# Random loss of mpg’s



## MRO1791 (Sep 2, 2016)

Welcome. I had a 2012 VW TDI before the Cruze. I have a Gen 1 and 3 Gen 2s.. it's the family car! One of Gen 2s is a manual. 
Ok, so an MPG drop. You did well to check MAF and air filter. On my Gen 1 there is also a MAP/MAT sensor (pressure and Temperature) that can get dirty with soot and lower MPGs. I've not yet seen it on any of my Gen 2s, but the high mileage is the manual, and I'm still under 20k. Also you could be in a period of Regen. If you are doing short drives the cycle might not be finished and it keeps restarting. That will drop MPG. Fuel quality can be a factor as well, lower cetane fuel will drop MPG. Are you seeing any other symptoms?


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## Bbruew (Jul 6, 2019)

Awesome thank you for the response. I don’t typically drive short distances. My drive to work is usually 30 miles each way and sometimes and usually a 10-15 mile drive everyday. I was usually seeing 43-46 per tank and right now I’m seeing like 39-40. What’s the regen you are talking about?
Also is the only sensor I need to check the MAF which is in the intake tube?
I have put cetane boost in this tank and haven’t seen a difference yet. I’d say my biggest symptom I’m seeing is any throttle it’s killing the mpg’s. Like more than normal.


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## oregon_rider (Jul 21, 2017)

Fuel filter.....


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## Bbruew (Jul 6, 2019)

oregon_rider said:


> Fuel filter.....


Hmmm. Dash does say 38% left and I think it’s “supposed”to be done at 30k. You think that could affect it as well?


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## Barry Allen (Apr 18, 2018)

oregon_rider said:


> Fuel filter.....


What does a fuel filter have to do with fuel economy?


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## oregon_rider (Jul 21, 2017)

Barry Allen said:


> What does a fuel filter have to do with fuel economy?


 If the fuel filter is restricted you will have poor running, lack of acceleration and potential misfire.

My logic is that if the engine isn't running properly it will be inefficient.....

The service intervals for some things on this car seem way too long to me. For exanple I will be changing the fuel filter every 20k miles and the air filter once a year.As a side note I change the oil and filter every 5k miles ....

Jeff


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## Bbruew (Jul 6, 2019)

oregon_rider said:


> If the fuel filter is restricted you will have poor running, lack of acceleration and potential misfire.
> 
> My logic is that if the engine isn't running properly it will be inefficient.....
> 
> ...


Yeah I use to change my filter on my tdi every other oil change which was 10k. My car isn’t running bad or anything so I’m still pondering of what my issue can be.


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## plano-doug (Jul 1, 2015)

Bbruew said:


> Yeah I use to change my filter on my tdi every other oil change which was 10k. My car isn’t running bad or anything so I’m still pondering of what my issue can be.


What are you using to determine your fuel mileage? Just the display on the DIC? Have you done any hand calculations to confirm your actual mileage?

As far as I know, there is no fuel flow rate sensor in the system - the computer doesn't really know exactly how much or how fast fuel is being used - it guesses it using a things like throttle position sensor, RPMs, vehicle speed, ouija board and crystal ball  

Seriously, unless I'm missing something, the computer is really only guessing at fuel mileage. So, if you haven't already, I encourage you to verify it using the odometer and gas receipts before you spend too much time worrying about it.

Doug

.


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## Bbruew (Jul 6, 2019)

plano-doug said:


> What are you using to determine your fuel mileage? Just the display on the DIC? Have you done any hand calculations to confirm your actual mileage?
> 
> As far as I know, there is no fuel flow rate sensor in the system - the computer doesn't really know exactly how much or how fast fuel is being used - it guesses it using a things like throttle position sensor, RPMs, vehicle speed, ouija board and crystal ball
> 
> ...


Very true. I am really just going by how the DIC is reacting towards throttle and it’s not reading like it has the past 24,000 miles lol. I just notice it’s different and obviously it’s saying my mileage is less.


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## oregon_rider (Jul 21, 2017)

As a precaution and preventative measure - I'd change the fuel filter and add some diesel additive on the next tank of fuel - or add in some b20 biodiesel to clean and lubricate things. The additive (or b20 bio) will ensure the injectors are clean and the fprv is happy... 

For additive - Stanadyne performance formula works well and meets GMs recommendations (no alcohol and no emulsifer) - and my local oreilly auto parts stocks it (to my surprise...).

jeff


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## Blasirl (Mar 31, 2015)

Bbruew said:


> Hey guys I am new here but want to introduce myself. I’ve owned a few multiple vw diesels back in the day but last year I bought my 6spd diesel hatch RS model and have loved it. I’m coming up on 25k miles and I’ve done all service myself and continue to do so as I use to be a mechanic and enjoy it. Now let’s get to the point.
> 
> I have noticed the last few tanks of fuel my fuel mpg’s have been down a good amount. I’ve noticed that any throttle I give it it dips into the single digits and I don’t ever recall it being that low. I just replaced my air filter and I have been running some clean diesel through the tank just to rule out fuel. I always checked the maf to make sure it wasn’t dirty or anything. Can anyone relate or help?
> 
> Thanks and I look forward to some input.


