car problems gone!! Crazy solution! Could it be? Options

[mk7celica]

So the ongoing power loss/hesitation falling on its face is gone finally after hundreds in parts and lots of labor on my part. and the problem still was there and seemed to get worse but did not do it all the time.
Work done all new ignition parts fuel pum fuel filter sock pulled fuel tank and cleaned and pull injectors and had them tested by a friend still problem. The same friend who tested the injectors found the right problem, Now this is going to blow your mind what is was.
I had a non vented fuel cap on the car. As soon as he took the cap off the car pulled hard and had no issues. I looked up the type of gas cap that is needed and it says vented fuel cap. My friend is a bit more smarter at mechanic work. He said he noticed the fuel return line was pouring gas back into the tank.
When he pulled the cap it stopped. He said somthing about presure in the tank messing with the fuel pump and prsure.


I hope this is the problem but the car seems to be fine with the new cap on.
Anyone ever heard of a nonvented gas cap causeing issues when a vented cap is called for? Somthing so simple who would have thought.
And Import face off/ trackday is Sunday. I got new struts,motor mounts and inserts. Car has no wheel hop turns out my back mount was broke
Thus makig wheel hop. And man the mount inserts are the bomb I LOVE EM !!
I am running 16lbs boost and no issues yet. Car is stronger than ever

[Bigblock]

Yes I've heard of that.A non vented cap can cause the fuel system to "fight itself" to get fuel.Air displacement helps feed the pump.
Without the air relief,the pump struggles.Usualy,you can tell a vented cap is needed if you here a hiss when the cap is removed.