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Hello,
I own the 3500 Promaster (High-Roof/Extended) 2018 with 285k mi and my gas mileage has dropped in the last 80k miles significantly.

I used to be able to get about 430mi on full tank (topped off) so about 18mi/g - most highway miles at little over speed limits - but it has gradually dropped down to about 380mi (16mi/g) - at best.

I mostly buy regular gas; however, I have tried to clean out the fuel injectors with premium gas and the cleaning liquid added almost every 3k miles. This has not helped much and it seems the gas mileage is only getting worse over time. I was told that as long as I maintain the vehicle at required times then there is no reason why the gas mileage should drop that low.

Is there something I can do to fix this issue? 2 mechanics told me that having the fuel injectors cleaned out from inside won't help - is that true? Or is that what's needed in this case? They were not cleaned out so far.

Any advice/suggestions will be much appreciated!!

Thank you for your time.
 

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1. Premium gas actually has a bit less BTU than regular so if regular is sufficient for your van you will get a bit better mileage with that.
2. The injectors are not likely to be the issue but they can be removed and cleaned if several applications of Techtron or another cleaner doesn’t help.
3. Winter lowers your fuel mileage as does driving in rain, higher speed, and many other factors.
4. Tires seem to be a large factor in these vans, tall, narrow, tires always return better mileage as does higher air preassure. I changed from the Vanco originals to Nexens and I think I am down about 1.5 mpg.

So what can you do? Burn regular gas, forget the injectors, slow down, replace those tires with a tall, narrow high pressure tire. I doubt there is anything else wrong unless your van is burning a lot of oil, shows signs of low compression, or is skipping or missing. All those can be tested.
 

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MMXVI - L2H2 in Indiana
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There was a recent post, I thought it was from Kip, having something to do with the air fuel mixture being enriched and recommended the owner get the mechanic check that.
 

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I have tried to clean out the fuel injectors with premium gas
The word "premium" is a marketing scheme to get you you pay extra money for fuel you don't need.
There is nothing "premium" or "ultra" about any of it.
It all has to do with the % of octane in the formula. And no, more octane doesn't make your car faster.
Octane resists combustion from pressure/heat alone. So, if you have forced induction (supercharger) and high compression, more octane in the fuel will prevent the fuel from detonating before the spark occurs (pre-detonation).
That is absolutely ALL octane percentages have to do with.
Unless you have forced induction and/or higher compression ratios that require a higher minimum octane %, there is no reason to use anything other than 87.
And it's been ages since I did all my homework on this, but I'm pretty sure they all have the same BTU.
However, in addition to the things Phil and RD referred to, (Winter weather and tire wear being at the top of the list), fluctuating levels of ethanol and water absorption in different batches and different gas stations could also be a culprit.
 

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Phil:
Wasn’t it ambient air temperature sensor out of spec?
RnR:
BTW A gallon of gasoline will usually contain from 115,000 to 125,000 BTUs. Rate of burn can be more important than BTU content actually but at low RPM’s more BTU’s gives more MPG generally. Ethenol and many other additives LOWER the BTU’s. I expect 10% Ethanol gas might have significantly less BTU than 115,000.
Ethanol has 76,100 BTU/gallon! More Ethanol less mpg!
Yes I know there are other factors.
 

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MMXVI - L2H2 in Indiana
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Phil:
Wasn’t it ambient air temperature sensor out of spec?
Found it,
I can feel your pain. And I hate to be the bearer of bad news. But something is amiss here. I lay the blame, as I usually do, on dealer incompetence.
I own 3 2014s, identical to a 2018 in every way, with a combined 1.3 million miles on them. So I have a good idea of how they should behave. The high idle should kick in a 40 degrees, not 20. The big complaint is downshifting going down hills, not at steady state highway. The throttles are all excessively sensitive, not insensitive. At 62mph in winter with cruise, you should be in the mid 18s for mileage. In summer, flat and level at 62mph in a long wheelbase van, you should be at high 19s low 20s regardless of weight loaded. The trans shifting should be smooth and little noticed. At 60mph, you should be at exactly 2000rpm.
The telltale for me is your range reading. I have verified that the computer knows it has a 24 gallon tank. That yours thinks different tells me something. The other red flag is the high idle - I’d be looking to verify that the idle air intake temp sensor is reading correctly, because if it’s not the engine will be enrichened all the time, which will result in bad fuel economy and possibly harsher shifts with more downshifts. Similar problems could be caused by a misreading throttle pedal, since how hard you are stepping on it informs shift logic.
I’d recommend going to a different dealer, and just ask them to plug it in and poke around the computer. They may find all kinds of things. Or, I’d make a stink with your dealer and take the tech on a ride in yours back to back with a ride on another one on the lot. Point out the differences. Have him compare your fuel trims to a new one’s fuel trims. Look for discrepancies.
 

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These posts are making me more concerned about our 2021 3500. We averaged only 12 mpg in our first ~7400 mile trip in the brand new van. It weighs ~8800 lbs, and it do have 245/70-17 BFGs on it (running 70 psi front and 80 rear). The roof is pretty streamlined with a low-profile rack. We usually drive around 70 mph and the best I saw on flat ground in top gear was usually about 16.

