causes of bad MPG - The 1947 - Present Chevrolet & GMC Truck Message Board Network

crm318 · 01-22-2008, 04:23 AM

my truck has never gotten hybrid milage, but lately, my truck has been getting worse and worse milae by the week. what are some things to look for on the engine that would be causing this?

jsbgump · 01-22-2008, 04:36 AM

When was the last time you did a tune up? Clogged air filter? Vacuum leak?

ChevyC102007 · 01-22-2008, 04:38 AM

Ive been having the same problem as well, I know were part of its at I have a bad choke thermostat and I believe my carb is out of adjustment (again

) those would be two things Id check first. Also if the misture isnt getting ignited fully that will cause bad mpg IMHO. But someone else on here would have a better idea Im sure. Just my .02 cents

67ChevyRedneck · 01-22-2008, 04:39 AM

....

.... or maybe it's the E10 winterblend fuel?

My silverado has dropped about 1 to 2 mpg as well.

**special-K** · 01-22-2008, 05:23 AM

Check your ignition.I know mine gets worse mileage with a key in the ignition.Seriously,how does it look under the hood?Smell fumey or see excess gas?Is it running smooth?The air filter and good spark with proper timing is the best you can do.

cdowns · 01-22-2008, 05:29 AM

under inflated tires bad alignment worn suspension components oversized tires poor choices of manifolds and camhafts as well as a bad right foot can and will cause milage issues

wibilly · 01-22-2008, 05:30 AM

check your tire presure, especialy if you have aluminum rims. they seem to lose presure as temps drop

**WorkinLonghorn** · 01-22-2008, 06:19 AM

Mine started getting crappy mileage and I checked the vacuum advance by sucking on the line to it and discovered ther diaphram was broken.Then when I looked in the distrib I found that the centrifical weights were all stuck too.
I replaced the whole distrib with a new HEI type and it runs great but and am SURE the mileage prob.is fixed.

Longhorn Man · 01-22-2008, 06:49 AM

I know you are in texas, and the weather isn't as much an issue as it is up here... but you'd be amazed at how much it kills the MPG to idle the engine for 20 min in the morning to warm it up.
Assuming that isn't the problem... check the basics as mentioned already. Tire pressure, plugs wires, cap and rotor... how old are they? If you have points, then inspect them (or say screw it and convert it to HEI) Look at the air filter, clean the carb, yadda yadda yadda.
If you are like me and are too lazy to pull the 2500 pounds of junk in the bed... that'll hurt your MPG too. I have a whole front cross member in mine, frim rotor to rotor, plus a set of cement steps that we tore apart off the back portch... puttin a good squat on the 'ole girl. I KNOW it ain't helping my situation... but, I have no place to dump it.
You don't need some concrete chunks do you?

**special-K** · 01-22-2008, 07:02 AM

Snow traction.

C. M. Wolf · 01-22-2008, 07:18 AM

So far the above posts cover most causes that I would know of. The alcohol in the fuel mix can be a real culprit forcing you to have to run hotter ignitions and even a preheat strip on the fuel line before the carb. Alcohol runs cooler, cleaner, but cooler. It's also a very strong detergent so you need to check your fuel filters more often or run a second inline fuel filter,(which still has to be checked more often).

350 engines are nutoriously known for not 'breathing well', when you replace your air filter, put the tallest you can in.

The size and type of tires you run have a lot to do with gas mileage also. The narrower the tires footprint on the pavement is the less resistance the truck has to move. The taller the tire is the less the engine turns per complete rotation of the tire. Which means the truck travels further per engine rotation allowing better milaeage, at least to a certain point. This can be taken to far and begin to defeat the mileage per horse power on the engine. (Running vehicles that look more like a kid's "HotWheels Toy Car" or Toy-Monster-Truck" may look good, but it sucks... no really, it sucks gas to the point that the sound of the whirlpool coming from your fueltank can be deafening!)

I'm running a holley/700 carb,(which I need to re-jet), on an eddy-0/50-AL intake, the holley has an electric choke that sucks and I'm told the best cure for the alcohol-fuel/mileage thing is to run an "Air-preheat-plate" under the carb. (Since I have headers, I don't want to bolt on any air-preheat tubes up to the air cleaner can, which would need to be regulated also to avoid vacume-lock) *sigh*

It might not be a good thing for the board's bandwidth to get me started on the pro's & con's of running alcohol fuels in V8's. (Let's just ignore the fact that alcohol is cheaper, cleaner-burning, and can be made from the grass-clippings from your yard using a "Warren Still", and the BATF has deregulated the mfg'er of alcohol fuels up to 10,000 gals per year of personal use without a license.... since the "Carter Administration"!)

