Tinkering with an old generator showed me how to spot bad diesel fast

I picked up a beat-up diesel generator from a yard sale to fix in my free time. It kept sputtering out after running for a bit, and I was sure the injectors were clogged. Instead of pulling them right away, I tried a simple trick my dad once mentioned. I drained a small sample of fuel into a clear jar and let it sit for a minute. If you see a layer of water or specks at the bottom, the diesel is contaminated. This quick check saved me hours of work because the fuel was just dirty, not the parts. Now I always test the fuel first on any diesel engine that acts up. What's your go-to method for checking fuel quality without a full kit?

3 comments

3 Comments

jason_wells4h ago

Paper towel test works in a pinch. Dab a little fuel on a white paper towel and let it dry. Good diesel leaves a light stain, but bad fuel with water or crap in it will leave separate rings or dark gunk spots. It's not perfect science but it shows contamination fast.

wren_butler743h ago

Check @jason_wells' test against the jar for water?

averyshah2h ago

That paper towel test Jason mentioned is so handy! My buddy Mike had his truck acting up last winter, sputtering on cold starts. He dabbed some fuel on a napkin like you said, and when it dried there were these clear separate rings and a dark sticky patch in the center. He figured out his jerry can had a cracked seal and let in condensation. After he swapped out the bad fuel, his engine sounded totally normal again.