Attempted a short economy run (5.3 vortec)

Pandabird · Aug 22, 2024

Went to visit the old folks the other day, roughly 200 miles back and forth. Went quite heavy on the pedal on the way there with the AC on full since it was a sunny day. Jumped a bit when I checked the average mpg (converted from L/100km for your convenience) it was at 14 MPG and I was like yikes! I've made a previous note about the AC because it makes a constant whiny noise when turned on. Anyhow I refused to believe i couldn't get it down to near 20mpg on my way home so I literally crawled it at ~50 mile speeds with very smooth pedaling and whatnot, AC turned off because the outside air was cool enough once it got dark. it went down to 20 alright. Actually ended up at almost 22mpg which is a really good result in my opinion.
Could of course have something to do with more uphill one way there and more downhill on the way back.

Anyways few notes since this is one of the first actual rides in my Tahoe not counting grocery runs.
Is ~500 rpm idling normal? Doesn't seem to struggle with it, it just looks low compared to what i'm used to.
Is that oil pressure gauge meant to be this active? it seems to idle right between the 0 and 40 but when I hit the gas it rises to 35 ish?
The automatic gearbox upgears downhill without touching the pedal, is this normal?
My cruise control is posessed by a demon, we have tons of speed cameras here so it's a no-no to use as is.
And does the AC truly add this much MPG? Why is it so whiny when in use though quiet when turned off (can't be a bearing since it still runs on that same belt right?)

RST Dana · Aug 22, 2024

Manual calculations?

Geotrash · Aug 22, 2024

Pandabird said:
Went to visit the old folks the other day, roughly 200 miles back and forth. went quite aggressively on the way there with the AC blaring since it was a sunny day and jumped a bit when I checked the average mpg (translated from L/100km for your convenience) it was at 14 MPG and I was like yikes! I've made a previous note about the AC because it makes the engine noise whiny when turned on so I had it turned off when showing off the engine to my friends. Anyhow I refused to believe i couldn't get it down to near 20mpg on my way home so I literally crawled it at ~50 mile speeds with very smooth pedaling and whatnot, AC turned off because the outside air was cool enough once it got dark. Results had me baffled. it went down to 20 alright. But it kept crawling down and down, little by little, fought some uphills, cursed at some traffic stops since they seemed to set me back. But once home I was almost at 22mpg.

Anyways few notes since this is one of the first actual rides in my Tahoe not counting grocerie runs.
Is ~500 rpm idling normal? Doesn't seem to struggle with it, it just looks low compared to what i'm used to.
Is that oil pressure gauge meant to be this active? it seems to idle right between the 0 and 40 but when I hit the gas it rises to 35 ish?
The thing upgears downhill without touching the pedal, no engine braking allowed
My cruise control is posessed by a demon, we have tons of speed cameras here so it's a no-no to use as is.
And does the AC truly add this much MPG? Why is it so whiny when in use though quiet when turned off (can't be a bearing since it still runs on that same belt right?)

Oil pressure gauge behavior is normal. A/C whine is not. The A/C clutch bearing turns when the clutch is disengaged and compressor shaft isn’t turning. But when the clutch engages, the shaft turns and a different set of bearings turns with it, along with the scroll inside the compressor.

My guess is you have a compressor on its way out.

tom3 · Aug 23, 2024

Could possibly be an idler pulley with the extra load of the compressor making it a bit noisy. More likely what the above post noted though. My older brother is always saying the biggest loss of gas mileage is using the brakes. You're driving technique on that return trip probably made a big difference.

swathdiver · Aug 24, 2024

Pandabird said:
Anyways few notes since this is one of the first actual rides in my Tahoe not counting grocery runs.
Is ~500 rpm idling normal? Doesn't seem to struggle with it, it just looks low compared to what i'm used to.
Is that oil pressure gauge meant to be this active? it seems to idle right between the 0 and 40 but when I hit the gas it rises to 35 ish?
The automatic gearbox upgears downhill without touching the pedal, is this normal?
My cruise control is posessed by a demon, we have tons of speed cameras here so it's a no-no to use as is.
And does the AC truly add this much MPG? Why is it so whiny when in use though quiet when turned off (can't be a bearing since it still runs on that same belt right?)

