[MPGeez]

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?

 

[MPGeez]

From personal experience Walmart sets the tire pressures low. They have set pressures for each rim size. I'd take a look there if you haven't already.

Oh yeah first thing I do is set my own pressure.
That was 10k miles ago so I've pumped it up a few times.

 

[MPGeez]

Have you seen this?

http://www.cruzetalk.com/forum/201-...-mileage-loss-fix-clean-map-sensor-again.html

Haven't seen that. That's job #1 now.

 

[MPGeez]

It is Walmart. Did they put the Assurance Fuel Max back on or plain Assurance?

I bought the non fuel max.
I didn't think tires could add 10mpg and still don't.
I cleaned my map sensor and will monitor it. I'll know on my first drive if that was it.

 

[MPGeez]

I saw about 3mpg from cleaning the MAP sensor. I'll try cleaning the MAF.
I'm noticing a larger than normal swing in mpg at cruise. I'm seeing 5-15 mpg swings on mostly flat road now.
That is not a tire problem to me.

 

[MPGeez]

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.

 

[MPGeez]

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]

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.

 

[MPGeez]

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.

 

[MPGeez]

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.