I have a set of 500 Efis and one of them is using noticeably more fuel than the other. There is visually black exhaust coming out at idle. Could it be a bad sensor? Wondering where to start looking

Check fuel pressure, should be right at 40 psi. Also need to measure the coolant temp sensor and see if the resistance changes as it gets warmer.

Thanks. I had another guy tell me the exact same thing. I'll start there and see what I find.