1999 Maxima getting 11 mpg - no CEL

[hot460]

My friend's '99 Maxima with 190K miles is burning a lot of fuel. It has gone from about 25mpg to 11mpg. There is no CEL and the only code that we have gotten is P0325 knock sensor. The car produces a lot of black smoke from unburned fuel. I feel an occasional "studder" driving a low rpm in OD at about 40 mph. This car has run regular 87 octane fuel since new and does not perform any differently on 93 octane. The local (Houston) dealer cannot find the problem since there is no code. Some history - A month ago the car started missing badly and the CEL blinked but did not stay on. It threw a P1320 misfire code so I had new plugs and Nissan coils installed. About 2 years ago I replaced the MAF sensor. Following the coil/plug replacement, the car runs smoother, but started using a huge amount of fuel. I have considered O2 sensors, but again, there is no code indicating they are the problem. Also, I have thought about a stuck injector. I would appreciate any advice from this forum to solve this problem. I hate to just start throwing parts at it.

[hot460]

Update on this issue:

I have replaced the knock sensor and had the injectors checked out (plugs were pretty "fouled" with all the excess fuel being dumped) with no affect. Still throws a knock sensor code. Having the fuel pressure regulator replaced now. This is a crazy / mystery issue. Anyone with any ideas, let me know.

[vonscorpio]

There are others much smarter at this than I am, however, you might check the mass air flow sensor. The occasional stutter, no error lights and fouled plugs sound like the same issue I was having with my M30 and I chased down everything I could think of. In the end, the helpful users here pin-pointed the issue: bad MAF. After putting in a new one I had horsepower I never felt before and a much smoother running engine, and significant fuel economy boost.

[NutriaforBreakfast]

Check the fuel regulator and fuel pump pressure
Look at your engine oil to see if it might have some gasoline in it