12 mpg...is it possible?
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 915
Likes: 0
From: salinas ca
12 mpg...is it possible?
Hi all. I have an 89 auto, with 150k on the od. While doing mostly city driving, not all that spirited i seem to average 12mpg. While reading around i seem to see that most people get 16+ in the city. Motor has a complete tune up, plugs, wires, o2 sensor, and new injectors. There does seem to be a slight miss at idle and i can't figure out why. I can't seem to find a vacuum leak, but there is a 160 thermostat in it so it hardly ever sees 180. Could it not be going to closed loop and only running open loop? At what temp does it engage closed loop? Car also has 2.59 gears in it.
So my question is is it really possible to be getting 12mpg avg in the city? Car gets 250 between fillups. And is it possible the miss could be the culprit of the horrible gas mileage? Before everyone starts flaming about how these are corvettes and not economy cars, I have an 02 camaro for the gas mileage not this thing. Just thought i would ask if most people get that bad of mileage.
So my question is is it really possible to be getting 12mpg avg in the city? Car gets 250 between fillups. And is it possible the miss could be the culprit of the horrible gas mileage? Before everyone starts flaming about how these are corvettes and not economy cars, I have an 02 camaro for the gas mileage not this thing. Just thought i would ask if most people get that bad of mileage.
Last edited by 1989vette; Jun 11, 2008 at 02:40 AM.
Race Director
Joined: Jan 2008
Posts: 16,028
Likes: 170
From: oregon
250 miles is not 12mpg, do the math on the miles traveled and gallons added. it will probably be more like 18-19mpg. the mpg computers can be off for various reasons, injector changes etc..
Last edited by racebum; Jun 11, 2008 at 03:17 AM.
Le Mans Master
Joined: Jan 2003
Posts: 5,858
Likes: 7
From: Santa Maria CA
Does pressing the reset button make it go higher? After a while, mine likes to "stick" at a certain reading that I know can't be right. Pressing the reset button tends to "free" the readout and it begins to read a more correct and higher (or lower if city) number. I'm assuming that's why GM gave us the reset button.
Last edited by 86PACER; Jun 11, 2008 at 03:09 AM.
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 915
Likes: 0
From: salinas ca
Does pressing the reset button make it go higher? After a while, mine likes to "stick" at a certain reading that I know can't be right. Pressing the reset button tends to "free" the readout and it begins to read a more correct and higher (or lower if city) number. I'm assuming that's why GM gave us the reset button.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
You can use the ECM's Field Service mode to see if it goes closed loop:
-warm up the engine to normal op temps and shut off
-insert a paper clip ends into ALDL pins A&B (upper right)
-start the engine and watch the SES flash rate, 2Hz at startup
-after a few minutes the flash rate should slow to 1Hz
-if the On duration is equal to the Off duaration the A/F is stoch
If the flash rate doesn not slow to 1Hz even after say 5min or so, the ECM is NOT in closed loop mode and you need to diagnose the problem.
-warm up the engine to normal op temps and shut off
-insert a paper clip ends into ALDL pins A&B (upper right)
-start the engine and watch the SES flash rate, 2Hz at startup
-after a few minutes the flash rate should slow to 1Hz
-if the On duration is equal to the Off duaration the A/F is stoch
If the flash rate doesn not slow to 1Hz even after say 5min or so, the ECM is NOT in closed loop mode and you need to diagnose the problem.
Safety Car
Joined: Sep 2001
Posts: 4,157
Likes: 554
From: Glenbeulah Wi
Hi all. I have an 89 auto, with 150k on the od. While doing mostly city driving, not all that spirited i seem to average 12mpg. While reading around i seem to see that most people get 16+ in the city. Motor has a complete tune up, plugs, wires, o2 sensor, and new injectors. There does seem to be a slight miss at idle and i can't figure out why. I can't seem to find a vacuum leak, but there is a 160 thermostat in it so it hardly ever sees 180. Could it not be going to closed loop and only running open loop? At what temp does it engage closed loop? Car also has 2.59 gears in it.
So my question is is it really possible to be getting 12mpg avg in the city? Car gets 250 between fillups. And is it possible the miss could be the culprit of the horrible gas mileage? Before everyone starts flaming about how these are corvettes and not economy cars, I have an 02 camaro for the gas mileage not this thing. Just thought i would ask if most people get that bad of mileage.
So my question is is it really possible to be getting 12mpg avg in the city? Car gets 250 between fillups. And is it possible the miss could be the culprit of the horrible gas mileage? Before everyone starts flaming about how these are corvettes and not economy cars, I have an 02 camaro for the gas mileage not this thing. Just thought i would ask if most people get that bad of mileage.
