Sentra Mass air flow problem

Tiny
NORMANH
  • MEMBER
  • NISSAN SENTRA
I have a 1994 Nissan Sentra LE, 152k miles, 1.6L multiport injected engine. I changed the oil and flushed the block with diesel fuel, and replaced the oil filter. The car has been using excessive amounts of oil, but not burning it; figured it might be fouled oil passages. When I started the car it had a difficult time starting and then died. When restarting the engine service light came on. The error code was and STILL IS 12 (Mass air flow sensor). I replaced the air flow sensor with a new one and the car started only to die a few moments later. I dismounted the MAS started the car, let warm up, and then remounted the MAF. The car would idle but die as soon as gas is given. Without the MAF the car will start and drive but bucks and stalls at about 30 acting like it is starving for fuel. I checked the F.I.C.D. Solenoid, the A.A.C. Valve and the injectors, they seem fine. The plugs, wires, distributor cap and rotor are two months old. The air filter has 4500 miles on it. The car operated great before the oil change.

I am stumped. Any ideas you could throw my way would be greatly appreciated
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Wednesday, July 25th, 2007 AT 2:31 PM

12 Replies

Tiny
JACK42
  • MEMBER
How did you flush the block? Any possibility that some diesel fuel got up in the intake manifold?
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Wednesday, July 25th, 2007 AT 3:24 PM
Tiny
NORMANH
  • MEMBER
Drained oil. Poured a half galleon of diesel into oil fill hole. Screwed in fill cap. Disconnected distributor cap coil input - the lead coming from the coil. Cranked motor over for 30 - 50 seconds. Drained diesel fuel. Changed oil filter, filled with new oil.

It would be unlikely that diesel had gotten into the intake manifold. If it had the motor would have smoked for a while.
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Wednesday, July 25th, 2007 AT 6:24 PM
Tiny
JACK42
  • MEMBER
Makes me wonder if the o2 sensor got gas washed or coated in diesel somehow and the ECM thinks the MAF because it cant control mixture. If the injectors werent unplugged, they were spraying fuel into the cyl's while you were cranking it
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Thursday, July 26th, 2007 AT 7:49 AM
Tiny
NORMANH
  • MEMBER
Went and bought Michells manual for the vehicle so that I could trace the wiring. There are no problems with the wiring. With the engine running the signal wire of the MAS should have between 0.7 and 1.1 volts; it has 0.23 - 0.25 volts. Not sure of what the relationship of voltage to richness of F/M is but the vehicle is running rich by the smell of the exhaust. Because of the low voltage readings I called the MAS defective and am getting another replacement in 3 days. Called a Nissan repair shop and they stated that they have seen this problem with a defective and/or after market MAS. I question the after market part because it sounds like a sales pitch. If a new MAS fails to cure the problem, I will build a consult cable and use Blazt Nissan data scan to further diagnose the problem. I have been manually reseting the computer after each test and manually setting the diagnostic scan from the computer. Even when I disconnect something like the TPS the computer only shows an error code 12. I wonder why? A defective ECM is a possibility, but per the dealer, a rare one.

The problem of building and using a consult cable is that I then will want to tinker with the ECM to change the performance to what I want. Because I am good with both computers and electronics this would not be the best option for me.

