1996 Mitsubishi 3000GT Engine misfire

The Engine Computer performs a number of self-tests, and when it detects a problem, it sets a diagnostic fault code. If the problem could have an adverse affect on emissions, the Check Engine light turns on. The first step in diagnosing any problem is to read those codes. There's over a thousand potential fault codes so we need to know the exact code number.

It sounds like you're replacing parts based on guesses. That's the most expensive and least effective way to solve a problem. Also be aware that there's always a long list of conditions that must be met to set a fault code, and one of those is that certain other codes can't already be set. That's because the Engine Computer compares sensor readings and operating conditions to each other to determine when there's a problem. When a code is set for something the computer uses as a reference, it suspends any tests that rely on that. If a new problem develops, you'll never know it because the Check Engine light is already on. Then, when the original problem is diagnosed and repaired, the self-tests resume, and that's when the new problem is detected, a new fault code is set, and possibly the Check Engine light turns on again. You might think the first problem was misdiagnosed, but in reality this happens real often when the first problem is ignored for a long time or it takes a long time to diagnose it.

Never heard of a two-way valve. Fuel systems have a check valve that blocks fuel pressure from bleeding off through the pump when the engine is turned off. If that valve leaks, you may have to crank the engine for a few seconds before it starts, but other than that, it will not cause a running problem or set a fault code.