Loss of MPGs

[nolahoe]

My ‘16 Z71 has a 2” level and 275/60/20s. I was seeing 14 city and 19 highway for a few months. After the dealership replaced the brake vacuum pump for a recall and my mileage dropped to 10 city. They insist that the brake vacuum has nothing to do with the mileage, but I noticed it the day I picked it up from the shop. Does anyone have any ideas what would cause a sudden drop in MPGs? Where should I start looking?


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[Pittz]

Intrestimg, but I will agree with the shop that one has nothing to do with the other.

 

[FrankU]

The cold weather will do that.

 

[BG1988]

My ‘16 Z71 has a 2” level and 275/60/20s. I was seeing 14 city and 19 highway for a few months. After the dealership replaced the brake vacuum pump for a recall and my mileage dropped to 10 city. They insist that the brake vacuum has nothing to do with the mileage, but I noticed it the day I picked it up from the shop. Does anyone have any ideas what would cause a sudden drop in MPGs? Where should I start looking?


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check the tires make sure it up to par... i normally keep mine at 45-50psi... just going from 32 to 45 will add 10% gain in mpg.. this is why manufactures use 38psi instead of 32

 

[Miami-Dade]

My ‘16 Z71 has a 2” level and 275/60/20s. I was seeing 14 city and 19 highway for a few months. After the dealership replaced the brake vacuum pump for a recall and my mileage dropped to 10 city. They insist that the brake vacuum has nothing to do with the mileage, but I noticed it the day I picked it up from the shop. Does anyone have any ideas what would cause a sudden drop in MPGs? Where should I start looking?


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A number of members here [including myself] had the recall with the dealer replacing the brake vacuum pump. No one had a issue with gas mileage dropping after the recall.

Could be the cold weather like Frank said. If you switched to a different brand of fuel that could be the issue.

 

[Doubeleive]

other possible causes, bad cat's, bad torque converter, I experienced mpg loss in both cases.

 

[nolahoe]

other possible causes, bad cat's, bad torque converter, I experienced mpg loss in both cases.

I’ll start with upping the tire pressure a little. How would I diagnose cat or torque converter issues? It’s not throwing any codes, but I am getting a rough shift here and there.


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[Doubeleive]

I’ll start with upping the tire pressure a little. How would I diagnose cat or torque converter issues? It’s not throwing any codes, but I am getting a rough shift here and there.


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you can buy a infrared thermometer, harbor freight has them starting at around $30-35, then you can check the cat temps, if you have a scanner that can read live data you can see what it is reporting for the cat temps also @swathdiver may have more info on the temps, I was getting pretty crappy mileage without any other explanation and had been checking my temps they were high but still within spec and about the time I started checking & monitoring them sure enough I got a cat code so I just replaced them, that was almost 2 years ago and the price of cat's has now doubled (probably corona related) so cross your fingers that's not it, right after the cats were replaced I gained 2mpg back. As far as transmission I could feel mine I was getting "shudder" at first at low speed around 30-35 mph, then it started doing it more and more often and at higher speeds and if I accelerated very quickly (pedal to floor) then I would get a 1-2 shift flare, but you may experience something totally different there are a lot of moving parts inside a transmission. I just had mine rebuilt recently and I have gained 2+mpg back as well. If your transmission is questionable it might be a good idea to swing by a (reputable) transmission shop and maybe have them test drive it or just tell them what is happening and see what they say.
how many miles are on you truck?
if you are at or over 100k the 02 sensors should be replaced (100k is the recommended interval)
have the plugs & wires been done? it may not have a big impact but they should also be done at or before 100k
air filter, clean the maf and throttle body that may help not necessarily for mpg but it can make a difference in general and with shifting because the maf reads the air and then everything else reacts accordingly.

 

[nolahoe]

you can buy a infrared thermometer, harbor freight has them starting at around $30-35, then you can check the cat temps, if you have a scanner that can read live data you can see what it is reporting for the cat temps also @swathdiver may have more info on the temps, I was getting pretty crappy mileage without any other explanation and had been checking my temps they were high but still within spec and about the time I started checking & monitoring them sure enough I got a cat code so I just replaced them, that was almost 2 years ago and the price of cat's has now doubled (probably corona related) so cross your fingers that's not it, right after the cats were replaced I gained 2mpg back. As far as transmission I could feel mine I was getting "shudder" at first at low speed around 30-35 mph, then it started doing it more and more often and at higher speeds and if I accelerated very quickly (pedal to floor) then I would get a 1-2 shift flare, but you may experience something totally different there are a lot of moving parts inside a transmission. I just had mine rebuilt recently and I have gained 2+mpg back as well. If your transmission is questionable it might be a good idea to swing by a (reputable) transmission shop and maybe have them test drive it or just tell them what is happening and see what they say.
how many miles are on you truck?
if you are at or over 100k the 02 sensors should be replaced (100k is the recommended interval)
have the plugs & wires been done? it may not have a big impact but they should also be done at or before 100k
air filter, clean the maf and throttle body that may help not necessarily for mpg but it can make a difference in general and with shifting because the maf reads the air and then everything else reacts accordingly.

I’ve got a trip this weekend that I’ll run with tires inflated to 40 psi. I’ve got a scanner that reads live data, my cat temps were sitting at 400 degrees - I haven’t looked up what it should be though.

I think what I’m feeling is actually a misfire and not a transmission slip. I’ve got 115k miles on it and had the plugs, wires, and coil packs replaced at 102k. I was getting the same misfire before and it went away for about 10k miles. I’m going to take it back to the shop and see if they can figure that piece out.

Once I’m back from this trip I’ll clean the maf and see if that gets me anywhere.

Thanks for all of your input!!


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[Doubeleive]

I’ve got a trip this weekend that I’ll run with tires inflated to 40 psi. I’ve got a scanner that reads live data, my cat temps were sitting at 400 degrees - I haven’t looked up what it should be though.

I think what I’m feeling is actually a misfire and not a transmission slip. I’ve got 115k miles on it and had the plugs, wires, and coil packs replaced at 102k. I was getting the same misfire before and it went away for about 10k miles. I’m going to take it back to the shop and see if they can figure that piece out.

Once I’m back from this trip I’ll clean the maf and see if that gets me anywhere.

Thanks for all of your input!!


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400 seem's low that must be just cruising around town? get it up on the highway and see what the temps are at highway speeds