1990 Nissan 300ZX Running Too Rich

[636Ryder]

I picked up a 1990 300ZX 5 speed non turbo last week. The car has not been running very well. Its running really rich. I get terrible gas mileage, sputtering and hesitation, and black soot coming out of the exhaust and onto the bumper. I've been getting 8-9mpg city driving and 10-11mpg on the hwy. This is pretty bad considering that most people avg 20-25mpg with this car. The previous owner had just installed an aftermarket exhuast (headers & muffler but not the cats) and I'm thinking the problem may have stemmed from the new exhaust. The check engine light is on and I'm inclined to think that it has to do with the o2 sensor going out. This weekend I'm going to pull back the passenger mat and take a look at which codes the ECU throws me. Is there anything else I should check out? Possible injector failure? Clogged air filter? Bad MAF? Is there anyway to upload a fuel map to the original ECU without replacing it? I'm pretty new to Nissans so any input is welcome! Thanks in advance.

[robomatic12]

It sounds like you are on the right track, check the ECU codes first. Then post back with your findings. A general tune up is probably a good idea if you bought the car with no maintenance records. Also check when the timing belt was done last, might be in need of replacement.

[marty1mc]

Check codes, pull the plugs, do a power balance test, ohm out your injectors, clean every connector for CAS,MAF, PTU, injectors, IAC, coil packs, temp sensor, TPS.
Also, check ignition timing and look for leaking injectors, vacuum leaks.

What Robo said shouldn't be taken lightly. The motor is an interference motor. If the timing belt breaks, it's toast. At a minimum, you are looking at bent valves. Nissan spec for a timing belt change is 60k miles OR 4 yrs (time or mileage). If you don't know when it was done, do it and fast.