Michael

crazy longhorn · 01-22-2008, 08:19 AM

Quote:

Originally Posted by 67ChevyRedneck

....

.... or maybe it's the E10 winterblend fuel?

My silverado has dropped about 1 to 2 mpg as well.

Top of the line"lead foot".....yup Im guilty of that

crm318 · 01-22-2008, 09:45 AM

well since i have the day off tomorrow, i will go over all of the points mentioned. all of my specs are mentioned below. the HEI is also a new.

BurnoutNova · 01-22-2008, 10:03 AM

also keep in mind as the outside temperature gets colder, your fuel economy will go down. combine 20 deg weather with E10 and you can drop 3-4 mpg.

**truckdude239** · 01-22-2008, 10:06 AM

gsa mileage whats that?

tim21391 · 01-22-2008, 10:50 AM

i didn't know these trucks got good gas mileage....you dont drive it to save gas you drive it to have fun.....ok im sorry man...i had a problem with my trucks gas problem....as they said above.....check air filter..spark plugs..spark plug wires..points and rotor and cap on the distributor....check tires...see if you have a leak or what not....also my truck was gettin bad gas mileage...check to see if your hittin on all cylinders i was missin on 1 and 4 clyinders so i got my valves adjusted and that fixed my problem....how to check to see if your firing on all cylinders you take the air filter off so you can get to the distributor...pull each plug wire off the cap...each one when you pull it off should make the engine make a different noise....and you should see the spark jumping from the plug wire to the cap...if you dont have spark you have a bad plug or wire....that could cause a bad gas mileage problem...but yeah pull each one off if it doesnt change the pitch of the engine then you are missin on a cylinder and you need to get your valves adjusted

Sweet72 · 01-22-2008, 10:54 AM

Quote:

Originally Posted by cdowns

under inflated tires bad alignment worn suspension components oversized tires poor choices of manifolds and camhafts as well as a bad right foot can and will cause milage issues

I really like your "shoot from the hip" responses cdowns

I really hate it when my "bad right foot" asses up good mileage!

mnunn454 · 01-23-2008, 05:13 AM

Mentioned above, but my 2 cents is with stuck choke, a vacuum leak or an ignition/timing problem.

BTW, the brakes aren't dragging on any corner, are they?

Spray-Bomb · 01-23-2008, 06:46 AM

Quote:

Originally Posted by crazy longhorn

Top of the line"lead foot".....yup Im guilty of that

X2

crm318 · 01-23-2008, 07:10 AM

the truck rides smooth and doesnt pull to the side. checking the tires wouldnt hurt.

Old Chevy Mech · 01-23-2008, 07:38 AM

Its 'possible" that the coil is dying. What do the plugs look like? If they do not look clean that might be a problem.

Frank

01-22-2008, 04:23 AM	#1
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	causes of bad MPG my truck has never gotten hybrid milage, but lately, my truck has been getting worse and worse milae by the week. what are some things to look for on the engine that would be causing this? __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies

01-22-2008, 04:36 AM	#2
jsbgump Gone Fishing Join Date: Aug 2005 Location: Fort Worth, Texas 76135 Posts: 12,361	Re: causes of bad MPG When was the last time you did a tune up? Clogged air filter? Vacuum leak? __________________ 68 Chevy Short Fleetside LSX Swap Under Way 3.73 posi 2.5/4 drop Frame of resto has been started 2022 Hyundai Tucson Limited 2019 Chevy Traverse Redline Edition Wifes Ride Build Thread http://67-72chevytrucks.com/vboard/s...=1#post3483616

01-22-2008, 04:38 AM	#3
ChevyC102007 Registered User Join Date: Sep 2007 Location: Emporium,Pa Posts: 248	Re: causes of bad MPG Ive been having the same problem as well, I know were part of its at I have a bad choke thermostat and I believe my carb is out of adjustment (again ) those would be two things Id check first. Also if the misture isnt getting ignited fully that will cause bad mpg IMHO. But someone else on here would have a better idea Im sure. Just my .02 cents __________________ "When you were born, you cried and the world rejoiced. Live your life so that when you die, the world cries and you rejoice." [Cherokee saying] 2001 Dodge Ram Sport 1500 4x4. (old ride) 1987 GMC V1500 Sierra Classic ("new" ride)

