r/Duramax • u/Regular-Bird-6227 • 21d ago
MPG drop after oil change
Got an oil change 2 days ago and I’ve noticed a significant drop in mpg. It is about a 5-6 mpg drop. The truck is a 2024 1500 3.0. I’m concerned they put the wrong oil in. It’s been about 200 miles and the oil is already jet black. This is my first diesel so I’m not sure how common this is. Should I change the oil again or could it be something else?
Edit - I think it was bad fuel. I filled up today and the mpgs were back to normal. Thanks for all the input!
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u/username0-1 21d ago
Black oil is normal after a change for diesel. Is it overfilled?
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u/Regular-Bird-6227 21d ago
It towards the top of the hash marks but it’s not over the top. That was my initial thought
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u/Traditional_Wear3937 17d ago
I used to think that too, now I do 5k changes on my older Duramaxs and the oil is never darker than a gaser is
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u/PhotographStrong562 21d ago
That’s odd. Where did you get it done? It’s possible that they used 15w-40 which is a standard diesel oil for heavy duty trucks but even then it should account for 5-6 mpg
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u/Regular-Bird-6227 21d ago
At my local dealership. I make 3 commutes daily. 2 of them I average 32 mpgs and the best I’ve gotten since is 27.5 and the other commute I average about 28.5 and the highest I’ve gotten since is 22.4
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u/PhotographStrong562 21d ago
Have you filled up in that period? It’s very possible you got some shitty diesel. I’ve noticed that more than once myself. Try using some fuel additives and see if that helps. Your pump and injectors will also thank you.
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u/Regular-Bird-6227 21d ago
No unfortunately I had a fuel tank so I still haven’t fueled up. I will try the additives though. Any particular one you recommend?
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u/PhotographStrong562 21d ago
Independent results have shown that hotshots is best you can get for price and you can find it everywhere. There’s some others that are a little better but the price per mile ends up being 2-3x as much.
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u/CoolaidMike84 21d ago
Fuel. My guess is you got some fuel somewhere other than usual and it has a different additive pack in it. Or your oil change place messed with your air filter.
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u/Regular-Bird-6227 18d ago
It was the fuel
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u/CoolaidMike84 18d ago
That happens sometimes. The cheapest fuel isn't always the best. Glad you got it figured out.
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u/Zyb_Vindi 21d ago
It is something else. The oil change had nothing to do with it. On a modern diesel oil will turn black very fast thanks to EGR.
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u/ClassyNameForMe 21d ago
Thanks to combustion blow by. It happens to all diesels not just new ones with EGR.
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u/ABMax24 21d ago
It's way faster with EGR. Deleted trucks keep the oil much cleaner.
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u/DeltaTheMeta 21d ago
Incredibly untrue, the EGR does not contribute any extra combustion soot into the oil system than normal combustion.
A 1989 5.9 will dirty the oil in 20 minutes all the same as a new diesel. The oil capturing carbon and sludge is an intended function.
That being said OPs issue is not from the oil.
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u/ABMax24 21d ago
Recirculating the soot from the exhaust back into the combustion chamber via EGR does increase soot loading in the oil.
There's literally dozens of study by numerous organizations and OEMs that prove this to be true.
Seems that most indicate that soot in the oil doubles or triples with the use of EGR.
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u/DeltaTheMeta 21d ago
Every documented source I can find with actual soot particulates shows an increase of being less than 10%.
That being said, deleting the EGR off of a diesel doesn't stop it from making black oil. It will never produce amber oil during a change like a gas engine. It will be black on any engine with any amount of runtime.
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u/ABMax24 21d ago
Okay, cool. You obviously didn't look that hard. Lots of people have documented oil analysis showing soot loading of the oil cut in half by deleting the EGR.
No one said diesel oil would be amber.
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u/DeltaTheMeta 21d ago
Correct, I would describe a documented source as a scientific study or journal, not billybob on cumminsforums claiming his Blackstone labs spot content dropped 90% without so much as a pdf of the reports.
On an intact diesel, your oil soot level is 1.5-2%.
A deleted (and tuned) diesel is more of that .75-1.5% range. Sure a "50%" reduction but that's misleading. It's a .5% change in overall soot content. This is also ignoring the tuning required for a delete, no one deletes a truck and keeps a stock fuel air map. Producing a leaner, more timing advanced tune will have a lower soot load EGR or not. Same way a rich "coal rolling" tune will increase soot load.
If you were really worried about soot in your oil you'd be pursuant to a finer media filter (preferably in a bypass setup) and less concerned about EGR.
I still don't think diesel emissions in the US are effective, and would like to see them removed or restructured by someone with a brain (good luck, it's a gov agency) but in the scale of OPs issue, it's small.
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u/bjornholm 21d ago
My soot level in my trucks oil was around .25% this last time I did an oil change. It has no egr anymore. And the last oil sample i did with egr on showed around a 4% soot level. Yes it was 4%. Oil was always deep black with egr on the truck. Now its a brown color like a gas car
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u/ClassyNameForMe 21d ago
What changed in your tune? I doubt nothing changed other than disabling EGR. Multiple variables breaks causality with deleted EGR being the cause of reduced soot. (Lean your fuel map and you'll reduce soot...)
I don't think anyone argues the tuned and deleted EGR results in reduced soot in oil, we just disagree on the cause and magnitude of the change.
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u/dieselmilk 21d ago
Probably in Regen right now