Have done everything, but fuel economy has DECREASED

Asked by Nov 02, 2011 at 02:49 PM about the 2003 GMC Envoy XL SLE 4WD

Question type: Maintenance & Repair

Purchased this vehicle 2 years ago.  In 2010, I was
getting great fuel economy, I had no complaints.  
500-600lms (300-360 miles) on a tank.  I changed
the air filter in August, and was not getting 600-
700 kms on a tank. (360-420 miles) on highway
use.  A personal best of 740kms on a tank actually.

Roll into 2011.  My mileage was poorer, (I assumed
it was the "Winter Gas, and the additives) then I
added 4 new tires in April, and always keep to
correct pressure.   Tires are Firestone
Destinations...

My mileage was barely 500kms now, on perfect
highway conditions.  I brought it in for a full tune
up...  One of the  O2 sensor was bad.  Had this
replaced. Replaced all plugs with OEM Champion
plugs.  Replaced the fuel filter, and air filter again.  
Also had a universal joint, and rear axle seal
replaced, as it was beginning to leak.  

IT's the 4.2L SLE 4x4 model.   I started a
spreadsheet to keep track of my fuel economy.    I
burn 15L-16L per 100km on highway (at speed
limit) . This is averaging 14-155MPG on highway.  

CITY MPG  - it is now getting 9.5MPG in the city.  It
burns 24L per 100KMS.   This means I can barely
go 300kms in town for $95.00 (30 cents a KM)

Compared to what I got last August/September,
(21MPG or 10.7L per 100km) this is just horrible..

Frustrated, don't know what to do next.  Love my
truck, need to remedy - any helpful advice?  So
much appreciated....

1 Answer

125

I did a custom tune of my PCM (Wait4Me Custom Tunes out of IN) on my 03 Chevy Trailblazer LT. I can pull 25 MPG on longer trips if I am steady on the pedal. I'd replace your plugs with AC-Delco's. What oil are you running? Amsoil is my 1st choice, then Mobil1. Tire pressure is VERY important and so is the AIrfilter (AC-Delco), which are 2 areas you've already covered. I also removed my roof racks to cut down on resistance. The grade/type of gas you use has an effect. Do you run a fuel system cleaner with your oil changes? I run Seafoam or Chevron for my fuel system. Hope this helps.

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