Something is leaking from my pipes on 1994 Nissan 300ZX

I have a 1994 Nissan 300zx non-turbo.I just had a fuel injector replaced on my 3rd cylinder. 3 days later after my car was sitting for about 7 hours i started my car an it was making really weird noises when i pressed on the gas. I then noticed that i lot of white smoke was coming from 1 of my tail pipes(i have 4) I turned the car off and i noticed that some type of fluid was leaking from the pipe where the smoke was coming from. Inside that pipe there was alot of black sludge. What could this be? Please help.

Asked by for the 1994 Nissan 300ZX
White smoke is a sure sign of leaking head gasket. Also check the oil filler cap and dipstick for milky lookin g oil this would be another sign. Have someone do a compression test on the engine to verify.