When I get the new new MAS, I will get back with you. In the mean time: what is your experience with the failure of ECM's; rare or common? Do you know of a quick test I could perform to determine if the ECM has failed and is pulling the MAS signal voltage down? And finally, in this model/year of car, do you know what the relationship between fuel/air/injector timing is to MAS signal level? The reason for this last question is because I used a battery and potentiometer to apply 1.0 volts to the MAS signal line and the car stalled.
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Friday, July 27th, 2007 AT 6:46 PM
Tiny
JACK42
  • MEMBER
Basically you are looking for the proper reference voltage and if the wiring checks out pin to pin back to the ecm it could be the problem. Ecm failure is kind of rare but not unheard of. Teh relationship betweem MAF signal and air fuel is that the ecm uses the amount of air measured by the MAF to adjust the pulse width of the injectors to lean/richen mixture and then compares that with the o2 sensor reading
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Saturday, July 28th, 2007 AT 8:52 AM
Tiny
NORMANH
  • MEMBER
The vehicle starts in a rich condition then the MAS sensor voltage rises shortening the injector pulse width, thereby leaning the F/M. When in closed loop, the o2 sensor then checks the mixture issuing a 1 or 0 voltage (on or off) which feeds back to the MAS secondary sensor (pin 27 on the ECM) voltage, which in turn raises or lowers the sensor voltage on pin 16 of the ECM and feeds back into the system. This thought helps because if the new MAS sensor also tests defective then the O2 sensor could also be at fault rather than the ECM. However, I found the resistivity of the sensor lead from the MAS to the ECM pin 16 to be high at 2 ohms which should be insignificant at the currents involved. Could the high resistivity play a part in the overall problem.
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Saturday, July 28th, 2007 AT 4:01 PM
Tiny
JACK42
  • MEMBER
That sounds high for a computer data line, could cause a problem
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Monday, July 30th, 2007 AT 7:42 AM
Tiny
NORMANH
  • MEMBER
Took car into dealer for diagnostics. They repeated they same tests that I had performed and did not put the car on a diagnostic computer. They however, did, with no knowledge identify the seller of the MAS that put in when the only identifying information read remanufactured MAS and gave a part number. Tried to sell me a factory MAS for over $500. I took the diagnostic back to the reseller (AutoZone) after talking to several repair shops on the way home and went to their shops supported reseller to purchase the part. Have to see when it comes in. I disbelieve the manufacturures claim that only a Nissan part will work (Nissan needs to tell this to the Tuners, who will laugh.

Now at wits end and may have to build my consult cable to do what I paid the dealer to do - put the car on a computer and give me a copy of the print out.

Do you think that the manufactures claim of "Only a Nissan part will work?" Second, when I pay a dealer to do a computerized diagnostic, just how do I get them to do it?
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Wednesday, August 1st, 2007 AT 9:43 PM
Tiny
JACK42
  • MEMBER
I personally believe in using factory parts. Cheaper electronics are just that. Cheaper. As far as diagnostics go, the tech will simply scan th ePCM, as far as fixing it, then he will go into further diagnostic if necessary.
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Thursday, August 2nd, 2007 AT 8:05 AM
Tiny
NORMANH
  • MEMBER
An OEM Mass Air sensor did not solve the problem. After it failed producing the same voltage range (.23 -.25) volts. The spec call for.7 - 1.1 volts on the signal wire. I put back in an after market MAS and used a potentiometer and resistor to apply the factory spec voltage to the signal wire. The car ran terribly and when shut off, it reset the ECM. I am currently building a consult cable ( about 90 % done), when it is finished I will look into the specs in real time.
1. Can you give me ideas of what to look for? 2. Could an O2 sensor create this problem?
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Saturday, August 4th, 2007 AT 1:41 AM
Tiny
JACK42
  • MEMBER
If it is shorted it could be giving false info to the ecm
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Saturday, August 4th, 2007 AT 9:00 AM
Tiny
NORMANH
  • MEMBER
It turned out that the ECM was defective. Even using a consult cable all the readings were correct except the signal line on the Mass Airflow Sensor. Checking further, the signal line with ignition off to ground was 0 ohms but at ignition on engine off the resistivity change to 3.6 ohms. I thought this was insignificant but changed the ECM. The car ran correct and has about twice the performance that it had before the problem developed. One other thing I think is pertinent to the issue; the transmission for about three weeks prior to the development would occasionally stick when coming to a stop. This also ended when the ECM was changed.

Thank for the help.
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Tuesday, August 14th, 2007 AT 5:12 PM

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