Question 1

How do I know if my generator fuel is bad?

Accepted Answer

Bad generator fuel shows signs including dark color, sludge at the tank bottom, water separation, microbial growth (biofilm), and a sour or rancid odor. ASTM D975 testing can confirm fuel quality by measuring water content, particulate count, microbial contamination, and oxidation stability. Fuel stored over 12 months without treatment should be tested immediately.

Question 2

How long does diesel fuel last in a generator tank?

Accepted Answer

Untreated diesel fuel begins degrading within 6-12 months. ULSD (ultra-low sulfur diesel) is particularly susceptible to microbial growth due to reduced sulfur content. With proper fuel treatment (biocide and stabilizer), conditioned storage, and annual polishing, diesel fuel can remain usable for 3-5 years. NFPA 110 requires annual fuel quality testing for emergency generators.

Question 3

What factors affect generator fuel quality?

Accepted Answer

Key factors include fuel age (degradation accelerates after 12 months), tank conditions (condensation from temperature cycling introduces water), microbial contamination (bacteria and fungi feed on hydrocarbons), filtration quality, and treatment regimen. Environmental factors like humidity, temperature swings, and sun exposure on aboveground tanks accelerate degradation.

Question 4

How often should generator fuel be tested?

Accepted Answer

NFPA 110 Section 8.3.8 requires annual fuel testing for Level 1 Emergency Power Supply Systems. Best practice for critical facilities (hospitals, data centers) is semi-annual testing. Fuel should also be tested after any delivery, after extended storage periods exceeding 6 months, or whenever generator performance issues suggest fuel quality problems.

Question 5

What is a fuel health risk score?

Accepted Answer

A fuel health risk score estimates the likelihood of fuel-related generator failure based on tank age, fuel age, testing frequency, storage conditions, and treatment history. Scores of 80-100 indicate healthy fuel with low failure risk. Scores below 50 indicate high risk requiring immediate testing and likely fuel polishing or replacement. Our free calculator provides a personalized score with specific recommendations.