# 1996 Altima - Terrible Gas Mileage?



## Xmsteel (Mar 6, 2011)

*Long story short:* I bought a 96 altima gxe with a bad engine and got it replaced.
*The problem:* I just filled up. I got 13 mpg. I drove it extremely conservatively, and don't know why it would get this bad. I actually got 16-17 when the engine was having issues, which is what led to getting it fixed (see below).

The replacement engine (150k Miles) had some issues, *so this is what has been done to it so far:*

New valves
New Head gasket
New Spark Plugs & Wires
New Distributor, Distributor Cap
New EGR Vacuum Solenoid
New Knock Sensor

The engine runs fine with no check engine light, but boy, the mileage took me by surprise. The only thing I can think of would be the cold weather (about 10-20F degrees here). 

Any ideas?


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## rogoman (Dec 16, 2004)

Two items that might have a major effect on MPG:

1 - O2 sensors. They may not set an ECU fault code because they are still working but may be out of proper calibration due to age or contamination.
2 - Coolant temperature sensor that feeds the ECU. Same reason as #1 above.


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## Xmsteel (Mar 6, 2011)

rogoman said:


> Two items that might have a major effect on MPG:
> 
> 1 - O2 sensors. They may not set an ECU fault code because they are still working but may be out of proper calibration due to age or contamination.
> 2 - Coolant temperature sensor that feeds the ECU. Same reason as #1 above.


Thanks for the reply.

Probably is the O2 sensors, mechanic said they might be bad but after they replaced the EGR solenoid it didn't read the code anymore.

There are only 2 sensors, correct? And I can replace them myself?


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## rogoman (Dec 16, 2004)

There are two O2 sensors; the front sensor screws into the exhaust manifold; the rear sensor screws into the rear CAT that's under the car. Easy enough to replace. However before replacing the sensors, first check the two engine grounds for tightness and oxidation build-up because the two sensors use shielded electrical connections which if not properly grounded could cause bad readings from the sensors.


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## b13er (Apr 7, 2010)

I saw a video on youtube where this 200sx was getting horrible gas millage and so they replaced the O2 sensor and then cleaned the MAF with CRC Mass Air Flow Cleaner and the fuel economy doubled.


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## Xmsteel (Mar 6, 2011)

rogoman said:


> There are two O2 sensors; the front sensor screws into the exhaust manifold; the rear sensor screws into the rear CAT that's under the car. Easy enough to replace. However before replacing the sensors, first check the two engine grounds for tightness and oxidation build-up because the two sensors use shielded electrical connections which if not properly grounded could cause bad readings from the sensors.


This is probably a really stupid question, but where exactly are the grounds?


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## rogoman (Dec 16, 2004)

There should be two ground connectors located at the top of the head near the fuel injectors.


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## Xmsteel (Mar 6, 2011)

Thanks. I took a look, they are dirty (as much as every other engine part), don't look like there are any problems and they are nice and tight.

Might decide to clean the connectors up when there isn't 8" of snow on the ground, but its probably the sensors themselves.

I found an upstream for about $12-$15 and a downstream for about $40. Does that seem about right?


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## rogoman (Dec 16, 2004)

That's about the going price. RockAuto has good prices. Go with the Denso brand; I think they are an OEM supplier for Nissan.


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## b13er (Apr 7, 2010)

You guys having 2 oxygen sensors is one reason I don't entirely hate having OBD1 (I have a 94 Sentra E); my one and only O2 sensor costs $10, and not only that but I can diagnose it as well:

I just turn the ECM's screw all the way clockwise, wait 2 seconds and turn it all the way counterclockwise, start the car, and then follow these instructions:









So can you do that with OBD2?

?


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## rogoman (Dec 16, 2004)

b13er said:


> You guys having 2 oxygen sensors is one reason I don't entirely hate having OBD1 (I have a 94 Sentra E); my one and only O2 sensor costs $10, and not only that but I can diagnose it as well:
> 
> I just turn the ECM's screw all the way clockwise, wait 2 seconds and turn it all the way counterclockwise, start the car, and then follow these instructions:
> 
> ...


According to the FSM, it'll work the same as on your car.


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## Xmsteel (Mar 6, 2011)

My car has gone as far on 1/4 as it did on half a tank last time, so maybe it was just a fluke or something.

I'll see what I get for mileage this time.


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