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Davo, Master technician
Category: Car
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Experience:  ASE Master Technician/Advanced Automotive Diagnostic Technician
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1996 Nissan Altima: 30 it drives well no studdering at all help please

Customer Question

I have a 1996 Nissan Altima that is getting an average of 12mpg when it should be like 25-30 it drives well no studdering at all help please????
Submitted: 11 years ago.
Category: Car
Expert:  TheMasterTechSource replied 11 years ago.
 Well there could be a wide variety of reasons for this problem--1 could be your station is switching from winter to summer fuel mixes--another reason could be what type of fuel you use--in other words--10% ethanol--and note: alcohol does not stay stable in fuel--therefore if your station is low on their reserves and you get as much as 30% ethanol--alcohol is merely a filler so that would translate to 30% reduction in fuel mileage--Hope this helps!!
Customer: replied 11 years ago.
Reply to THETECHSOURCE's Post: I have added the can mix to the fuel and have gotten gas from a variety of places and no change the car has had a tune up recently and the transmission has been replaced no one can figure this out and it is driving me nuts
Expert:  Davo replied 11 years ago.
 Have you had it checked for codes?Checked the air filter?
Customer: replied 11 years ago.
Reply to Davo/diablo666's Post: I don't know what you mean by have it checked for codes??? I have had the gas and air replaced though..
Expert:  Davo replied 11 years ago.
 Have someone hook up a scan tool and check for computer codes.Autozone offers this as a free service if you have one local.Post back here with the results,don't buy anything just yet.Post numbers not just the description they tell you it is.The check engine light can be off and still have a stored code.
Customer: replied 11 years ago.
Reply to Davo/diablo666's Post: I went it passed all code tests.... I am so confused
Expert:  Davo replied 11 years ago.
 What if any repairs have been made to the vehicle other than the fuel and air filters prior to the problem occuring?Timing belt maybe?
Customer: replied 11 years ago.
Reply to Davo/diablo666's Post: have had the car for only 6 months the transmission was replaced thats all i know!
Expert:  Davo replied 11 years ago.
 Has the gas mileage been terrible since you got it then?How many miles are on the car?
Customer: replied 11 years ago.
Reply to Davo/diablo666's Post: the gas mileage seemed to be good when i bought it then kinda just sucked there are about 78000 miles on the car
Expert:  Davo replied 11 years ago.

Are you the only driver of this car?Is it driven moderately to try and get economy or are you in a hurry to get somewhere all the time?Have you changed the tires or anything else to dress up the car?

Customer: replied 11 years ago.
Reply to Davo/diablo666's Post: new tires and this is a commuter car it is not beat on at all it drives hwy all day. that is why i dont understand the gas mileage its not stop and go at all the only other thing it has aftermarket is the radio and a car starter.
Expert:  Davo replied 11 years ago.
 Try testing the MAF sensor to see if it's in specs.They usually will set a lean code if they are faulty but you need to start checking somewhere.You may want to pull the hose off the fuel injection pressure regulator to see if its leaking gas.Also check the upsteam O2 sensor maybe its getting sluggish.Test the coolant sensor circuit also maybe the pcm thinks the car is never warmed up.

OPERATION


The Mass Air Flow (MAF) sensor directly measures the amount of air flowing into the engine. The sensor is mounted between the air cleaner assembly and the air cleaner outlet tube.


The sensor utilizes a hot wire sensing element to measure the amount of air entering the engine. The sensor does this by sending a signal, generated by the sensor when the incoming air cools the hot wire, to the ECM. The signal is used by the ECM to calculate the injector pulse width, which controls the air/fuel ratio in the engine.



TESTING


See Figures 1, 2, 3, 4 and 5


Click image to see an enlarged view
Fig. 1: The MAF sensor is attached to the air cleaner housing



Click image to see an enlarged view
Fig. 2: To backprobe the MAF sensor connector, first remove the rubber boot



Click image to see an enlarged view
Fig. 3: For this vehicle, the MAF voltage should be approximately 0.2 volts with the ignition ON. This sensor is within specifications



Click image to see an enlarged view
Fig. 4: With the engine running at operating temperature, MAF voltage should be 0.85-1.35 volts. This sensor's reading is at the high side of the specification



Click image to see an enlarged view
Fig. 5: Measure voltage by backprobing the MAF connector as illustrated





  1. Using a multimeter, check for voltage by backprobing the MAF sensor connector, as illustrated.



  2. With the ignition switch ON and the engine stopped, voltage should be less than 1.0 volt.



  3. With the engine idling at operating temperature, voltage should be 1.3 -1.7 volts.



  4. With the engine running at approximately 4000 rpm, voltage should be approximately 4.0 volts.


It is important to watch for a linear voltage rise in response to increases in engine rpm, up to about 4000 rpm.




  1. If voltage is not within specifications, check the power and ground circuits.



  2. If the power and ground circuits test okay, the MAF sensor may be faulty.














Customer: replied 11 years ago.
Reply to Davo/diablo666's Post: since i know nothing about cars i will give the instructions to my car guy and have him check it out... I will get back to you thank you sooo much ... also would it be worth it for me to have them run a diognostic scan on the car as well??? thanks rich
Expert:  Davo replied 11 years ago.
 If you know someone that can check all the sensor inputs you may find the problem.Something is wrong and a sensor should be giving a clue.If they can scan and understand the sensors it would be well worth it in my opinion.It probably wouldn't hurt to have the valve timing checked if all the sensors check out ok.That is in case it jumped or the belt was changed and wasn't installed properly.

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