[Garret78]

My 2006 Malibu Maxx SS (3.9L V6) is averaging 13.67 mpg. My last fill up was 11.6 mpg!
I purchased this vehicle a month ago and track my mileage on every fill up. This is not the information coming from the computer, these are my calculations based on miles driven between fill ups.
I replaced the EVAP purge valve and got a new gas cap after a code. The CEL hasn't gone off since.

Any ideas on why the vehicle is getting HORRIBLE gas mileage?

 

[cp-the-nerd]

There are tons of things that can affect MPG. The state of maintenance on the engine in general, spark plugs, O2 sensors, catalytic converters, MAF sensor, clogged intake or throttle body. What possibilities have you eliminated?

 

[Scooter75]

I came from a Maxx with the 3.9. Super great reliable car that I lost to a deer. MPG will depend a lot on type of driving. If driving around the city 13-14 isn't that far off and definitely not horrible compared with the car's capability. The car is rated for 16 city 24 highway I got about 17 in mixed driving. Since you have 15 years of unknown history I'd start with plugs and O2 sensors. I found the O2 sensors finicky make sure you get OE. You should be able to get a couple MPG back.

 

[Garret78]

There are tons of things that can affect MPG. The state of maintenance on the engine in general, spark plugs, O2 sensors, catalytic converters, MAF sensor, clogged intake or throttle body. What possibilities have you eliminated?

I haven't elimiinated any of the above. I'm a bit of a newb, but I'm very capable of moderate repairs/maintenance.
What would you start with? At 96k miles I should start with plugs? If I remove the O2 sensors, can I visually inspect them for wear? What about the fuel injectors?

 

[cp-the-nerd]

1. Absolutely check and replace the plugs with OEM spec, you're due.
2. Open and inspect the air intake box and filter. Remove the filter and look for clogs, install new OEM paper filter if necessary.
3. Pull the MAF sensor and visually inspect the filament for crud (do NOT touch the filament, buy a dedicated touchless MAF cleaner spray).
3. Open and inspect the throttle body. Spray a rag with throttle body cleaner and thoroughly clean it out if dirty.
4. Add a bottle of techron injector cleaner to a half tank of gas and drive the car aggressively to clean out ports and injectors.

Techron injector cleaner:

https://www.amazon.com/Chevron-Techron-Fuel-System-Cleaner/dp/B009L91EWO/ref=sr_1_5?crid=2TPW2GESNOC4T&dchild=1&keywords=techron+plus+fuel+system+cleaner&qid=1603471879&sprefix=techron+plu+%2Caps%2C149&sr=8-5

CRC MAF Cleaner

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CRC Throttle Body Cleaner

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[cp-the-nerd]

If none of that works, then examine/replace the downstream O2 sensor. After that check the cats.

 

[Darthz]

Also make sure your tire pressure is up and correct.... makes a difference even on short highway jaunts for me and my 3.5.

I cant believe they consume that much more.... wonder if they have significantly different tire sizes.... in my 85 montecarlo..... I had 245 60 15 all the way around.... swapped to stock 195's?... made a huge difference in milage for me.

My 3.5 JUST averaged 7.8 l/100km on a 260 km trip 3 days ago.... and i have a maf code and busted up loud exhaust. ... pure highway. ... 87 octane. I am around 11.5 to 12 for pure city driving.

Some great suggestions above.... ya ypu want the best milage... gotta get a bit finky with maintenance

 

[repairman54]

At 96k your due for plugs. Change the O2 sensors also, use only AC plugs and sensors, the upstream sensors that are closest to the engine as they control fuel mixture. There is no looking at the sensors to see how they are like a spark plug. O2 sensors get lazy / slow responding as the miles go on.
I've gotten back 2-3 mpg on putting in fresh O2 sensors.
Clean MAF
We all have been down this road and this is what has worked for us.

 

[cp-the-nerd]

Change the O2 sensors also, use only AC plugs and sensors, the upstream sensors that are closest to the engine as they control fuel mixture.

Dang, I had it backwards and said downstream. I over thought it.

