I have a 2000 blazer 4x4 4.3l 2door automatic. I drive the same exact road everyday, to and from work. The fuel mileage for no reason for about the past 6 months fluctuates. I reset my tripometer every time I fill the tank, and I use a calculator and figure up my fuel mileage. The strange thing is, is that 1 fill up it gets 14 mpg., then the next fill up it gets 17-18 mpg., and I'm doing nothing different. I've read other posts about this, but cannot seem to find an identical problem such as mine. I've changed plugs,wires,cap,rotor,cleaned maf and egr, fuel pump and even replaced the fuel tank cap, with no change. I'm afraid there maybe a problem that might take out other components with it before I find out what is causing this.

 

changes in weather affects it a bit.

fill up with the same grade at the same location? if not that can affect it a bit.

air level in your tires affects it a bit.

stop and go action affects it a bit.

that are a ton of factors. a bit here and a bit there can add up quick. it's easier to ruin your MPGs, thna it is to save em...


what are you fearing will go bad if you fuel ecconmy bounces? Throttle cable?

you could have a bit of crud living in your engine that a taste of seafoam wouldn't hurt.
cleaning your throttle body couldn't hurt.
take a look at your colling system: sticky/stuck tstat can cause you engine to run 'off'
how the old fuel filter treating you?

 

I'm afraid of other components such as the EGR and IAC valve clogging up from a rich mixture when it's getting 14 mpg.....
What exactly do you mean when you say the engine will run "off" because of a sticking thermostat?

 

as in "not run like it should" "it's off" "not right"

you already cleaned your egr, so unless you got a really bad gasket seal, that shouldn't be an issue... you IAC you can tackle if you do the TB cleaning or by itself if you are so inclined. <-- that is another good one i over looked

 

Have you noticed anything else at all that is a bit off besides your gas mileage?
It may be something small but it could be key to locating the problem. For example having a bit less power on take off or on hills then normal, RPM fluctuation's particularly at higher speeds, occasional rough idle or hard starts. Exhaust color or smells can be a great diagnostic tool as well.

The same goes for fluid levels and smells. Inspecting the spark plugs can also tell you tons about what is going on inside your engine.

Lots of issues start gradually and tend to not be noticed easily when you drive the car every single day. When gas mileage suffers then there is usually some other symptom present.

 

It has always had a rough idle and some power loss compared to my wife's 2002 blazer 2wd 2door, and it does smell kinda strong or richer than my wifes truck. It also has a stumble or miss or something at about 1,000 rpms.It has always done this even before the complete tune up, and it would shutter at about 1,800 rpms when driving on the interstate, but if I kicked it down to the lower or passing gear the shuttering would stop, but the strange thing is it only did this shuttering during the summer, it stops when the temperature outside seemed to get below around 40 degrees, so the shuttering has stopped for now because its winter time and we have had lots of snow so everyday has been like 10 - 30 degrees for a couple of months now.

 

i have a 97 blazer an has some what the same prob. its very hard to start when outside temp is hot but the first try it starts then dies then takes 5 to 10 trys to start but in the mornings when cool outside it starts fine.. an ive noticed the fuel mileage has went down do you know what that could be??

 

another thought would be to check vacume hoses . these can play an important part in regular operation .

 

Too many possiblities to adress-most have been mentioned-concider as well additioninal warm up time as with cold trans& rear end! My fuel economy dropped from a concistant 24+ to a realistic 19 avg once the cooler weather got here! In a 97 with a 19 gal tank
I can still get 22mpg-using Marvel and I live in a flat area...so

 

i have no idea how you guys are getting that gas mileage.
i just used this
http://www.squawkfox.com/tools/gas-mileage-calculator/
and i get under 14mpg and i keep the rpm low, mainly highway, and in 2wd...i usually loose 50km a tank when i start using 4x4 a little. and its been like that since i got it 5 years ago. bran new plugs, rotor, wires and cap, fuel system is clean.
i think im going to run some seafoam through her again and see if that does anything but i doubt it lol