I have a 2000 Chevy Blazer 4WD that the voltage is dropping down to about 12V when there is a heavy load or if at lower RMPs (600). I have replaced the battery and the issue continues. The alternator has been removed and tested at AutoZone and passed. I plan to replace the alternator anyways. My question is about the economy however... I used to get 17/18 MPG but recently with this power issue it has been declining down to 12 MPG. Could a power issue like this cause bad fuel economy?

 

Um-no not usually! Replace the alternator-that is a given! As the regulator is in the alternator.Check the tensioner& replace the belt at the same time(since its all apart) Mileages issues this time of year can usually be related to cold weather influences:not only is the motor cold,but so is the entire driveline is as well-hence poorer economy(warm up time etc) But 12mpg is too low for this car even in a cold climate unless driven in 4WD extensivlly! Back in NH at -20* in 4x4:12 was good! Cold has a major impact!

 

In my Experience it is possible that your MPG will go down if you dip below 12v because you computers need to reset and that takes about 3 or 5 miles to do above 12 v with some cars and trucks ( your truck gets stupid and needs to relearn)
But I have not had this problem with my 2000 blazer yet my caravan yes my Eagle talon yes
now minde you there is other things that will do this also like a air filter or even your 4WD auto being on when you 4Wd auto is on it locks your front axle in but dose not go in to 4X4 unless there is slipage