# 18 DIESEL mpg down



## mr overkill (Dec 1, 2013)

So ever since I had the transmission replaced 6 weeks ago the cars mpg has been down. This weekend I took a drive from NJ to Baltimore (this is where I usually get my highest milage 62mpg)(66 with some help) and it struggled. I'm down an average 10mpg. 15 if I was trying to maintain 65. 

Tires are at 42 cold, rotated, checked to make brakes are not holding, fresh oil change, fresh air filter, cleaned map sensor. The egr should be closed 100% of the time 😉 .
I am in 9th on the highway 2200 at 80. 


Just weird. Somethings changed with the car and I cannot figure out what yet


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## snowwy66 (Nov 5, 2017)

Who did the trans?
Was it a rebuild or a parts trans from the wrecking yard. Or a dealer oem?

What oil is in the motor? Did you switch brands? As much as people like to argue that oil is oil. It's not.

Gas isn't gas either. Or fuel in your case.


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## mr overkill (Dec 1, 2013)

snowwy66 said:


> Who did the trans?
> Was it a rebuild or a parts trans from the wrecking yard. Or a dealer oem?
> 
> What oil is in the motor? Did you switch brands? As much as people like to argue that oil is oil. It's not.
> ...


 Gm replacement. Dealer replaced 
Same oil as day 1. Amsoil euro 530


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## Goodluckgluck (Nov 13, 2021)

Could it have been possibly regenerating?


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## mr overkill (Dec 1, 2013)

Goodluckgluck said:


> Could it have been possibly regenerating?


For 6 weeks?


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## Diesel4Ever (May 30, 2018)

mr overkill said:


> For 6 weeks?


I had a TDI with a bad thermostat that would constantly regen. No codes but I could see it on my maestro equipped pioneer receiver. I know they’re different cars but you may have a bad sensor or damaged wire causing something like that. If you have an OBD2 monitor like Torque you may be able to see what’s happening.


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## mr overkill (Dec 1, 2013)

Diesel4Ever said:


> I had a TDI with a bad thermostat that would constantly regen. No codes but I could see it on my maestro equipped pioneer receiver. I know they’re different cars but you may have a bad sensor or damaged wire causing something like that. If you have an OBD2 monitor like Torque you may be able to see what’s happening.


 I have it monitored. It's doing Regens at 825 miles(max milage before forced regen)


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## mr overkill (Dec 1, 2013)

mr overkill said:


> I have it monitored. It's doing Regens at 825 miles(max milage before forced regen)


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## snowwy66 (Nov 5, 2017)

A sign of plugged DPF maybe? Or def not working?


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## 15cruzediesel (Sep 30, 2019)

Since it is a replacement and not a rebuild it is possible the transmission is still tight and needs a few more miles to loosen up since the rebuild. Maybe 500 miles for the gears??

If you ever bought a new car it takes some time for the drive train to break in before fuel mileage is correct.

With that being said. I'd monitor MAP, MAP desired and boost to see how hard it's working on high mileage roads. (Ideal conditions) Along with your instantaneous mileage. 

All three should be very low when the car isn't working hard and mileage should be high.


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## shimmy816 (Aug 22, 2020)

could it be a relearn issue? BTW I have the same 2014 cruze diesel .... love it..


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## mr overkill (Dec 1, 2013)

shimmy816 said:


> could it be a relearn issue? BTW I have the same 2014 cruze diesel .... love it..



Yea right now I'm averaging what my 14 was getting loloo. I'll track it down sooner or later


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## skietzman (Apr 9, 2015)

Why was the transmission replaced? If you there is an update or tech bulletin out there for the trans, it is most likely that you received that update. Also, an update may have been mandatory for the new transmission and it's warranty. I am always leery of what the dealer does.


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## mr overkill (Dec 1, 2013)

Lost 9th gear then the trans quit. The new trans did the new tcm update amd it actually makes it shift great. 4th and up. But a tcm reflash does not touch ecm files


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## mr overkill (Dec 1, 2013)

So this tank. On a clean ecm. 500 miles on 13.1 gallons= 38.16. 80% + highway


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## Barry Allen (Apr 18, 2018)

I could be paranoid, but I believe my 2018 6MT Cruze sedan is declining in fuel economy. The last couple of fill-ups have been during the start of summer weather where using AC is mandatory, but I could swear the fuel economy isn't achieving what it used to.


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## Barry Allen (Apr 18, 2018)

I swear, highway driving is now achieving about 42-44 mpg on a known route where I used to be able to get 52-54 for the trip. I know it's summer and I'm using the AC, but summer fuel should also have slightly more BTUs than winter blends (unless there is lots of biodiesel in the fuel). It's like something is wrong with my car and I have the 6MT transmission. Attached here is a chart of all my fuel-ups. It's declining.


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## 6speedTi (May 18, 2018)

With a noticeable drop you need to look at regeneration cycles being forced for some reason. Fuel quality such as cetane ratings. Big changes in cetane rating added with a possible second issue might affect your mileage. Fuel injectors, fuel pressure and any other fuel delivery problem. Air filter maybe? Could have a mass air flow sensor reading at the edge of its parameters?
O2 Sensor acting up? Many things to look at but start by hooking up a scanner and see what you have. Do the easy stuff first. Are you using an Oil type air filter like K&N? Oil will eventually clog up the mass air flow sensor. Something just happened to affect the mileage that much.


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## mr overkill (Dec 1, 2013)

Regens are averaging from 500 miles to 700 miles. All fresh filters. Always used amsoil 530. Checked /cleaned map,Mas, o2


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