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1999 Chevy Suburban 10mpg with every fuel-saving MOD alread

Tiny
STEVE HALVORSON
  • MEMBER
  • 1999 CHEVROLET SUBURBAN
  • V8
  • 4WD
  • AUTOMATIC
  • 111,000 MILES
My 99 suburban 1/2 ton with 5.7 is getting 10-11mpg, city or highway. It has a 3"lift, but my friends with the same lift get 16mpg's still. I have a cold-air intake, a Superchip performance chip, I have tried setting the chip to economy and performance, neither help mpg's in the slightest, I have a performance muffler, a new high-flow catalytic converter, a new fuel system (the $1000 complete spider-shaped fuel delivery thing), complete new tune-up with performance wires and plugs, a fresh fuel filter, fresh belts, EVERYTHING! It runs great, but seems a little lacking in power, maybe its normal to lose significant speed going uphill, but most of all, I haven't gained so much as a 1/2 an mpg with all this stuff. Every fluid in the truck is fresh, it runs cool around 140, but 10mpg's! HELP!
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Wednesday, March 24th, 2010 AT 5:44 PM

7 Replies

Tiny
DOCFIXIT
  • MECHANIC
  • 18,828 POSTS
Is thermostat removed 140 is not near hot enough for good fuel economy. Need to be 190-210 if running that cool ECT signal to PCM will richen fuel mix.
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Thursday, March 25th, 2010 AT 3:01 PM
Tiny
STEVE HALVORSON
  • MEMBER
  • 5 POSTS
Even when towing heavy loads uphill it rarely reaches 180 degrees, so is my thermo broken or wrong somehow? Is there a way to test or correct this? No thermostats or anything have been removed, it appears to work correctly as well, on hot days or under heavy load it appears to run hotter, but it never reaches 200 not ever. ? Thoughts? -- THANK YOU IN ADVANCE!
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Thursday, March 25th, 2010 AT 7:10 PM
Tiny
DOCFIXIT
  • MECHANIC
  • 18,828 POSTS
Replace thermostat with 195 deg
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Friday, March 26th, 2010 AT 8:32 AM
Tiny
STEVE HALVORSON
  • MEMBER
  • 5 POSTS
Is there a way to adjust what it thinks the temperature is or do you mean there is a thermostat that always says its 195 degrees? I apologize i'm not very educated on this stuff. So am I buying a special thermostat? If so is there a brand to look for? Thanks again! Whatever you tell me to do I will try it and let you know the results
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Friday, March 26th, 2010 AT 12:54 PM
Tiny
DOCFIXIT
  • MECHANIC
  • 18,828 POSTS
Just a plain ol" thermostat allows engine to warm to operating temp then opes up allows coolant to flow maintaining temp. Foe engine to run that cool the thermostat is stuck in open position
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Friday, March 26th, 2010 AT 12:58 PM
Tiny
STEVE HALVORSON
  • MEMBER
  • 5 POSTS
Thank you I will have a new one put in a let you know if it works!
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Friday, March 26th, 2010 AT 1:01 PM
Tiny
STEVE HALVORSON
  • MEMBER
  • 5 POSTS
Got the thermostat put in last night, it already reads much higher operating temperatures than the other one ever got to, but i'll have to take in on a road test to see how it works on the highway, if it cuts back on fuel and gets me back to where I need to be. Thanks again, i'll let you know after 200 miles or so if there's a difference.
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Saturday, March 27th, 2010 AT 10:46 AM

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