My truck has been sitting for over a year. I’ll be replacing the engine soon. Should I drain the old fuel or fill it up with fresh gas and fire it up? It’s at about 1/4 full right now.

 

Yes, would highly recommend clearing the old fuel.

One might get away diluting with a lot of fresh fuel, but why add another unknown degree of difficulty to the challenge of lighting things off, unless one has an abundance of time and a warehouse full of frustration to burn through?

Wayback story, ok about 8 years ago - let a Jeep sit for almost two years, trying to start it, would hit and miss, teasing with the almost but not quite. Drained the fuel - came out brownish and smelling like varnish. With fresh fuel, it piped right off.

 

When I was troubleshooting my fuel issues I drained out the gas from both tanks. I removed the front tank all together and once I checked inside of the rear tank and saw how rusted up and trashed it was, I just replaced the rear tank and fuel pump. I did try cleaning that rear tank and it was just too much trouble. I just felt like the more and more I tried to clean it, the more rust I saw. It was just more effective to replace that rear tank and fuel pump.

 

A year is not enough to worry about the gas. 4 years is about where you should start worrying. At 6 years, it's a catastrophe.

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I've left my Bronco parked for ~2 years, hopped in, set the cruise around 100mph for a couple of hours, wheeled for a weekend, and then cruised home even faster.

 

I agree with Steve, my truck sat for 4 years at one point with nearly full tanks. It started up, did some work to it, used all the old gas.