Some products are a waste, most good “Fuel System Cleaners” contain PEA, which is a detergent and not a solvent. You shouldn’t need Fuel System Cleaner(FSC) to pass emissions. If that were the case, simply run a few takes of premium and take the test with premium fuel in the tank.
Some good ones are as follows(PEA containing):
Red Line SI-1 (I = as in Igloo)
Gumout REGAN High Mileage
^Those are generally the ACTUAL best, not just ‘favorite’.
Other’s function simply as an Upper Cylinder Lubricant, especially in apps that don’t like alcohol(ethanol) containing fuels that USA uses these days. Lucas fuel treatment product is one of these, too expensive for it’s uses as an UCL, not a great cleaner AFAIK(as far as I know).
Sea Foam is ‘okay’ as a cleaner, really more of an UCL because the effects wear off quite fast after use, if you noticed an improvement in idle and throttle response in the first place that is. I get that ‘temporary’ improvement in feel/idle but it goes away fast with Sea Foam use. I only use that VERY occasionally. Personally, I use one of the above in the list before ‘most’ oil changes. Depending on your Oil Change Interval(OCI = mileage between oil changes) every 3,000 may be too frequent; where as 1 oil change every 7,500-10,000 would do well to have a FSC added to the tank prior to the oil change. Best not to introduce fresh oil to more contaminants right away by adding the cleaner after an oil change or at the same time = shorter oil life.
For this reason, most people prefer to use Marvel Mystery Oil(MMO) as an UCL, 2 ounce/5 gallons is the general rule of thumb, BETTER to slightly under-dose than OD and cause your octane to drop some. MMO is cheapest, but has some lubricant and solvent properties. Those are maintenance type fuel additives between oil changes. Another popular one, for older engine at least(FI not DI) is oil that is TC-W3 certified, basically meaning an asheless lubricant(no metal additives to leave behind soot of any kind that is). Use the cheapest between TC-W3 and MMO, TC-W3 is EASIER to overdose mind you. Only 2 ounces / 10 gallons if using TC-W3. FYI = find those certified oils in boating departments of stores or a boat store. Marine shop, etc.
If you have a really neglected engine and can change things such as the fuel filter after additive use then a product that can be purchased online called Kreen is a sleeper product, growing in popularity on forums such as BITOG(google that).. It can be used to clean up a ‘mildly-moderately’ “carbon deposit/varnish covered engine. Can be added to fuel or oil. Of course, if you have actual sludge in your engine, best to manually clean with valve cover(s) removed if possible, then clean the bottom end(with oil pan removed), before continuing clean up efforts with solvents. If you don’t clean manually, don’t use additives. That simple. Instead use a High Mileage oil or synthetic oil on 3,000 MAX oil change intervals for a while before using solvents). That way hardened chunks won’t break free clogging oil passages or the pickup screen/tube in the oil pan.
Getting ahead of myself, but there are products that will have measurable benefits to fuel system cleaning. Some of those can be used added to oil(not on a continual basis but for a specific purpose to address a KNOWN issue like severe examples above, better safe and under-dose/gradually build up treatment than to kill shot your engine/fuel system with Overdose.
Some ‘cleaners’ really aren’t that great at cleaning, if you want solvent B-12 chemtool is powerful. They also make a parts cleaner solvent, which ‘could’ be used to fill a crankcase(not to start the engine mind you!) after an oil change in order to soak the bottom end(crud on oil pan/crankshaft/oil pick up tube or screen)…some have soaked their engine after dropping the used oil with parts cleaner for a couple of days with some sort of heater on the pan, then drop the drain plug on that > refill with cheaper conventional oil at idle > change oil a 2nd time then you are G2G.
First, find out what you want to address, then pick your product. For fuel system cleaning use one of the initial products I mentioned. Any UCL product is better for ‘maintenance dosage’ use. In other words, minimal concentrations. Read each products instructions for use as cleaning application(some UCL ask to over dose really in the ‘hopes’ of using their product to clean when its best in conservative use).
Combustion Chamber Cleaners are also available(Mopar CCC, Amsoil Power Foam, GM Top-End cleaner)…if you suspect heavy varnish/lots of carbon deposits in combustion chamber(poor performance/pinging under moderate-heavy acceleration), those products mentioned are ACTUAL foam spray cleaners that you spray into the Throttle Body(dont do that on engines equipped with MAF sensors, must be ingested a different way if at all), and you basically soak the engine by cutting it off after ingesting entire contents of can. Ideal in summer/engine full temp. Cans usually state to only soak for 10-15 minutes and then restart, but its best to wait 2 hours or so before restart so product can soak/penetrate as much crud as possible. Let it idle for a few minutes, rev the throttle some, then go for some Wide Open Throttle driving(safely mind you!) to expell what was disolved.
When using a top-end cleaner, I use them AFTER fuel system cleaning, since the fuel system’s crud will enter the combustion chamber. It doubles as a way to clean the TB too(using top-end cleaner via the TB).
After expelling, I change the oil. As some deposits etc will be remaining in oil, lots of solvent use may bring viscosity down so prolonged use after ‘may’ be somewhat risky.
Any questions or clarifications let me know.