RPMs is normal for emissions compliance. Oil pressure fluctuations is normal as it's based on load/speed.

Transmission uses best gears for going up a hill or down a hill with cruise control on with varying precision depending on the model year.

Running cruise control makes the engine more fuel efficient.

My compressor started whining back in 2018 and still does to this day. My 2012 Sierra's compressor started whining about 2 years ago. Both blow cold and do not drag on the motor; so far. The '13s is quiet still. I hear them whining all the time at the gas station, other people's cars. Must be using the same bearings as the 1990s GM power steering pumps!

yonsayvitch · Oct 21, 2024

I think GM's compressors are a weak point. My 2008 Suburban is on its 3rd one. Funny how these new Tahoe's/Suburban's don't get better MPG. Mine has about 332,000 ( 2nd engine has 212,000 ) and I can still get 22mpg if paying attention. Mix town and highway. Normal average around 16. Looking at buying a 2025 Tahoe Z71 diesel but I don't want to wait until April or May. So we are settling with the 5.3.

Marky Dissod · Oct 21, 2024

Pandabird said:
Is ~500 rpm idling normal? Doesn't seem to struggle with it, it just looks low compared to what i'm used to.

Yes. Pretend for a moment that you are a car company trying to eke out every fraction an MpG you possibly can,
and don't forget that you DO NOT WANT that vehicle to last too long.

Pandabird said:
Is that oil pressure gauge meant to be this active? it seems to idle right between the 0 and 40 but when I hit the gas it rises to 35 ish?

Yes. More oil pressure is needed when the engine is working/playing, compared to idle.

Pandabird said:
The automatic gearbox upshifts downhill without touching the pedal, is this normal?

Yes, if the downhill is moving the vehicle instead of the engine, it allows for upshifts to happen even sooner than on flat ground.

Pandabird said:
... does the AC truly add this much MpG?

It's certainly possible, most GM vehicles run hotter with AC OFF than with AC ON.
So it's not just AC parasitic drag, if the motor oil and ATF are a bit colder, they're a bit thicker,
which lowers MpG a bit.

swathdiver said:
Running cruise control makes the engine more fuel efficient.

Any driver with more than a passing familiarity with hypermiling,
knows how to get better fuel mileage by AVOIDING cruise control - by sacrificing time, especially uphills.
That said, on flat ground or downhill, if cruise control keeps the TCC locked and stays in the highest gear possible,
then yes, cruise control can use the engine efficiently.

j91z28d1 · Oct 22, 2024

yonsayvitch said:
I think GM's compressors are a weak point. My 2008 Suburban is on its 3rd one. Funny how these new Tahoe's/Suburban's don't get better MPG. Mine has about 332,000 ( 2nd engine has 212,000 ) and I can still get 22mpg if paying attention. Mix town and highway. Normal average around 16. Looking at buying a 2025 Tahoe Z71 diesel but I don't want to wait until April or May. So we are settling with the 5.3.

it is a bit sad every newer Gen of ls/lt gets worst fuel mileage and is less reliable than the one before it.

the trucks do get heavier and they do make a bit more hp but they aren't actually any faster. glorified mini vans is all 90% of them get used as while getting 12mpg. haha.

Pandabird · Oct 22, 2024

j91z28d1 said:
it is a bit sad every newer Gen of ls/lt gets worst fuel mileage and is less reliable than the one before it.

the trucks do get heavier and they do make a bit more hp but they aren't actually any faster. glorified mini vans is all 90% of them get used as while getting 12mpg. haha.

Some people had warned me about gasoline engines and hauling but I was pulling roughly 4000lbs with a lot of wind resistance and the mileage wasn't bad at all. I was prepared for maybe 12mpg but in reality it was more like 16+.

I like the tow/haul mode! I made a previous mention about the upshifting but apparently in tow/haul mode the vehicle finally uses engine braking. Very useful if I want to drive in lapland where there's miles of miles of downslopes (a lot of people *destroy* their brakes on these)

yonsayvitch · Oct 22, 2024

Yeah crossing my fingers on reliability. If it's similar to the 08 I'd be happy. Looking forward on seeing that 1/2 million mark and then some.

Attempted a short economy run (5.3 vortec)

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