This is a bit long winded but just had to share it.

Just tought id post this as it may help someone . I have a 4 cylinder 3ltr mercruiser inboard that i had installed new back in 2007 . ive always looked after it and has now done 1500 hours .

Motor runs fine but has slowly lost a bit of its get up and go over the years so i decided to do a compression test . Unfortunately the news wasnt good . the results were as follws 115psi 120psi 115psi and number 4 cylinder was down to 90psi . I repeated the test 3 times with 2 different compression guages with the engine both hot and cold and the results were all the same . To say i was devistated was an understatement .

I am mechanically minded so set about to diagnose the problem . did a bit of investigating but bottom line was that i had no pressure coming back up the dipstick tube so decided it must be a cyliner head issue .

Off to my local mercruiser mechanic and got price of a new head $5k + install. Recon my head was a cheaper option .

During the discussion  he was raving about the success he has had with internal engine cleaner . I had heard of seafoam in the past and this sounded like the same stuff , different brand . To be honest i recon it was all crap but thought what the hell ive got nothing to lose . So spent the $22 and went home with a bottle of the mystery juice . 

He told me to take out spark plus and squirt a generous amount in to each cylinder , let sit over night , crank motor with plugs still out to expell the liquid then re install plugs .

After that start motor , bring to temp and slowly squirt cleaner into carby keeping revs up . Engine should now be clean .

Well i sort of did what he said but only waited 6 hours after filling cylinders . Had absolutely no expectations of anything changing my low compression issue .

Cranked the motor to expell the liquid from the cylinders and decided to do a cold compression test before i put plugs back in . Decide to test the bad cylinder first . Well F**k me i now had 145 psi in that cylinder up from 90psi . That cant be correct so i did it 3 more times with same result . Then did other 3 cylinders with same result . tried again with my other compression guage and still the same . At this stage im gobsmacked and went and had a cuppa to think about it .

After cuppa i decided to run motor to get to operating temp and test again , results were the same . Then i ran motor and squirted down the carby as told , let sit for 30minutes , ran motor again then let sit for an hour . 

Did another few compression tests and now i have 150psi on 2 cylinders 155 on the third and 160psi on the one that was 90 . Absolutely amazing .

As a back yard mechanic ive always been able to fix things and always thought fuel addatives and the like were a wast of time and money . Boy have i changed my mind .Assume i had stuck rings and carbon buid up internally .

I have no affiliation with the product and Im not saying this is a cure for all things but in my situation it worked and having googled it im not the only one that has had good resuts with seafoam or in my case mercury power tune engine cleaner . Its also a testament to the mechanic that told me to give it a go - there are good ones that are not just out to rip you off. would have been easy to say need a new head .

Hope this helps someone

Cheers Paul