2001 Nissan Altima Rough idle, bad mpg

NCSHERM

MEMBER
2001 NISSAN ALTIMA
4 CYL
FWD
AUTOMATIC
100,000 MILES

Has rough idle, bad mpg. Lot of air comes out oil cap when removed than car will die. Revs up fine Drives fine on Hi way, seems like nothing is wrong. Good speed and passing. I changed cap, plugs, rotor. Check light came on and back off, code said nock sensor, vacuume leak, misfire, low fule psi. Local mechanic said bad fuel injector, another said pcv valve (that I cant reach). Good spark and fuel. Didnt check fuel psi. Cant afford to spend $ on lots of parts to guess at problem.

2 Replies

RASMATAZ

MECHANIC
75,992 POSTS

Misfires can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.

A Random Misfire code usually indicates a vacuum leak or bad gas.

If a misfire in a specific cylinder should lead you to check the spark plug, fuel injector and compression

A lean fuel condition can be caused by:

* Low fuel pressure due to a weak pump or leaky fuel pressure regulator. (Use a fuel pressure gauge to check fuel pressure at idle)

* Dirty fuel injectors. (Try cleaning the injectors)

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Use a vacuum gauge to check for low intake vacuum)

* Leaky EGR valve. (Check operation of EGR valve)

* Leaky PCV Valve or hose. (Check valve and hose connections)

* Dirty or defective Mass Airflow Sensor (MAF). (Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires)