# 2005 Altima bad gas mileage...again



## Surflost8 (Feb 21, 2015)

I know this issue has been discussed before but I have yet to see something solved. I just bought this 2005 Altima 2.5 a couple months ago with 135k miles and have been getting poor gas mileage from the beginning. I drove it from Detroit to South Carolina and averaged between 24 and 26 mpg while keeping it under 75 mph on the hwy. in town I'm getting 19 mpg. I drive slow and don't rev it. I don't let it idle a long time. I just changed the oil out with 5w-30 and tire pressure is at 35psi. I already changed the air filter but it didn't help. There's no CEL on and the car is peppy so I don't think it's a spark plug or O2 sensor issue. I was going to change the spark plugs tomorrow bc that hasn't been done since 66k miles. Is there anything else I should check that could be causing this? I've heard the pre cat can cause bad mileage but I want to be sure before I spend that much money. I figured if there was a bad O2 sensor then a CEL would come on. I do not have a bunch of junk in the trunk either FYI. 
Any not dumb ideas are welcome. Thanks.


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## rogoman (Dec 16, 2004)

Here are some possible causes for bad MPG:

- The spark plugs may be dirty; replace them. Use only the OEM NGK plugs. Using aftermarket types like Bosch may make the engine run worse.

- Consider replacing the O2 sensors; a marginally operating sensor will not necessarily set a fault code in the ECU. There are two sensors; one on top of the pre-cat and one on bottom. 

- The fuel injectors may be dirty. Run some good injection cleaner, like Techron or Redline SL-1, through the system; give the cleaner about a week or two to do it's job

- There may be a vacuum leak in the intake system. To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be around 21 InHg.

If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps. 

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Here are some vacuum gauge readings and their indications:

Low & steady: Late ign timing/valve timing, low compression

Very low: Vacuum leak

High & steady: Early ignition timing

Gradual drop in reading from idle to higher RPMs: Excessive back pressure in exhaust system 

Intermittent fluctuation at idle: Ignition miss, sticking valve 

Needle fluctuates as engine speed increases: Ignition miss, blown head gasket, leaking valve or weak valve spring 

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If the CAT is plugged up, it will affect the performance of the car. The easiest test for a plugged up CAT is done with a vacuum gauge. Connect the gauge to a full vacuum source on the intake manifold or throttle body. Note the reading at idle, then raise and hold engine speed at 2,500 rpm. The needle will drop when you first open the throttle, but should then rise and stabilize. If the vacuum reading starts to drop, pressure may be backing up in the exhaust system.


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## Surflost8 (Feb 21, 2015)

I will try replacing the plugs and fuel additive. I've read that spark plugs don't really affect gas mileage as long as they are all working. Either way, I'll replace them. 

My car does the weird thing of taking a few extra seconds to crank if the car has been sitting for like an hour after it was previously running for a bit. Not sure if that may have anything to do with poor mileage. Possible fuel leak? Evidently this is a common problem so I'd be surprised if that is true. 

I would like to replace O2 sensors but given that they are 100$ plus each I will try plugs and additive first. I don't have a vacuum tester but I'll get one if necessary. 

Thank you for your valuable input!


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## smj999smj (Jan 1, 2006)

The rear oxygen sensor is only a monitoring sensor for the ECM to determine if the catalytic converter is working efficiently and has no affect on gas mileage.
You can get a OE front oxygen sensor from Rockauto.com for $60. NTK makes the oxygen sensors for Nissan. You'll need NTK#24320. For spark plugs, use NGK Laser Platinums #6240; those are the original equipment plugs.


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## quadraria10 (Jul 6, 2010)

The ratings on Fuel Economy are 21 city 23 combined and 27 highway. You are not that far off. In Canada mileage tends to get a bit worse in winter, partly due to winter gas formulations with higher ethanol content, not to mention idling and lots of air con use to remove humidity. You are only off 2 mpg from the rated figures, maybe if you drove at 60 mph or waited for warmer weather you would hit the actual ratings. Not bad for a 9 to 10 year old car in my opinion.


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## Surflost8 (Feb 21, 2015)

Smj - Good info. Thank you.
Quad - I'm pretty sure this is unusually low mpg for this car. I know for a fact that this very car has done much better (it was my moms car). Other 2005 Altima drivers have reported mid twenties in city and over 30 on hwy. mine is a good 20% lower than that.


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## quadraria10 (Jul 6, 2010)

You are welcome. I was thinking the same way as you when I first got my X trail which has the same 2.5 engine. Simple truth I found out, was that my left foot is the main factor regarding gas consumption. I tend to take off a little faster than I should from stops, and I like to floor it getting on to highways. Its a peppy engine that is fun to push a bit for a 4 cylinder. If its any consolation you should be aware that most of the new model gas mileage claims are fiction and reality is a fair bit lower.


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## Surflost8 (Feb 21, 2015)

I don't think that 19 mpg city and 25 hwy is normal for the Altima. Even if it has 136k miles. Anyone with an older Altima care to chime in here?


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## cj1 (Oct 21, 2014)

Check fuel trim, see if adding fuel more than normal. If not it may be as good as it gets.


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## quadraria10 (Jul 6, 2010)

2005 Nissan Altima Annual Reports | Fuelly

Check the above for a real life example of one that owner has kept records for 5 plus years. In fact look at all of the 2005 2.5 same for 2004 and 3s and 2s. Its also instructive to look at the monthly graphs for specific cars. You will see fluctuations of 7 to 8 mpgs depending on time of year and different fill ups. 
2005 Nissan Altima (Nissan Altima) | Fuelly 
or look at this one for recent history
Green Hornet (Nissan Altima) | Fuelly

or study them all
2005 Nissan Altima Mileage

My point is, you haven't had the car that long, and the past couple of months have been cold no matter where you have been living in the US. Historically this is the time of year everyone gets their worst mileage. You should not be rushing to conclusions that something is wrong, especially when you have no check engine lights appearing.


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## Surflost8 (Feb 21, 2015)

cj1 said:


> Check fuel trim, see if adding fuel more than normal. If not it may be as good as it gets.


What is fuel trim? Never heard of that.


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## smj999smj (Jan 1, 2006)

You would need a scantool capable of reading datastream info to see fuel trim. 100% being ideal, it is used to show how much the ECM is richening or leaning the fuel injection to achieve the ideal mixture. Generally, 97-103% is fine, but a fuel trim greater or lower than that would indicate there is an issue causing the ECM to over-correct the fuel injection to create the ideal mixture.


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## Aussie Steve (Apr 13, 2018)

I have a 2006 Altima S with about 166k on the clock . I am only getting just under 17 MPG around town , with only about 24 MPG on a highway run...... I had new plugs put in with no improvement , what else causes such lousy MPG in this model . Many Thanks ....


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## LnR68 (8 mo ago)

Surflost8 said:


> I don't think that 19 mpg city and 25 hwy is normal for the Altima. Even if it has 136k miles. Anyone with an older Altima care to chime in here?


I have had my Altima for 16 years and mileage is going down and I treat her good, she has never given me probs til now. But then, she's my lil racecar!!!


Surflost8 said:


> I know this issue has been discussed before but I have yet to see something solved. I just bought this 2005 Altima 2.5 a couple months ago with 135k miles and have been getting poor gas mileage from the beginning. I drove it from Detroit to South Carolina and averaged between 24 and 26 mpg while keeping it under 75 mph on the hwy. in town I'm getting 19 mpg. I drive slow and don't rev it. I don't let it idle a long time. I just changed the oil out with 5w-30 and tire pressure is at 35psi. I already changed the air filter but it didn't help. There's no CEL on and the car is peppy so I don't think it's a spark plug or O2 sensor issue. I was going to change the spark plugs tomorrow bc that hasn't been done since 66k miles. Is there anything else I should check that could be causing this? I've heard the pre cat can cause bad mileage but I want to be sure before I spend that much money. I figured if there was a bad O2 sensor then a CEL would come on. I do not have a bunch of junk in the trunk either FYI.
> Any not dumb ideas are welcome. Thanks.


Dic you ever figure it out?


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## LnR68 (8 mo ago)

LnR68 said:


> I have had my Altima for 16 years and mileage is going down and I treat her good, she has never given me probs til now. But then, she's my lil racecar!!!
> 
> Dic you ever figure it out?


Did!


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## marius.noah (7 mo ago)

You are only off 2 mpg from the rated figures, maybe if you drove at 60 mph or waited for warmer weather you would hit the actual ratings. Not bad for a 9 to 10 year old car in my opinion.



Nox Vidmate VLC​


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## VStar650CL (Nov 12, 2020)

There's one big, very easy thing that often causes deteriorating gas mileage on older Nissans. That's IAVL (Idle Air Volume Learn). It's a procedure that calibrates the actual MAF and TPS readings to the ECM so it can correctly map airflow versus throttle angle. Without knowing the exact amount of air entering the engine, the ECM can't meter the correct amount of fuel. The IAVL is set at the factory, but all silicon devices can "drift" from specification as they age. The MAF is no exception. TPS's rarely drift much, but I've done "before and afters" on some older cars which didn't have an IAVL since Hannibal knocked at the gate, and in one instance found a MAF that had drifted a full 30%. Considering that your car takes in roughly 9000 gallons of air for every gallon of gas you burn, that's a _lot_ of error.

If you have a bluetooth scanner rig, the NDS-III app will do an IAVL painlessly in a few minutes. Or there's a manual procedure (although it can be very difficult to execute properly on I-key cars). At the dealership we do it as part of every 30K or induction service, and it should always be done after changing a MAF or PCV Valve, or after cleaning the Throttle Body.


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