Nissan Repair: Low mpgs and sporadic stalling, mass air flow sensor, mass air flow

Question
QUESTION: Dear Von,

I have several problems with my 1996 Nissan Maxima:
1. I get about 15-17 mi/gl city driving; however when it is highway – 28 mi/gl.
2. At traffic lights the car would stall.
3. I noticed that RPMs is acting sporadically.
4. If I drive say at 40 mi/hr and stop pushing the gas pedal the RPM goes down to about 800-900 within 2 seconds

Here is what was recently done on this car:
1. New  Mass Air Flow Sensor – after market
2. New Idle Control Valve
3. New knock sensor
4. Cleaned the whole air throttle body
5. Tune up was done about 10K miles ago

I had this car checked by a guy whose specialty is electronics – he found nothing wrong: ECU, Oxygen sensors, TPS all good.

Thank you for your time

ANSWER: Igor,

Email me back with the model.  But from a Nissan perspective, I would check the distributor for oil on the optical shutter wheel.  If you find oil on the optics replace the distributor.

---------- FOLLOW-UP ----------

QUESTION: It 1996 GLE, the engine model is VQ30(DE).

Additional info - my wife says that the stalling happens mostly when the engine is cold. I have 128K miles on this car.

Am I wrong suspecting MAP, TPS, injectors? Could the ECU be fried by the faulty Idle Control Valve?

Thank you very much!


ANSWER: Igor,

The computer is pretty bullet proof.  If the battery is jumped and the terminals are reversed is usually the only time an ECM gets cooked.  I would check the codes and see what is going on.  Autozone or Checker will read the codes for free and let you know if any are resident.  If the car is stalling and the plugs are recently replaced then I would be looking at the coils, mass airflow sensor or idle air control valve.

---------- FOLLOW-UP ----------

QUESTION: Right after I passed the inspection today, the Check Engine light came on! At Autozone they diagnosed P0172 and P0175. According to Maxima manual this Malfunction is detected when ...
1. Fuel injection system does not operate properly.
2. The amount of mixture ratio compensation is too large. (The
mixture ratio is too rich.)
The following items should be checked:
1. Front heated oxygen sensor
2. Injectors
3. Exhaust gas leaks
4. Incorrect fuel pressure
5. Mass air flow sensor

I will start with the O2 sensor.

However I am not sure that all this explains the stalling when the car is cold. It actually happened to me today - the RPM jumped initially to 1400 and then dropped to 400 after which the car stalled.

What would be you best guess on the cause of "cold" stalling? The temperature sensor was replaced a month ago.

Thank you!

Answer
Igor,

If you have not replaced all the oxygen sensors do so.  I changed the three on my wife's Maxima and was being cheap about it.  I ended up changing one at a time until I had replaced all three.  Anyway, after changing them the strange idle and engine stall went away.  But, I did have to disconenct the battery to reset the codes.