Welcome Aboard

Don't forget to introduce yourself and your Cruze here.


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## Bbruew (Jul 6, 2019)

Has anyone done a video on the gen 2 filter swap or is it as easy as unscrew housing and change filter and o rings and screw back on?


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## oregon_rider (Jul 21, 2017)

There are instructions in the owners manual on how to remove, replace and prime the fuel filter. It must have a lift pump - because you prime by turning on in "service mode" and then cranking the motor (no longer than 40 seconds at a time).

jeff


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## Bbruew (Jul 6, 2019)

oregon_rider said:


> There are instructions in the owners manual on how to remove, replace and prime the fuel filter. It must have a lift pump - because you prime by turning on in "service mode" and then cranking the motor (no longer than 40 seconds at a time).
> 
> jeff


Copy. Thanks.


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## dieselboy731 (Feb 4, 2019)

Bbruew said:


> Awesome thank you for the response. I don’t typically drive short distances. My drive to work is usually 30 miles each way and sometimes and usually a 10-15 mile drive everyday. I was usually seeing 43-46 per tank and right now I’m seeing like 39-40. What’s the regen you are talking about?
> Also is the only sensor I need to check the MAF which is in the intake tube?
> I have put cetane boost in this tank and haven’t seen a difference yet. I’d say my biggest symptom I’m seeing is any throttle it’s killing the mpg’s. Like more than normal.


Regen is the cleaning of the diesel particulate filter. The engine injects fuel into the cylinder which then the exhaust stroke pushes it out the exhaust system while the diesel is still burning to heat up the exhaust to 1100 degrees.


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## Bbruew (Jul 6, 2019)

dieselboy731 said:


> Regen is the cleaning of the diesel particulate filter. The engine injects fuel into the cylinder which then the exhaust stroke pushes it out the exhaust system while the diesel is still burning to heat up the exhaust to 1100 degrees.


how long does the regen process last for?


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## Bbruew (Jul 6, 2019)

oregon_rider said:


> There are instructions in the owners manual on how to remove, replace and prime the fuel filter. It must have a lift pump - because you prime by turning on in "service mode" and then cranking the motor (no longer than 40 seconds at a time).
> 
> jeff


Do you have to go through the water draining process if you are replacing the filter? meaning is the water seperator in a different cavity from the fuel filter.


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## oregon_rider (Jul 21, 2017)

Bbruew said:


> Do you have to go through the water draining process if you are replacing the filter? meaning is the water seperator in a different cavity from the fuel filter.


 This is also covered in the owner's manual. The water is captured in the bottom of the fuel filter housing.

When you change out the fuel filter and drain the housing completely it will be purged of water.


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## Bbruew (Jul 6, 2019)

oregon_rider said:


> This is also covered in the owner's manual. The water is captured in the bottom of the fuel filter housing.
> 
> When you change out the fuel filter and drain the housing completely it will be purged of water.


I read the owners manual. I’m just confused.


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## Barry Allen (Apr 18, 2018)

plano-doug said:


> As far as I know, there is no fuel flow rate sensor in the system - the computer doesn't really know exactly how much or how fast fuel is being used


It's calculated by known injection quantity. Engineers know exactly how much each fuel injector injects down to the microgram, based on injection cycle and fuel pressure.


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## plano-doug (Jul 1, 2015)

Barry Allen said:


> It's calculated by known injection quantity. Engineers know exactly how much each fuel injector injects down to the microgram, based on injection cycle and fuel pressure.


Okay, that makes sense, at least for new/clean injectors. Thanks.

Doug

.


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## Barry Allen (Apr 18, 2018)

plano-doug said:


> Okay, that makes sense, at least for new/clean injectors.


My knowledge is that for gasoline injectors, most modern ECUs can measure and adapt as the injectors wear. During steady-state engine operation the ECU can monitor O2 sensors and do some internal "testing" of injectors. By increasing or decreasing the fuel quantity injected by one injector, the ECU can use the O2 sensor signals to adjust total fuel quantity based on those results. If the ECU detects one specific injector that is running lean, the injection cycle of that injector can be increased to bring the fuel quantity injected up to match other injectors. Or, an injector flowing more fuel than others can be trimmed down with a shorter injection pulse to decrease fuel flow.

I do not know if our diesel engines are capable of doing this.


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## oregon_rider (Jul 21, 2017)

Bbruew said:


> how long does the regen process last for?


There is some variability - manual states that it can take up to 20 minutes. I recall some here stating that it can finish in 10-ish minutes.

jeff


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## Barry Allen (Apr 18, 2018)

Bbruew said:


> how long does the regen process last for?


It depends on what kind of driving you are doing and other factors. At highway speeds on a hot day I have seen a regen cycle finish up in under 10 minutes. When doing city driving I have seen it go on for almost a half hour because the car is constantly speeding up and stopping, interrupting the cycle.


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## dieselboy731 (Feb 4, 2019)

Bbruew said:


> how long does the regen process last for?


Can take up to half an hour 


Sent from my iPhone using Tapatalk


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