1985 Pontiac Fiero 1985 Pontiac Fiero Idle and other proble

First of all, I'd like to start by saying I've googled the hell out of every problem I've had with this car, before anybody tells me to use the search function.

This is a four cylinder "iron duke" with 145,000 miles. I bought the car pretty cheap and I've had these unsolvable problems since then.

"edit: ALSO, I've tried to read the codes off this car, as sometimes the check engine light will turn on or off, usually triggered by a random event, such as accelerating, letting off the gas, turning the radio on, turning my headlights on, ect. But the only code it will read is 12, which appears to just indicate the diagnostic as started."

1st.

The car won't idle. I've tried everything, cleaned and replaced (with used) fuel injector. Cleaned the entire throttle body. Checked my vacuum routing. Replaced my EGR valve, replaced my PCV valve, new plugs, checked my wires, replaced distributer cap, timed vehicle, replaced rotor, replaced air filter, checked MAF, replaced MAP sensor, and. There's probably more but nothing I can think of at the moment. Edit: also replaced fuel filter.

It's a really annoying problem having to pop my vehicle into neutral and keep my foot on the gas the entire time. Although, I've noticed SOMETIMES it will idle just out of no where. The only times it will idle though, it's pretty rough, and it's burning so rich white smoke comes out of my exhaust.

Also, sometimes when starting cold, the vehicle doesn't want to stay running unless I'm giving it gas, and if I give it too much, it'll die. I can't shift it into gear either or it will automatically die. It gets way too much fuel when at low RPM's though. It also smokes white upon cold start, but will not idle.

One more thing, I thought my fuel pump was bad, but it won't pump fuel into the injector until my oil pressure reaches 40. To my understanding, there's some sort of fuel relay switch being triggered by the oil pressure, but I know next to nothing about it.

2nd, probably pretty easy fix. My tachometer doesn't work, sometimes it will, sometimes it wont. But at the times it does, it will frequently jump up and down.

I thought there was a third, but I forgot.

Anyways, first and foremost I worry about the idling problem, tachometer isn't a priority to me at the moment. PLEASE if anybody can be of ANY assistance it's much appreciated. I've taken it to one mechanic and he suggested I buy a fuel injector (which I've replaced with a used one, 80,000 miles) and a fuel pressure regulator, which doesn't make any sense to me aside from the fact that it gets way too much fuel when not driving.