when I run into those who "know better" I back off.
I'll say it again: The product has it's uses but this isn't one of them. Using it this way only lightens your wallet and shortens the life of your cat without benefit. The product is mostly light oil. Where do you think all the smoke comes from? Since when is combusting oil in either the cylinders or converter good? Rich mixtures kill cats so oil won't? If it removes any carbon it'll only be a tiny amount. If all you have is a tiny amount you're doing more harm then good because a small amount works to protect the piston crown. Good thing it comes right back, which makes the "treatment" even more of a waste. If you want to clean excessive carbon use water injection, an overnight piston soak, walnut blasting, or any of the other *proven* methods. Once it's gone keep it at bay by regular use of a PEA based fuel additive.
Aside from all the other tests you could've done a simple way for verifying it's lack of effectiveness is to examine the motor, including the valves, before and after using a borescope. I have a surplus medical instrument I use for those jobs. It was originally designed for, well, lets just say it's been shoved inside far worse places than a motor. I would've suggested doing this except it's already been done by myself and many, many other people who aren't as gullible as most of the public. The results are always the same: it does squat. People who swear by it don't do science. They accept anecdotal evidence from others. Being ignorant is not the same as being a dumbass but imo anyone who uses something without understanding the science behind it, based on what others say who also don't, is.
At least you did some "research" even if it wasn't the best kind. And like most people you did it after the fact. Frankly, if I was going to rely on what others say about a product's effectiveness (and I never do) I'd sure as hell require a lot better than 50% odds before any cash came flying outta my wallet. And even if the product did work I'll point out injecting anything into the brake booster line results in a very poor distribution of it across cylinders. If you feel a need to dump stuff into your engine do it upstream of the plenum, through the throttle body for example.
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