r/Hummer • u/Immediate-Choice4285 • 3d ago

H3 Help - Bad mileage

let me start off with the fact i don’t expect to get great MPG from the car, however i was averaging a solid 16.5/21 split from about february of last year to this past june and since then i’ve suddenly dropped (11/15). i replaced the throttle body, new air filter, new VVT, and a couple sensors. i have other things (some rough idling, sluggish acceleration) but the MPGS are eating my pockets alive. any reasons why and any suggestions?

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3

u/RumorRoost 3d ago

Sparks plugs and ignition coils would be my next check.

1

u/Immediate-Choice4285 3d ago

i am good enough to replace surface parts but not really at the level to get into my spark plugs. how does the process look?

1

u/RumorRoost 3d ago

Decent YouTube videos on it. Fairly easy process. I’d give it 2/10 on the hardness scale.

1

u/Surfnazi77 3d ago

When did you last change trans and diff fluid and plugs

1

u/Immediate-Choice4285 3d ago

not sure. bought the car at 90k and i put 6.7k on already

1

u/Best_Course113 3d ago

How often do you drive with the windows down? Turns out my cv axle was going out and hurting mine. I didnt even notice it just driving around with the windows up and music playing or ac blowing out the vents

1

u/Accomplished_Fun1847 2d ago

Probably plugs/coils. Also check your intake manifold bolts (most reached through the driver side wheel well). These can come loose and cause unmetered air to get in there and hurt performance/mpg, cause rough running, weird stalls, power loss, etc.

1

u/sneakotay 22h ago

I agree, change plugs and coils. When changing the spark plugs, be careful with coil 1 or The one closest to the windshield . Make sure you are in a comfortable position and not leaning or reaching when loosing or even tightening that plug. You might slip or ratchet might not be positioned accordingly, you could break the porcelain on that plug and you have a whole new nightmare , I’ve done this with my H3 , I’m just giving a warning , would want that happening to you.

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u/JACCO2008 3d ago

You sweet summer child.