Gas mileage dropping faster than the stock market, less than 200 to the tank...

[YanaCakes]

Before I sell my soul for gas....I have a 2012 Tahoe LT, 5.3 4WD. The truck has a 3.5 inch lift and sits on 33 inch tires. I was getting around 300-330 to the tank. In the last 2ish years, it has progressively declined to where I can barely get 200 to the tank. I do mostly city driving for reference.

I had an intermittent evap code that comes and goes by itself. About a month ago, I replaced the purge valve and charcoal canister. Also did a smoke test, all good. Code hasn't come back. Gas mileage seemed to be improving andddd then tanked..... Back to under 200 to the tank. This tank may be closer to 150-175 with how it's looking. No other codes, truck runs great, no rough idle.

Please help.

 

[OR VietVet]

You need to get a live data scanner attached and see what the fuel trims are. If you do mostly city driving, your mileage will suck. When did the lift and bigger tires get put on and how soon after that did the mpg go down? What gear are you running?

 

[j91z28d1]

alcohol precent?

 

[Fless]

Ditto on looking at short- and long-term fuel trims.

Get a scanner on it that can read the alcohol percentage that the computer thinks is in the system. Do you run E85? Or just the gas with lower ethanol content (E10, E15)?

If you're running E10 or so, not E85, but the computer's "virtual sensor" thinks the engine is drinking E85, it's going to push more fuel into the combustion chamber. If the system alcohol vs actual (pump) numbers aren't close, then reset the alcohol % with the scanner. Then change the fuel-contaminated oil.

Let us know what you find.

 

[Doubeleive]

bad cat's will cause a mpg loss also, just something to consider as well. check the temps front & rear and maybe do a backpressure test. easy enough and you can then rule it out. when mine first went bad I had a solid 2mpg loss before a code finally showed up just as I was questioning/testing the cat's because nothing else made sense, everything was solid.

 

[blondie70]

Check for dragging brakes also.

 

[rdezs]

I will double down on the catalytic converters being plugged. If so and you continue, about the time the spark plugs start to get fouled you'll notice a drop in power. When this happened to me some years ago, this last symptom hit fast. Driving home from 30 miles away, suddenly I was having to hold the pedal to the floor just to do 40 mph. By the time I got home it wouldn't do 20 miles per hour. Definitely test it before it gets that bad, made a hell of a mess of the inside of my engine. Not to mention O2 sensors. At least back then you could buy a "catalytic converter test pipe" that bolted right in for $29.... Somehow I got left in place for another hundred thousand miles

 

[YanaCakes]

You need to get a live data scanner attached and see what the fuel trims are. If you do mostly city driving, your mileage will suck. When did the lift and bigger tires get put on and how soon after that did the mpg go down? What gear are you running?

Thank you for that advice. I am running stock gears. I got the lift and tires done about 6 years ago. And the fuel issue started in the last few years. I got a simple Bluetooth scanner but I’m not sure what I am looking for or if the scanner is any good.

 

[YanaCakes]

alcohol precent?

According to my scanner, around 4%

 

[YanaCakes]

Ditto on looking at short- and long-term fuel trims.

Get a scanner on it that can read the alcohol percentage that the computer thinks is in the system. Do you run E85? Or just the gas with lower ethanol content (E10, E15)?

If you're running E10 or so, not E85, but the computer's "virtual sensor" thinks the engine is drinking E85, it's going to push more fuel into the combustion chamber. If the system alcohol vs actual (pump) numbers aren't close, then reset the alcohol % with the scanner. Then change the fuel-contaminated oil.

Let us know what you find.

I do use regular. I have a little Bluetooth scanner but I don’t know how accurate it is so I’m gonna grab a decent one. I will definitely keep you updated because this has baffled

 

[YanaCakes]

bad cat's will cause a mpg loss also, just something to consider as well. check the temps front & rear and maybe do a backpressure test. easy enough and you can then rule it out. when mine first went bad I had a solid 2mpg loss before a code finally showed up just as I was questioning/testing the cat's because nothing else made sense, everything was solid.

You know, I was thinking about the cats the other day but thought the light would be on. I will check.

 

[YanaCakes]

Check for dragging brakes also.

I replaced the pads, rotors and hubs two year ago because I was getting drag. And rechecked everything a few months ago, greased pins, etc. But… my brakes squeak, they sounded the same before I changed everything. I’m attributing it to crap brakes. Once NYC winter is over and the salt is gone, I’m gonna do ceramics and see.

 

[YanaCakes]

I will double down on the catalytic converters being plugged. If so and you continue, about the time the spark plugs start to get fouled you'll notice a drop in power. When this happened to me some years ago, this last symptom hit fast. Driving home from 30 miles away, suddenly I was having to hold the pedal to the floor just to do 40 mph. By the time I got home it wouldn't do 20 miles per hour. Definitely test it before it gets that bad, made a hell of a mess of the inside of my engine. Not to mention O2 sensors. At least back then you could buy a "catalytic converter test pipe" that bolted right in for $29.... Somehow I got left in place for another hundred thousand miles

Oh no! I want to try to avoid that and I’m sorry that happened to you. And there is a gas smell when I start it. I think someone else mentioned that too. I will look into it.

 

[YanaCakes]

You need to get a live data scanner attached and see what the fuel trims are. If you do mostly city driving, your mileage will suck. When did the lift and bigger tires get put on and how soon after that did the mpg go down? What gear are you running?

Thank you for your service

 

[OR VietVet]

What is stock gears? An inexpensive decent live data scanner would be a Foxwell NT301. Check it out. You also might try a slower burning higher octane fuel, like 89. Also does a better job of cleaning deposits. Might even try some Techron additive. Over the long haul, the cleaner the better and more efficient. If you are lifted and have bigger tires and have a higher gear like 3.42 or higher, you are geared to high, especially with just city driving.

 

[petethepug]

 

[Fless]

Here's a somewhat detailed video on diagnosing a clogged cat. The last half deals with the analysis of how Wes arrived at the decision to check the backpressure, along with his sketches of how things work -- or should.

 

[YanaCakes]

What is stock gears? An inexpensive decent live data scanner would be a Foxwell NT301. Check it out. You also might try a slower burning higher octane fuel, like 89. Also does a better job of cleaning deposits. Might even try some Techron additive. Over the long haul, the cleaner the better and more efficient. If you are lifted and have bigger tires and have a higher gear like 3.42 or higher, you are geared to high, especially with just city driving.

This one?! And I have 3.42 ratio.

 

[j91z28d1]

According to my scanner, around 4%

what scanner do you currently have? the cheap ones don't usually read that.

 

[YanaCakes]

Here's a somewhat detailed video on diagnosing a clogged cat. The last half deals with the analysis of how Wes arrived at the decision to check the backpressure, along with his sketches of how things work -- or should.

Wow, thank you so much for sharing the video. It was super helpful. I am going to try this when I get a decent scanner.