MPG drop

9928 Views 34 Replies 17 Participants Last post by MPGeez, Sep 7, 2017

MPGeez

Discussion Starter · Jun 21, 2017

All of a sudden my MPG has dropped by 10mpg at cruise. I used to get 60 at 50mph. Now it's 50 at 50. Only major change was a new set of Walmart Goodyear assurance tires in stock size.
Is that the probable cause? We're the Oem Tires that much better?! Or is there a checklist to go through to find an issue? I'm at 55k miles. Mpg got worse at 45k along with the tire swap.
Any ideas?

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MPGeez

Discussion Starter · #21 · Jun 28, 2017

TX CTD said:
That's exactly a tire problem. They don't roll quite as well and they weigh more so it takes more power to turn them.

You must not understand what I said.
Cruising I can watch the mpg fluctuate from 60-45 and all over in between. 10k miles ago the fluctuation was 57-63 in the same circumstances.
That huge variance is NOT attributable to rolling resistance (a constant). It's a variable. There is something causing the PCM to add and subtract fuel in larger increments. Load isn't fluctuating while this is happening.
I believe I have a dirty or faulty sensor. The tires may drop he mpg a few but they won't cause that.

Reactions: 2

mkohan

· #22 · Jun 29, 2017

Did you have the emission recall done recently? I had the recall done at the beginning of the month. Prior to recall, hand calculated mileage was about 1.5-2 MPG better than DIC mileage. Today I filled up for the second time since recall work. Hand calculated mileage 5 MPG better the DIC. On the first tank I noticed a difference but chalked it up to the manual regen. Maybe there is a calibration problem when the computer is reset during recall work. just wondering

Reactions: 2

TX CTD

· #23 · Jun 29, 2017

If you think a 5lb heavier tire isn't a variable then I can't help. It could also be sensors or wind or the fuel mix or the season of the year.

MPGeez

Discussion Starter · #24 · Jun 29, 2017 (Edited)

TX CTD said:
If you think a 5lb heavier tire isn't a variable then I can't help. It could also be sensors or wind or the fuel mix or the season of the year.

The heavier tire will impact mpg, I'm not disputing that. Rolling resistance is a constant ONCE APPLIED. It will not cause mpg to VARY more than before but will cause it to LOWER.
There is no wind. I watch both directions and average them to get flat terrain mpg, I've tried multiple brands of fuel and I've gone through all four seasons over the 10k miles I've had the issue.
Again this has been going on for 10k miles. It's not a simple variable, im not tooting my own horn but I'm rather knowledgable with vehicles.
I've narrowed it to a mechanical issue (not a tire problem!).
I'm looking for Cruze diesel specific oddities that can cause this.

MPGeez

Discussion Starter · #25 · Jun 29, 2017

mkohan said:
Did you have the emission recall done recently? I had the recall done at the beginning of the month. Prior to recall, hand calculated mileage was about 1.5-2 MPG better than DIC mileage. Today I filled up for the second time since recall work. Hand calculated mileage 5 MPG better the DIC. On the first tank I noticed a difference but chalked it up to the manual regen. Maybe there is a calibration problem when the computer is reset during recall work. just wondering

No, haven't done the recall. But I am calculating actual mpg. And it correlates quite well to the Dic.

Cruz15

· #26 · Jun 29, 2017 (Edited)

15 mpg is pretty big range. Did you check all four tires to make sure they are all exactly the same tire? Could be voltage on something in the fuel system? Have you had any cel s from the fuel system before? Has dealer checked for wondering voltages on the critical sensors like maf, map and oxygen sensor? Im stabbing here. Its the stuff I would look at anyway.
Have you checked how your oil pump is running how about oil pickup?

starspangled6.0

· #27 · Jun 30, 2017

Perhaps you've been watching the MPG drop during a regen cycle?

obermd

· #28 · Jun 30, 2017

Cruz15 said:
According to CR the Cinturato P7 All Season Plus should give better fuel mileage than the Assurance Max.
It is rated as the lowest rolling resistance tire in our category of tires. They weigh exactly the same. Dia is .2" larger, I would have to put a pair on and test.

The FuelMax give better fuel efficiency. I rode on both.

MPGeez

Discussion Starter · #29 · Sep 6, 2017

Finally took it to the dealer after a couple CELs. They supposedly did the PCM update/recall and told me the mpg should be fixed and the codes were part of it. They also said they did a manual regen.

On the drive home I had a regen immediately. The next morning I popped the same cel.
Great job guys.

After doing research the code for the EGT sensor has a lot to do with the fuel trims. And is what I keep popping now. Each time I clear it the mpg drops. If it's set the mpg is 75% better.

Ordered a new EGT #2 sensor to fix it myself since the dealership can't do a simple job and doesn't even do what they say.
Will possibly report back in a week with progress.
Just goes to show you that even in a new to you unknown vehicle the best mechanic is still yourself. Just wish this site had more info but when the car is rare and most aren't diy I guess it's expected.
If only there was a reasonably priced self tuning/datalogging setup this would have been over a long time ago.

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Reactions: 1

Taxman

· #30 · Sep 6, 2017

TX CTD said:
If you think a 5lb heavier tire isn't a variable then I can't help. It could also be sensors or wind or the fuel mix or the season of the year.

Tires are only variables when you put them on or take them off.

While they're on the car and maintaining steady inflation pressure, they're a constant.
Or, in other words, OP is not running alongside the car at 50mph switching tires when his constant velocity fuel consumption is changing substantially.

oldestof11

· #31 · Sep 7, 2017

All of this talk about tires...

I went from stock setup getting 45ish mpg @ 80mph to 36mpg @ 80mph with 245/35/20s on Nankangs.

Tires may be partly to blame but not all of it.

raffmanlt

· #32 · Sep 7, 2017

Trying to figure mine out also. I have had a steady decline for a long time. Before and after tire change. Wierd mine didnt have the special tires when I bought it new. Currently avg of 30 mpg instead of 40 mpg us. I just cleaned map and it really needed it but doubt that was the total difference also. Just took off underside plastic havent put back on yet wonder what that will do to mpg?

Patman

· #33 · Sep 7, 2017

New tires overall will kill your MPG for a little while till broken in(drag created by new tire tread unlike the "old tires" ) given all else is OK.

AlbertaDiesel

· #34 · Sep 7, 2017

MPGeez

You mentioned in one post you were planning to clean the MAF. Did you ever end up doing it?

MPGeez

Discussion Starter · #35 · Sep 7, 2017

AlbertaDiesel said:
MPGeez

You mentioned in one post you were planning to clean the MAF. Did you ever end up doing it?

I took out the air filter and checked over the MAF. Both looked relatively new inside still. The MAF had zero buildup and was so clean inside there was no reason to do it.

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