Flubber
Banned
So if the americans didnt enter the war officially they would still help the allies, what impact would this have on the situation in 1919 ?
As has already been explained to you, the only way the war can last until 1919 is if the US supports the Entente in some fashion and supports them in a more direct fashion than it did between 1914-1916.
By the spring of 1917, the Entente was running out of money and the collateral needed for additional loans. Because the US Treasury would not sanction unsecured loans, the UK's war spending was about to take a substantial hit and, because it's war spending was subsidized by the UK, France's spending will be effected even worse.
It looked as if the first few months of 1917 would be a race between the Entente's war winding down as their loans dried up and Germany's new USW campaign triggering some kind of US intervention. Then the idiotic Zimmerman Telegram entered the equation and the US entered the war. With US entry, the Entente received the uncollateralized loans it needed to last through 1918.
If you want the war to last past 1917, you either need the US to join the war or the Entente to find the monies it needs.