01-22-2008, 04:39 AM	#4
67ChevyRedneck Hittin E-Z Street on Mud Tires Join Date: Nov 2004 Location: Greenville, SC Posts: 23,090	Re: causes of bad MPG .... .... or maybe it's the E10 winterblend fuel? My silverado has dropped about 1 to 2 mpg as well. __________________ Jesse James 1967 C10 SWB Stepside: 350/700R4/3.73 1965 Ford Mustang: 289/T5-5spd/3.25 Trac-Loc 1968 Pontiac Firebird: Project Fire Chicken! 2015 Silverado Double Cab 5.3L Z71 2001 Jeep Wrangler Sport 4.0L 5spd 2020 Chevrolet Equinox Premium 2.0L Turbo 2011 Mustang V6 ~ Wife's ride American Born, Country by the Grace of God 1967 CST Shop Truck Rebuild! My 1967 C-10 Build Thread My Vintage Air A/C Install Project "On a Dime" Trying my hand at Home Renovation! 1965 Mustang Modifications!

01-22-2008, 05:23 AM	#5
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	Re: causes of bad MPG Check your ignition.I know mine gets worse mileage with a key in the ignition.Seriously,how does it look under the hood?Smell fumey or see excess gas?Is it running smooth?The air filter and good spark with proper timing is the best you can do. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

01-22-2008, 05:29 AM	#6
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: causes of bad MPG under inflated tires bad alignment worn suspension components oversized tires poor choices of manifolds and camhafts as well as a bad right foot can and will cause milage issues __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY Last edited by cdowns; 01-22-2008 at 05:31 AM.

01-22-2008, 05:30 AM	#7
wibilly Senior Member Join Date: Nov 2005 Location: northwestern wi Posts: 2,111	Re: causes of bad MPG check your tire presure, especialy if you have aluminum rims. they seem to lose presure as temps drop __________________ common sense isn't so common anymore

01-22-2008, 06:19 AM	#8
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,886	Re: causes of bad MPG Mine started getting crappy mileage and I checked the vacuum advance by sucking on the line to it and discovered ther diaphram was broken.Then when I looked in the distrib I found that the centrifical weights were all stuck too. I replaced the whole distrib with a new HEI type and it runs great but and am SURE the mileage prob.is fixed. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

01-22-2008, 06:49 AM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Re: causes of bad MPG I know you are in texas, and the weather isn't as much an issue as it is up here... but you'd be amazed at how much it kills the MPG to idle the engine for 20 min in the morning to warm it up. Assuming that isn't the problem... check the basics as mentioned already. Tire pressure, plugs wires, cap and rotor... how old are they? If you have points, then inspect them (or say screw it and convert it to HEI) Look at the air filter, clean the carb, yadda yadda yadda. If you are like me and are too lazy to pull the 2500 pounds of junk in the bed... that'll hurt your MPG too. I have a whole front cross member in mine, frim rotor to rotor, plus a set of cement steps that we tore apart off the back portch... puttin a good squat on the 'ole girl. I KNOW it ain't helping my situation... but, I have no place to dump it. You don't need some concrete chunks do you?

01-22-2008, 07:02 AM	#10
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	Re: causes of bad MPG Snow traction. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

01-22-2008, 07:18 AM	#11
C. M. Wolf Registered User Join Date: Jan 2008 Location: MT Posts: 85	Re: causes of bad MPG So far the above posts cover most causes that I would know of. The alcohol in the fuel mix can be a real culprit forcing you to have to run hotter ignitions and even a preheat strip on the fuel line before the carb. Alcohol runs cooler, cleaner, but cooler. It's also a very strong detergent so you need to check your fuel filters more often or run a second inline fuel filter,(which still has to be checked more often). 350 engines are nutoriously known for not 'breathing well', when you replace your air filter, put the tallest you can in. The size and type of tires you run have a lot to do with gas mileage also. The narrower the tires footprint on the pavement is the less resistance the truck has to move. The taller the tire is the less the engine turns per complete rotation of the tire. Which means the truck travels further per engine rotation allowing better milaeage, at least to a certain point. This can be taken to far and begin to defeat the mileage per horse power on the engine. (Running vehicles that look more like a kid's "HotWheels Toy Car" or Toy-Monster-Truck" may look good, but it sucks... no really, it sucks gas to the point that the sound of the whirlpool coming from your fueltank can be deafening!) I'm running a holley/700 carb,(which I need to re-jet), on an eddy-0/50-AL intake, the holley has an electric choke that sucks and I'm told the best cure for the alcohol-fuel/mileage thing is to run an "Air-preheat-plate" under the carb. (Since I have headers, I don't want to bolt on any air-preheat tubes up to the air cleaner can, which would need to be regulated also to avoid vacume-lock) sigh It might not be a good thing for the board's bandwidth to get me started on the pro's & con's of running alcohol fuels in V8's. (Let's just ignore the fact that alcohol is cheaper, cleaner-burning, and can be made from the grass-clippings from your yard using a "Warren Still", and the BATF has deregulated the mfg'er of alcohol fuels up to 10,000 gals per year of personal use without a license.... since the "Carter Administration"!) Michael