 

[Garret78]

1. Absolutely check and replace the plugs with OEM spec, you're due.
2. Open and inspect the air intake box and filter. Remove the filter and look for clogs, install new OEM paper filter if necessary.
3. Pull the MAF sensor and visually inspect the filament for crud (do NOT touch the filament, buy a dedicated touchless MAF cleaner spray).
3. Open and inspect the throttle body. Spray a rag with throttle body cleaner and thoroughly clean it out if dirty.
4. Add a bottle of techron injector cleaner to a half tank of gas and drive the car aggressively to clean out ports and injectors.

Techron injector cleaner:

https://www.amazon.com/Chevron-Techron-Fuel-System-Cleaner/dp/B009L91EWO/ref=sr_1_5?crid=2TPW2GESNOC4T&dchild=1&keywords=techron+plus+fuel+system+cleaner&qid=1603471879&sprefix=techron+plu+%2Caps%2C149&sr=8-5

CRC MAF Cleaner

Amazon.com: CRC 05110 Mass Air Flow Sensor Cleaner - 11 Wt Oz.: Sports & Outdoors

Buy CRC 05110 Mass Air Flow Sensor Cleaner - 11 Wt Oz.: Mass Air Flow - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

CRC Throttle Body Cleaner

Amazon.com: CRC 05078 Throttle Body and Air-Intake Cleaner - 12 oz.: Sports & Outdoors

Buy CRC 05078 Throttle Body and Air-Intake Cleaner - 12 oz.: Carburetor & Throttle Body Cleaners - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

Thanks!
I just replaced plugs and wires. Intake and filter were cleaned/replaced with oil change after purchase (1 month ago). I regularly blow and vac the filter and vac out the intake box. MAF visually appears clean. I did a fuel induction and throttle bottle cleaning. I'll throw a bottle of injector cleaner in today. I also did this immediately after purchase.
Fingers crossed.

 

[Garret78]

At 96k your due for plugs. Change the O2 sensors also, use only AC plugs and sensors, the upstream sensors that are closest to the engine as they control fuel mixture. There is no looking at the sensors to see how they are like a spark plug. O2 sensors get lazy / slow responding as the miles go on.
I've gotten back 2-3 mpg on putting in fresh O2 sensors.
Clean MAF
We all have been down this road and this is what has worked for us.

Is there any difference between O2 sensors? I'm seeing a Bosch @ $140 and a Walker @ $125. Does the extra $15 make a difference?

 

[Drunken Elvis]

You have a code for the fuel system? Perhaps you should take care of that first.

 

[repairman54]

Is there any difference between O2 sensors? I'm seeing a Bosch @ $140 and a Walker @ $125. Does the extra $15 make a difference?

Use original AC brand O2 sensors only, other do not last or give issues. Bosch and Walker are junk.
Rock Auto has AC original sensors for less than $40 each.

 

[Darthz]

Use original AC brand O2 sensors only, other do not last or give issues. Bosch and Walker are junk.
Rock Auto has AC original sensors for less than $40 each.

X2 on bosch o2 being junk.... in my other life (Camaro tech) bosch o2's are junk.... we in that application use ngk ....if i remember correctly or denso...

 

[cp-the-nerd]

AC = ACDelco brand OEM parts, in case that's confusing. With rare exception, use factory parts for replacements.

 

[Garret78]

Replaced plugs and wires. Had throttle body cleaned. Put Techron fuel injector cleaner into tank. Just came inside from replacing 02 sensor (upstream/ACDelco). Fingers crossed.

 

[Garret78]

So ... I'm still sitting at 13 mpg UGH! When I run the car there is a kind of exhaust/oil smell coming in from the vents (noticed more now that I'm running the heater). I got a FIXD OBD2 device and it indicates potential 02 sensor issues and potential EVAP system. Again, I've replaced EVAP solenoid/purge valve, replaced upstream 02 sensor, replaced sparks/plugs and had the throttle body cleaned. Also, I've run a bottle of Techron fuel injector cleaner through. Any suggestions for next steps?

 

[cp-the-nerd]

What are you basing your fuel economy estimates on? If you're using the gauge cluster average readout, have you reset it since doing all the work?

 

[Garret78]

What are you basing your fuel economy estimates on? If you're using the gauge cluster average readout, have you reset it since doing all the work?

NOT using the gauge cluster. I track my mileage and calculate mpg everytime I fuel up.

 

[Drunken Elvis]

What are "potential" ussues withe EVAP and O2 sensors? I suggest you take care of the EVAP problems first.