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Old 08-24-2012, 11:51 AM   #1
flacopower
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Default 7 MPG drop while using AC

My commute is roughly 46 miles RT with 70% being highway. Per my scangauge 2, I've been averaging 19mpg per tank while driving very modestly. Yesterday, my AC belt snapped off at the very beginning of my commute home. So I just rolled the windows down and drove like normal. I replaced the belt last night.

Well today I checked the scangauge and was shocked to see I had gotten 25MPG yesterday (including the morning commute w/ AC on). I left the AC off today and returned 26.5MPG. On the highway, as soon as I turned on the AC the mileage dropped immediately back into the teens. I know AC causes parasitic loses, but doesn't 27% seem really high? Is this a sign my AC is not operating correctly? Additionally, what can I do to decrease the MPG drag of my AC?

CLIFFS: Belt snapped yesterday so no AC, registered 7MPG higher (per Scangauge 2) than normal commute.

Is my AC operating correctly?
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Old 08-24-2012, 11:57 AM   #2
NitroTrike
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It would have to be a pretty dry rotted belt to snap, so your compressor may be really tight and about to seize.

However, I'm not convinced any scan tool can accurately measure MPG given the variable of fuel returned to the tank etc.
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Old 08-24-2012, 12:35 PM   #3
flacopower
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not trying to sound defensive, but the scangauge is pretty accurate. by most estimation within 5% when i calculate miles driven/gallons to fill up tank
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Old 08-24-2012, 12:36 PM   #4
flacopower
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please tell me more about my compressor being tight. what does it mean, how do i measure, any preventative measurses?

this is the second belt in two and a half years and the last one was an oem belt
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Old 08-24-2012, 12:58 PM   #5
zashoe
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I agree about the compressor being tight. belts generally don't just snap. I'm honestly not sure how to test it. but they aren't too much at a salvage yard.
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Old 08-24-2012, 06:39 PM   #6
NitroTrike
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I'm surprised it's that accurate. I wonder what algorithm the Scangauge uses to "know" the fuel pump flow and the gazillion of other variables that don't even have sensors to monitor.

Anyway, back to the compressor issue. Was the belt just cleanly snapped, or was it burned / stretched at the break? If it was burned, the belt was just loose and the crank pulley spun in it until it let go. Was it squealing at all prior to breaking?

Has the A/C recently been charged? An overcharge will cause compressor stall / lockup too, but I'm not sure how much harder it will be to turn before this happens.

Nothing really explains the 27% difference though - but a silly question, since it's a bugeye.. have you had the fuel line recall / update done?
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Old 08-25-2012, 11:28 AM   #7
WhiteBgeye02
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w/ the AC on and your given situation which i dont really know it could be putting you in OL fueling and dumping more fuel in w/ the AC on. like being on the very edge of the engine load scaling and then when the AC goes on the engine load goes up and moves over to the next cell. is your car tuned? stock map? that info would be helpful. also how tight was your belt? belts even after dry rotting last years.
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Old 08-26-2012, 02:14 AM   #8
Uncle Scotty
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in cant believe a 7mpg difference witout something being broke somewhere....ac clutch bearings going bad or idler pully bearings or the tune being on the edge of ol

is the fuel filter clogged???
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Old 08-26-2012, 01:39 PM   #9
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Fuel filter was changed a couple years ago, I'm still on stock tune. My only mods are downpipe and intake. Only code is p0136 (rear 02 sensor wiring)
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Old 08-26-2012, 04:48 PM   #10
flacopower
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Just cleaned air filter and maf.

Also going to change pcv valve.

Do you guys think it could be the front o2 sensor (even if I'm not throwing a code?) I was getting an overboost code a while back...
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Old 08-26-2012, 06:47 PM   #11
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change the fuel filter...reset the ecu
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Old 08-26-2012, 08:31 PM   #12
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Ok... Will do tomorrow. Is there anyway to be able to tell if the fuel filter is bad?
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Old 08-26-2012, 09:19 PM   #13
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I noticed a pretty sizable drop with a/c on as well, this was while cruising at 70/75 however. At 90 today, with a/c on it was in boost the whole time lol. Oh dear. You could pretty much see the gauge going down.
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Old 08-27-2012, 01:42 AM   #14
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Quote:
Originally Posted by flacopower View Post
Ok... Will do tomorrow. Is there anyway to be able to tell if the fuel filter is bad?
Fuel filter was changed a couple years ago

i think that's a REAL good indication, there
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Old 08-27-2012, 01:44 AM   #15
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Quote:
Originally Posted by tebriel View Post
I noticed a pretty sizable drop with a/c on as well, this was while cruising at 70/75 however. At 90 today, with a/c on it was in boost the whole time lol. Oh dear. You could pretty much see the gauge going down.
yet another reason i like my 06....i can go 90 all day long at 100*F, ac on '2' and get 23-24mpg
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Old 08-27-2012, 10:31 AM   #16
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Well yeah that's probably what I was getting. This drop is down from 30mpg.
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Old 08-27-2012, 10:39 AM   #17
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i just made a lon trip (10 hours each way to the beach and back home) on the way there i got 29-30mpg with no ac. on the way home i was getting around 21-22 with ac on the whole way. gf was cryin cause it was hot out. so it does drop a good bit
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Old 08-27-2012, 01:12 PM   #18
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Quote:
Originally Posted by NitroTrike View Post
I'm surprised it's that accurate. I wonder what algorithm the Scangauge uses to "know" the fuel pump flow and the gazillion of other variables that don't even have sensors to monitor.
all it needs to know is injector on time and vehicle speed.
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Old 08-27-2012, 08:52 PM   #19
knotcher
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don't trust the scangauge...test it a bit more before you try and fix anthing. teh AC doesn't pull that much and shouldn't make that much of a difference
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Old 08-28-2012, 09:23 AM   #20
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Quote:
Originally Posted by flacopower View Post
My only mods are downpipe and intake. Only code is p0136 (rear 02 sensor wiring)
You need a tune for a downpipe
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Old 10-22-2012, 01:04 PM   #21
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Quote:
Originally Posted by NitroTrike View Post
It would have to be a pretty dry rotted belt to snap, so your compressor may be really tight and about to seize.
What would cause a compressor to become tight? Was the AC system recently recharged?
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Old 10-22-2012, 02:41 PM   #22
Andrewxxcarlson
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Quote:
Originally Posted by indytruckboy

You need a tune for a downpipe
^^^^^
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Old 10-22-2012, 10:01 PM   #23
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Quote:
Originally Posted by indytruckboy View Post
You need a tune for a downpipe
not for the bugeyes ya dont
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Old 10-24-2012, 12:18 AM   #24
silverbulletwagon
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Default weird

there is major drag somewhere?
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Old 10-26-2012, 01:09 AM   #25
Grippen
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My 05 does the same exact thing as yours. When my a/c is on the car is gets 2-3mpg less, give or take a few. Not sure...
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