2021 Blazer Fuel Capacity

According to this seemingly authoritative site, the 2021 Blazer was not available with a 19.4 U.S. Gallon capacity fuel tank, came with 19.0 or 21.0 gal tank, depending on trim, engine, etc.

Model year 2020 was available with 19.4 gal tank.

See here: https://www.autopadre.com/gas-tank-size/chevrolet-blazer

The reason this matters is that fuel filler pipe and tank vent arrangements vary quite a bit between the model years and tank sizes.

I would suspect that there is an issue with the fuel tank vent… plugged up or pinched… somehow not allowing proper vent of air.

This site has (possibly) a layout diagram of your truck’s fuel filler pipe, and more troubleshooting tips on more complicated solutions:

https://chevyguide.com/how-to-fix-chevy-fuel-tank-vent-problem/

The internet being what it is, there are likely many other sites that have diagrams available.

Regarding “The fuel gage reads full before I start pumping in the extra 4 gallons.” I had a Dodge pickup with a - IIRC - 26 gal tank that read at the gauge as full at 20 gal or so. I dunno if this is a common trait amongst truck type vehicles or not; today I have a Hummer H2 that doesn’t seem to do this.

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Do you always wait until the fuel light has been on steady for a few miles, then blinking for more miles before getting to the fuel station on the fumes?

Most of us know that can lead to issues by drawing the dregs from the bottom of the tank into the fuel lines.

I would expect to fill up while there was still at least two gallons in the tank, so should not need the full tank capacity.

Pumping extra in takes up the expansion space and can lead to blown fuel tanks. This happened in the past when a manufacturer built tanks with the breather pipe in the wrong location and had to recall vehicles to swap out the tanks.

Most of the automotive fuel tanks have in-line fuel filters, mechanical floats for fuel level measurements and mechanical fuel gauges..

The fuel filter has a builtin cartridge that may already be soaked and partially clogged.. Coupled with the type of the mechanical float used inside the gas tank together with the sluggishness or responsiveness of the mechanical metering used in your fuel gauge when combined with variations in ambient temperatures and the eye parallax may be the reason that causes the differences in your observations...