Strange problem

I just encountered a very weird problem and wanted to see what you guys think. I was planning on going to a test and tune day at a nearby strip this weekend. 2 days ago I took the car out and adjusted the suspension to how I wanted it and took it for a ride. I warmed the tires up in 1st gear with a small break stand at about 5k rpms.

All of the sudden I hear a POP, backfired through the intake. Drove it 15 miles home going about 30 mph max, blowing black smoke out the tails. If i gave the car more that 1/4 throttle it would sputter and backfire. Obviously im burning excess fuel for some reason.

heres the weird part. The next day I start the car up and the thing runs PERFECT. I had went through all the injector connections, vacuum lines, MAP sensor and everything the night before and it still ran like ****, and the next day it runs fine...boggles me.

I pulled codes, and nothing. Any ideas guys Im stumped.

Yea I pulled codes, which got me nothing.

Also, I put a fuel pressure gauge on my schrader valve and at idle I have about 38psi, which I always have. When I shut the car off the needle dives down to zero in about 5-8 seconds. should the pressure drop off that quick?

There is not fuel in the vacuum hose that goes from the regulator to the manifold. Heres another weird thing. The fuel pressure did that when the car was running like crap. and it does it now, when the car is running fine

Is there a definite way to check for a bad fuel injector?