Sudden drop in MPG for 2015 Malibu 2.5L - Chevy Malibu Forum: Chevrolet Malibu Forums
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post #1 of 10 (permalink) Old 09-05-2018, 06:49 PM Thread Starter
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Sudden drop in MPG for 2015 Malibu 2.5L

I have had this car for about a year now, purchased with 67k and now has 84k. I have always had 30.5 to 33 ish MPG, but over the last few weeks I cannot get my average above 25-27MPG on the DIC or when I crunch the numbers at the pump. What could this be? Note ** oil is changed regularly, tires are not low, air filter recent.
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post #2 of 10 (permalink) Old 09-05-2018, 08:38 PM
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Re: Sudden drop in MPG for 2015 Malibu 2.5L

Do you reset the DIC at each fill-up or has it been running continuously? Resetting will allow it to show you the running total, which would be 1 tank. Letting it run continuously will show the average over the life, with some suggesting that it removes anything over 1000 miles old from the calculation. That idea has not been validated.

Air filter.

Spark plugs.


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post #3 of 10 (permalink) Old 09-06-2018, 04:48 AM Thread Starter
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I can reset it and it will start out high but then drop down to around 25. But even just taking your trip mileage total and dividing it by the gallons going in it only adds up to 24-25 when it used to be 30-31
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post #4 of 10 (permalink) Old 09-06-2018, 10:13 AM
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Re: Sudden drop in MPG for 2015 Malibu 2.5L

This is a direct injection engine. 3+ years and 84,000 miles is more than enough time for carbon build-up in the intake ports/valves to have effect on power and efficiency. I suggest running a top engine cleaner like seafoam or CRC spray cleaner, and perhaps Techron Concentrate injector cleaner added to a few gas tanks as well.

A top engine cleaner is necessary because direct injection motors are not self-cleaning the way older port-injected engines are. Decades ago, when my timid-driving mom's old buick needed emissions checked, my dad would take the car and drive it like he stole it for 10 minutes to "blow out the carbon." This doesn't work with the fuel injectors inside the cylinders (i.e. direct injection), so cleaner needs to be applied via the air intake to dissolve carbon deposits in the ports and on the valves.

So run seafoam and techron concentrate and drive the car like you stole it. Run two seafoam treatments if you feel inclined. Then see how your fuel economy changes. Lots of seafoam tutorials on youtube if you need help.


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post #5 of 10 (permalink) Old 09-06-2018, 10:22 AM
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Re: Sudden drop in MPG for 2015 Malibu 2.5L

Quote:
Originally Posted by DrivenDaily View Post
Do you reset the DIC at each fill-up or has it been running continuously? Resetting will allow it to show you the running total, which would be 1 tank. Letting it run continuously will show the average over the life, with some suggesting that it removes anything over 1000 miles old from the calculation. That idea has not been validated.

Air filter.

Spark plugs.
He said he hand calculated MPG as well in the OP.

The 1000 mile MPG average was what I deduced in the 7th gen Malibu after I had driven the car over 10,000 miles without resetting it, and yet each 300 mile tank of gas affected the average MPG in mathematically impossible significance. Ergo, the average MPG screen must be capped at a round number like 1000 miles, give or take. This is probably not applicable to the 8th gen.

The newer gauge cluster programs have an actual lifetime mileage counter in some models as well as fuel economy per trip mileage A and B.


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Last edited by cp-the-nerd; 09-06-2018 at 10:25 AM.
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post #6 of 10 (permalink) Old 10-20-2018, 07:54 PM Thread Starter
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Quote:
Originally Posted by cp-the-nerd View Post
This is a direct injection engine. 3+ years and 84,000 miles is more than enough time for carbon build-up in the intake ports/valves to have effect on power and efficiency. I suggest running a top engine cleaner like seafoam or CRC spray cleaner, and perhaps Techron Concentrate injector cleaner added to a few gas tanks as well.

