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Fuel mileage

Tiny
TRAVS.PITTS
  • MEMBER
  • 2004 JEEP GRAND CHEROKEE
  • V8
  • 4WD
  • AUTOMATIC
  • 110,000 MILES
My 2004 jeep grand cherokee overland with a 4.7l v8 ho. Has recently lost a lot of fuel mileage. It started out at 18-20mpg. My wife drove it to ny pulling a trailer when we moved there from kansas. After arriving and unhitching the trailer. The fuel mileage went down to aprox 8-10mpg. Just before leaving for NY. SHe had all of the usual maintenance done. I.E. Oil change, and changing spark plugs. She also had the throttle positioning sensor changed.

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Tuesday, April 5th, 2011 AT 12:41 AM

6 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Check to see if there are any codes on theengine. Make sure that the plugs are the correct ones as this can effect mileage. Also changeteh fuel filter and check fuel pressure. Finally check vacuum at idle if it's around 15" or so you may have a plugged converter. It should be around 20" at idle.
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Tuesday, April 5th, 2011 AT 1:05 AM
Tiny
TRAVS.PITTS
  • MEMBER
  • 4 POSTS
@ apox what rpm should it idle
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Tuesday, April 5th, 2011 AT 1:18 AM
Tiny
TRAVS.PITTS
  • MEMBER
  • 4 POSTS
Also no codes are showing up. The plugs are right and come pre gapped. I plan on changing the fuel filter very soon. And how do I check vacuum?
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Tuesday, April 5th, 2011 AT 1:22 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
I can't seem to find the idle speed in the manual. I would guess about 700 or so in gear. So it wold make it 850-900 idle. If it's higher it may have a vacuum leak.
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Tuesday, April 5th, 2011 AT 1:32 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Find a port to hook up a vacuum gauge and start teh car and read the gauge.
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Tuesday, April 5th, 2011 AT 1:34 AM
Tiny
CJ MEDEVAC
  • MECHANIC
  • 11,004 POSTS
This is what you need to "TEE" into a MANIFOLD VACUUM PORT. This is how you do it. Remove the existing line off of the INTAKE Port/ Nozzle/ T. It/ Thingee. Plug it back on to the "TEE" on your test rig. Then plug your EMPTY TEST RIG HOSE BACK ON TO the INTAKE Port/ Nozzle/ T. It/ Thingee, that you took the original line off of. Original stuff will still operate, and you are now tapped into vacuum. This pic is of my 1977 JEEP CJ 5---This how to do it, YOUR STUFF DEFINITELY LOOKS DIFFERENT! MY JEEP IS NOT RUNNING, IF IT WERE, IT WOULD BE REGISTERING A "ROCK STEADY NEEDLE" AT 22 INCHES OF MERCURY (Hg)---I hope this helps--a cool "little", "compact", "most everything you need" tune up kit can be found at "Harbor Freight" for under $40. Good Huntin'--The Medic
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Tuesday, April 5th, 2011 AT 3:31 AM

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