Car cranks but wants to start when i release the key but wont run

How about some more info as well! Does it have a stock harness, any mods ETC!

Your avatar would lead us to believe a 74.
It is a 74 dart I just had to change the engine wire harness because the other on smoked it self the new one is from a 73 dart with a slant 6 car so I had to modify the harness to the v8 style. I am going to check the voltage tonight do you guys know what it should be reading ?