One thing I noticed is that it will drop into 5th gear for almost any reason. Even on almost flat ground and it always seems hesitant to up shift again. It did this even when I was driving it empty with factory wheels and tires and no roof rack. I experimented with Tow/Haul mode and it didn’t seem to make much difference.

Another thing I noticed is that, with cruise control on, which I use almost all the time, it seems to be injecting fuel to control speed down even slight downhills. I also noticed that the cruise control is very “crude”…it usually doesn’t accelerate until it’s about 5 mph below the setting and then overshoots by about the same amount…I think this is also wasting fuel. Does anyone else have similar experiences or ideas? I’d chalked it up to the weight but it seems some people are getting much better mileage even close to fully loaded. I’ll try to check the IAT and fuel trim readings but I’d expect some codes on a brand new van if that was an issue. Thanks!
 

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12 mpg seems pretty low. I have a 2021, built out with full tanks it weighs about 7900. We average right about 17 mpg. Probably 80% of our 18,000 miles have been on two lane highways, don't really care for interstates, so our speed is lower than your 70 mph.

I agree about the cruise control, it's not real precise.

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Clogged air filter seems very unlikely on such a new van. Something blocking the air intake plumbing perchance? Engine idle normal when warm?
 

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2018 3500 EXT Camper Conversion in CT (TX for now due to Covid)
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These posts are making me more concerned about our 2021 3500. We averaged only 12 mpg in our first ~7400 mile trip in the brand new van. It weighs ~8800 lbs, and it do have 245/70-17 BFGs on it (running 70 psi front and 80 rear). The roof is pretty streamlined with a low-profile rack. We usually drive around 70 mph and the best I saw on flat ground in top gear was usually about 16.
A few observations:

Its hard to get very good mileage with the cruise engaged. I get 14-16 with cruise and 16-20 when I control it, because I allow the van to slow a little uphill and go a little faster downhill than the cruise will.

The tires are not going to help. I love BFGs but have always lost a few mpg with them on other cars. My Subaru lost about 3 on the highway from near 30 to 26.

Anything on the roof is not as low profile as you think and probably cost you 1 mpg on the highway, the faster you go the more it is.

While I can get ~20 at 60, I can only manage at best about 17 at 70 under perfect conditions(an no cruise). In the city the weight makes it tough to get a lot better than 14 in my van (about 7700 pounds, so lighter than yours). There is a big difference in economy as you go faster. 70 is 16% faster than 60, but results in about 36% more drag. 75 is 25% faster but is more than 50% higher wind resistance.

The gearing makes certain speeds difficult. below 60 mph the engine doesn't make enough power to hold top gear. Its really hard to get good economy shifting back and forth. The exact speed where there is not enough power to hold top gear can change a little with tire size. Sometimes going a little faster is paradoxically better for economy.

All that said, 12mpg is very low. Unless you were driving with a lot of elevation changes or very fast. (Driving the speed limit on a tollway in texas at 85mph gave me about 12mpg)
 

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Are you using high mileage oil? If so the heavier oil may contribute to lower mileage.
Maybe jack it up one wheel at a time and see if you may have some dragging brakes or worn wheel bearings.
 

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A few observations:

Its hard to get very good mileage with the cruise engaged. I get 14-16 with cruise and 16-20 when I control it, because I allow the van to slow a little uphill and go a little faster downhill than the cruise will.

The tires are not going to help. I love BFGs but have always lost a few mpg with them on other cars. My Subaru lost about 3 on the highway from near 30 to 26.

Anything on the roof is not as low profile as you think and probably cost you 1 mpg on the highway, the faster you go the more it is.

While I can get ~20 at 60, I can only manage at best about 17 at 70 under perfect conditions(an no cruise). In the city the weight makes it tough to get a lot better than 14 in my van (about 7700 pounds, so lighter than yours). There is a big difference in economy as you go faster. 70 is 16% faster than 60, but results in about 36% more drag. 75 is 25% faster but is more than 50% higher wind resistance.

The gearing makes certain speeds difficult. below 60 mph the engine doesn't make enough power to hold top gear. Its really hard to get good economy shifting back and forth. The exact speed where there is not enough power to hold top gear can change a little with tire size. Sometimes going a little faster is paradoxically better for economy.

All that said, 12mpg is very low. Unless you were driving with a lot of elevation changes or very fast. (Driving the speed limit on a tollway in texas at 85mph gave me about 12mpg)
Thanks…those observations help and line up with some of what I was thinking. Especially about the cruise control. And the speed…the few times we were on 50-60 mph 2-lame roads and in a snowstorm on an interstate where we were only going about 60 is where we seemed to get about 16 mpg. And we did cross the all the mountain ranges between the east and west coasts twice on this trip!
 

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Are you using high mileage oil? If so the heavier oil may contribute to lower mileage.
Maybe jack it up one wheel at a time and see if you may have some dragging brakes or worn wheel bearings.
This was all in the factory fill, so so special oils and I just changed it with the same (Penzoil Platinum 5-20). I can check the rolling resistance areas you mentioned but that seems like a long shot on a new van. I was also seeing similar low mileage when the van was totally new and empty…
 
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