I have a 1995 isuzu rodeo and was wondering what the oil pressure should be for it? Picked it up from a local shop and now black smoke is coming out of the muffler. They were suppost to be replacing a thermostat and a alternator belt. The mechanic said that i have a bad oil leak, i haven't had to put oil in it. It is acting like it is having trouble picking up speed.It was running better before i took it to the mechanic. Could you please help me determine what this might be?{single mother tired of being taken advantage of}