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Discussion Starter · #1 ·
Took my 2009 GMC Duramax in for an oil change today & when i got it back my mileage went down the crapper...the truck has 4000 miles on it...I understand it could have gone into regeneration mode....But on the highway my engine oil life monitor dropped 8% in 15 miles...can someone explain this???
 

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What was the mpg before and after the oil change? And more importantly was the new oil put in the crankcase API CJ-4 or CJ-4 plus rated? If not then there could be a possiblity of the DPF getting clogged.
 

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Discussion Starter · #3 ·
I lost 3mpg...oil is full...& yes used 5w40 synthetic CJ4
 

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man there are a lot of threads on this lately.

the % jumps around. don't worry about it. very common.

change your oil based on mileage intervals, not what the computer tells you.
 

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Discussion Starter · #5 ·
Yeah i always change oil on a mileage basis...very suspicious this happens after the first service....wondering if they were in my computer
 

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Yes I wouldnt trust the oil life %, rather keep track on your own based on mileage and engine loading, etc.

If you're reading the drop in MPG from the DIC (which isnt as accurate as hand calculation) readout then you might consider resetting it. Bring up the MGP readout then press and hold the "check" button on the bottom of the controls until the MPG goes to "0". Then you can start over and see what it reads. Keep in mind that the reading is an average.

2009 Silverado LMM SBCC 4X4 all stock.
 

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Discussion Starter · #7 ·
Will Do...Thanks for the quick replies guys...
 

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The reason the oil life dropped is because you must have went into regen right when you left the dealership.

The reason it drops so fast is due to fuel dilution. Fuel gets into your oil, and the computer accounts for this by taking % off your oil life. IMO when you're stock with an LMM you should have a mileage that you want to change it out normally at, and do it either when the DIC says, or when your mileage hits, whichever comes first. The computer takes into account fuel dilution which you picking an interval does not. I don't know about these other guys, but I don't want an extra half gallon of diesel fuel in with my oil :eek:

search around, there's threads about guys pulling out close to an extra gallon of oil than what they put in. Thats the kind of "unspoken" crappy thing about the DPF and regen. Since I'm DPF free, I just go by the DIC, and change it when it gets down below 30% which was still like 7000 miles on my last change. Plenty for me, and I don't even run synthetic.
 

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No prob.

Don't be surprised if you see 20-30 MPG when you're cruising at highway speeds with no load and you reset it.......just push the throttle to floor and you'll see 9-10 MPG. Good reality check.
 

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Discussion Starter · #10 ·
All you guys on here already told me more than the techs at the GM dealership told me....They didn't have a suggestion to what could cause this...thanks again
 

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That's because to the guys at the dealer, trucks are their job, not their hobby.

Like Jesse said, I'm DPF-less and PCV-Less, and EGR-less, and my oil lasts a loooong time.
 

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Discussion Starter · #12 ·
How very true
 

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Engine oil life

That's because to the guys at the dealer, trucks are their job, not their hobby.

Like Jesse said, I'm DPF-less and PCV-Less, and EGR-less, and my oil lasts a loooong time.
The DPF is in the exhaust please explain how it affects oil life, inquiring mind want to know. Also doesn't the dealer reset the oil life when they do an oil change otherwise how does the computer know the oil was changed
 

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I had the same thing with my 2008 Duramax. Took it to the Dealer for an oil change and with less than 2000 miles on the oil change, I got the Change Oil message. Second oil change went approx. 20 miles from dealership to home and was down to 76% oil life remaining. MPG went down and had a slight loss of power. It took 3 different trips to the Dealership before they concluded it was the DPF that was bad. It was constantly in regen mode, which caused the oil life to continuously run down. They changed the DPF to correct the condition. I'm now at 4000 miles on the last oil change with 46% oil life remaining.
 

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Jeez how do you guys get such great milage? My oil is freshly changed, and I'm only gettin 14.5 miles per gallon and the dash says its getting 22.5. Maybe I just have too many miles. I have put in a few additives so maybe it'll improve a lil bit.
 

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Discussion Starter · #16 · (Edited)
Jeez how do you guys get such great milage? My oil is freshly changed, and I'm only gettin 14.5 miles per gallon and the dash says its getting 22.5. Maybe I just have too many miles. I have put in a few additives so maybe it'll improve a lil bit.
I run fuel additive every tank...(stanadyne)...buy it by the case...case treats 1500 gallons for $120...cheap piece of mind
 

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The DPF is in the exhaust please explain how it affects oil life, inquiring mind want to know. Also doesn't the dealer reset the oil life when they do an oil change otherwise how does the computer know the oil was changed
Because in order for the DPF to regenerate, the ECM adds extra fuel late in the combustion cycle to create high EGT's. Some of this fuel makes it into the oil and dilutes the oil with diesel fuel. If you watch next time your truck goes into regen, your oil life will drop at least a few % from when it starts to when it ends. This is the computer compensating for oil dilution, and why without your DPF you won't need your oil changed as often.

And yes the dealership should reset your oil life when you have it changed there. It's really easy to do, if they didn't do it I definitely wouldn't let them work on your vehicle :eek:
 

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They most likely "flashed" updates to the ECM which would have reset the computer. GM requires all diesels to be flashed with the most recent updates when rigs are brought in for service. This way, they can check if a tuner has ever been installed which is easy to verify on the LMM.

Give it some time and your mileage will return to normal...
 

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^^^ good point!! That could definitely be what happened.
 

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Discussion Starter · #20 ·
They most likely "flashed" updates to the ECM which would have reset the computer. GM requires all diesels to be flashed with the most recent updates when rigs are brought in for service. This way, they can check if a tuner has ever been installed which is easy to verify on the LMM.

Give it some time and your mileage will return to normal...
I out right asked them if they flashed the computor...They said no...The lube bay is behind a glass wall...you can watch them change your oil,but i left for a coffee for 10 minutes....may have done it when i was not watching.
 
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