A top engine cleaner is necessary because direct injection motors are not self-cleaning the way older port-injected engines are. Decades ago, when my timid-driving mom's old buick needed emissions checked, my dad would take the car and drive it like he stole it for 10 minutes to "blow out the carbon." This doesn't work with the fuel injectors inside the cylinders (i.e. direct injection), so cleaner needs to be applied via the air intake to dissolve carbon deposits in the ports and on the valves.

So run seafoam and techron concentrate and drive the car like you stole it. Run two seafoam treatments if you feel inclined. Then see how your fuel economy changes. Lots of seafoam tutorials on youtube if you need help.
After doing this and cleaning the mass air flow sensor my mpg is back in the 29.5 range, thanks! Don’t know if I should replace the spark plugs to see if that helps anything.
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post #7 of 10 (permalink) Old 10-22-2018, 11:40 AM
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Re: Sudden drop in MPG for 2015 Malibu 2.5L

Hey! Thanks for updating, and I'm really glad my advice helped! I would suggest running seafoam every couple years or 30k miles for maintenance.

As you approach 100k miles, changing spark plugs wouldn't be a bad idea. If there was a significant amount of carbon build up, it could certainly affect the efficiency of the spark plugs as well.


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post #8 of 10 (permalink) Old 10-22-2018, 05:28 PM
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Re: Sudden drop in MPG for 2015 Malibu 2.5L

While on my twice a year 543 year road trip to visit the parents, was cruising at 74MPH blissfully...and outta no where, CAR suddenly over revs, dashboard throws blinking codes and slows to 45MPH...I pull over, whip out Torque Pro, and see these 3 codes: *Current Fault - P2097 - Powertrain Post Catalyst Fuel Trim System too Rich bank1; Pending Fault P0089 - Powertrain Suction control valve stuck; Historic Fault - P228C - Powertrain...Tap Web view for more information...

So I continue my drive and note that I have to shift in M mode and NOT let the car exceed about 2,175rpm...It took 2.5 minutes to get up to 63MPH slightly downhill for the remainder of my trip to the dealer. I am told that I need a "high pressure fuel pump" to the the tune of 685 dollars. Am picking up my car tomorrow morning and then driving back to VA.

Just want folks to know that I previously experienced a HUGE drop in MPG around end of September and took my car to the dealer and was told there was nothing wrong with my car after driving in with the "P228C - Powertrain...Tap Web view for more information" idiot light on...My Gen 8 has 41,183 miles...I purchased a 1,686 dollar 100K "extended warranty" March 16, 2013 but obviously the paperwork is stuffed into a drawer at home...I asked the SC Chevy dealer why do I have to pay anything at all and they said we're not aware of any "extra warranty status" in the database....

Pissed in South Carolina...Hoping to get back to Virginia at normal highway speeds Wednesday...Point is this, if your car suddenly takes a nose dive in MPG and you take it to an expert who says "nothing is wrong"...find a new mechanic....and be smart...send your paperwork to the CLOUD! CPS
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post #9 of 10 (permalink) Old 10-22-2018, 06:01 PM
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Re: Sudden drop in MPG for 2015 Malibu 2.5L

I hope you get back safely and quickly, and that you find the paperwork. If you know who it's through, you might be able to call the company and get them to give you the info the dealer needs to hopefully cover it under that warranty.
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post #10 of 10 (permalink) Old 10-23-2018, 08:59 AM
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Re: Sudden drop in MPG for 2015 Malibu 2.5L

Thanks DD; I apologize for ranting earlier, mainly b/c my wife wouldn't let me buy an SS earlier...then I saw cp-the-nerd's post...guess it triggered me b/c he probably 'got better gas mileage than me' the past few months...(Which was the entire reason I was not "allowed" to follow 'his' lead...) I told the wife to hold up looking through my den drawers b/c she was getting too close to my exorbitant "Mod" receiptstucked under the bottom drawers...again, those purchases were b/c of folks like you and cp-the-nerd's fault...LOL...I just picked up the car, test drove it around SC and am happy b/c the mileage apparently is gonna be back pushing 37ish on the highway after the 68 mile test! (Plus the car has it power/acceleration back...like it was brand new.) CPS
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