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 915
Likes: 0
From: salinas ca
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Le Mans Master
Joined: Oct 2004
Posts: 7,706
Likes: 485
From: Everett WA
C4 of Year Finalist (appearance mods) 2019
So ..... uh........ I'm wondering how you get 18-19?? Do you guys run a special 14 gal tank?
Intermediate
Joined: Jul 2007
Posts: 32
Likes: 0
From: Hurricane UT
I guess being an accountant and not a racer I am missing a basic principal but if he has a 20? gal gas tank and he has to refill it after 250 miles (and lets assume he still has a gallon left in the tank) isn't 250 / 19 gal = 13.2 mpg.
So ..... uh........ I'm wondering how you get 18-19?? Do you guys run a special 14 gal tank?
So ..... uh........ I'm wondering how you get 18-19?? Do you guys run a special 14 gal tank?
A. Odometer start:
B. Odometer stop:
C: B minus A = total miles
D. Number of gallons at next fill-up:
E: C divided by D = AVG MPG
Yes, I'm somewhat OCD, but if you want the Excel template, I will email to you. I also track price per gallon...ouch!
Le Mans Master
Joined: Feb 2005
Posts: 6,482
Likes: 566
From: Clearwater Florida
Huh !?!?!?
I guess being an accountant and not a racer I am missing a basic principal but if he has a 20? gal gas tank and he has to refill it after 250 miles (and lets assume he still has a gallon left in the tank) isn't 250 / 19 gal = 13.2 mpg.
So ..... uh........ I'm wondering how you get 18-19?? Do you guys run a special 14 gal tank?
So ..... uh........ I'm wondering how you get 18-19?? Do you guys run a special 14 gal tank?
Supporting Vendor
Joined: Mar 2008
Posts: 1,762
Likes: 14
From: Cumming Ga
I heard from eric and asked wat new injectors he put in.. He thinks they are SVO (bosch24) if thats the case his car thinks it has 22's. plus the Bosch injectors flow almost 26lb/hr at 43.5. He is getting me the part # to confirm. The engine would definitely drink fuel if the Ecm was still stock.
Last edited by FICINJECTORS; Jun 12, 2008 at 05:50 AM.
Race Director
Joined: Jan 2008
Posts: 16,028
Likes: 170
From: oregon
It works out to 13.8888, 18 gall tanks if i remember correctly?
Well i normally reset it after each top off. DO you reset often? Between fill ups? Mine too seems to "stick" sometimes around 12.5. Even if i know i'm doing a little better than that, although it is the AVERAGE.
Well i normally reset it after each top off. DO you reset often? Between fill ups? Mine too seems to "stick" sometimes around 12.5. Even if i know i'm doing a little better than that, although it is the AVERAGE.
Race Director
Joined: Jan 2008
Posts: 16,028
Likes: 170
From: oregon
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 915
Likes: 0
From: salinas ca
well i would love it if this were the problem, but it isn't. Right now the avg is 11.2 mpg with all city driving. Reset it last night drove on the highway for 30 min to test the mpg. All highway, curising at 65 with 2.59's got me 21.8 mpgs. That seems low with a 700r4 and 2.59s.
Safety Car
Joined: Oct 2007
Posts: 3,816
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From: :45 minutes from everywhere / E-I-E-I-O
well i would love it if this were the problem, but it isn't. Right now the avg is 11.2 mpg with all city driving. Reset it last night drove on the highway for 30 min to test the mpg. All highway, curising at 65 with 2.59's got me 21.8 mpgs. That seems low with a 700r4 and 2.59s.
I can get mine (LT1) to hold 30 on a stretch if I hold just at 55. 65 cuts it to a little over 27.
Race Director
Joined: Jan 2008
Posts: 16,028
Likes: 170
From: oregon
well i would love it if this were the problem, but it isn't. Right now the avg is 11.2 mpg with all city driving. Reset it last night drove on the highway for 30 min to test the mpg. All highway, curising at 65 with 2.59's got me 21.8 mpgs. That seems low with a 700r4 and 2.59s.
did you measure how much fuel was added to the car to fill, how much you drove and then fill again to get these numbers? my car constantly tells me i'm getting 14-15mpg when the gallons of fuel i add say otherwise. my injectors are above stock lbs & the ecu has been reprogramed which is most likely my cause but seriously. check facts, not dash boards. fill the car, drive, keep track of miles. go fill again and see how many miles you drove versus gallons added. do the math and you will have your mpg. excessive rich conditions normally trigger check engine lights.