01-22-2008, 09:45 AM	#13
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	Re: causes of bad MPG well since i have the day off tomorrow, i will go over all of the points mentioned. all of my specs are mentioned below. the HEI is also a new. __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies

01-22-2008, 10:03 AM	#14
BurnoutNova Registered User Join Date: Mar 2004 Location: CA Posts: 2,057	Re: causes of bad MPG also keep in mind as the outside temperature gets colder, your fuel economy will go down. combine 20 deg weather with E10 and you can drop 3-4 mpg.

01-22-2008, 10:06 AM	#15
truckdude239 Senior Member Join Date: Sep 2005 Location: Lewisville, Nc Posts: 10,238	Re: causes of bad MPG gsa mileage whats that? __________________ David fuller Ase Certified Mechanic Click here to help support our board!! 1971 Chevy c-10 under going a 4.8l LSx swap Build Thread http://67-72chevytrucks.com/vboard/s...d.php?t=421305 2007 Honda Accord my daily 145kmiles 2002 Honda Accord 4 door With 330k(sisters car) 2005 toyota Avalon 228k( brothers car) 2002 Sububran 5.3 245k 2000 Tahoe 5.3l 378 General manager for Marco's Carwash & lube

01-22-2008, 10:50 AM	#16
tim21391 Registered User Join Date: Jul 2007 Location: Salina, Kansas Posts: 1,998	Re: causes of bad MPG i didn't know these trucks got good gas mileage....you dont drive it to save gas you drive it to have fun.....ok im sorry man...i had a problem with my trucks gas problem....as they said above.....check air filter..spark plugs..spark plug wires..points and rotor and cap on the distributor....check tires...see if you have a leak or what not....also my truck was gettin bad gas mileage...check to see if your hittin on all cylinders i was missin on 1 and 4 clyinders so i got my valves adjusted and that fixed my problem....how to check to see if your firing on all cylinders you take the air filter off so you can get to the distributor...pull each plug wire off the cap...each one when you pull it off should make the engine make a different noise....and you should see the spark jumping from the plug wire to the cap...if you dont have spark you have a bad plug or wire....that could cause a bad gas mileage problem...but yeah pull each one off if it doesnt change the pitch of the engine then you are missin on a cylinder and you need to get your valves adjusted



Register or Log In To remove these advertisements.

01-23-2008, 05:13 AM	#18
mnunn454 Registered User Join Date: Dec 2006 Location: Rhode Island Posts: 602	Re: causes of bad MPG Mentioned above, but my 2 cents is with stuck choke, a vacuum leak or an ignition/timing problem. BTW, the brakes aren't dragging on any corner, are they? __________________ 69 C-10 SWB Fleetside 454/TH400 (For Sale) http://67-72chevytrucks.com/vboard/c...ct=1299&cat=12 70 El Camino 454 TH700 29 Model A 4-DR Sedan Deluxe

01-23-2008, 07:10 AM	#20
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	Re: causes of bad MPG the truck rides smooth and doesnt pull to the side. checking the tires wouldnt hurt. __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies

01-23-2008, 07:38 AM	#21
Old Chevy Mech Keepin it runnin! Join Date: Oct 2007 Location: Deep River, CT Posts: 1,036	Re: causes of bad MPG Its 'possible" that the coil is dying. What do the plugs look like? If they do not look clean that might be a problem. Frank __________________ 1970 K-5 4X4 Blazer 1970 K-20 4X4 Long Bed "Liberty Means Responsibility, that is why most men dread it" Last edited by Old Chevy Mech; 01-23-2008 at 07:39 AM.

Register or Log In To remove these advertisements.