Generators that won't start or run: is the fuel bad?

When your generator won’t start or stops running unexpectedly, the natural first step is to call a technician. Unfortunately, one of the most common (and often premature) conclusions they’ll draw is: “The fuel is bad.” Some even go so far as to claim it’s damaged critical components.

But is the fuel the problem?

In many cases, the answer is no. And when technicians jump to that conclusion without deeper investigation, it can lead to costly missteps, like disposing of usable fuel or replacing parts that aren’t broken.

Before you go down that road, here are the essential diagnostic steps to follow—and why skipping them could cost you thousands.

The Cost of a Misdiagnosis

Misidentifying the cause of a generator problem doesn’t just waste time—it can drain your budget fast.

• Fuel Disposal: Disposing of just 500 gallons of diesel can cost $6,500–$8,500, plus labor, transport, and replacement fuel.

• Unnecessary Repairs: Replacing injectors or fuel pumps without cause could run $1,000–$5,000 or more.

And that doesn’t include the cost of downtime while you chase a fix that won’t solve the real issue.

Step-by-Step Diagnostics Before You Blame the Fuel

Instead of assuming the fuel is at fault, work through these basic steps. After each one, attempt to start the generator—if it runs, you’ve found the issue.

1. Replace the Fuel Filters (and Inspect Them)

If the generator shuts down or won't reach proper RPMs, replacing both the primary and secondary fuel filters is a smart first move.

After removal, cut the filters open and inspect the media:

• Black, slimy buildup suggests microbial growth.

• Neutral-colored slime points to water contamination.

If these signs are present, your issue may involve microbial contamination—treating for both microbes and water will be necessary. But if replacing the filters solves your issue, you’ve fixed the problem for the cost of a few filters, potentially saving thousands.

2. Check the Fuel Line and Foot Valve

Start by blowing air back through the fuel line to ensure it’s not obstructed. If the line includes a foot valve, this test won’t work—so extract the line and clean it instead.

Also, inspect and clean the foot valve or check valve, usually found between the tank and the primary filter or lift pump. Reinstall and test the system again.

3. Look for an Inline Fuel Filter

Some fuel systems include a third, inline fuel filter that resembles an automotive-style filter. If you’ve replaced the main filters and cleared the lines, try replacing this one too.

What About Microbial Contamination?

Microbial contamination isn’t always the direct cause of failure, but it’s often a contributing factor. If your filters showed slime or black residue, you should also check the water separator filter, a common site for microbial activity.

Look for:

• Black slime or mold

• Sour or rotten odor

• Leopard-spotting on the paper element

• Visible particles or clumps

• Cloudy or milky water in the bowl

If you spot any of these, have the fuel tested for microbes using a method like ATP-by-Filtration. Bell Performance can help guide you through testing and treatment, including water removal and biocide application.

When It Is the Fuel

Sometimes fuel is the issue, but not as often as you’re told. Confirming a real fuel problem means identifying one of the following:

• Low cetane or incorrect flash point, causing poor combustion

• Excess particulates, clogging filters

• Critically low lubricity, potentially damaging injectors or pumps (rare but possible)

If a technician claims “bad fuel” without identifying a specific issue like these, the diagnosis is suspect—and probably wrong.

What Fuel Can’t Do

Let’s be clear: some claims about “bad fuel” are just myths.

In one real-world case, a technician blamed recurring fuel pump failures on “fuel eating up the filters.” That’s simply not possible. The filter media is impervious to normal fuel and common additives, such as stabilizers or biocides. These products are used worldwide, every day, without degrading filters.

So if you hear claims like this, it’s worth pushing back—or getting a second opinion.

A Common but Serious Problem: Gasoline in Diesel

One type of contamination that can seriously damage your system is gasoline in diesel. Even small amounts (2% or less) can lower the fuel’s flash point below ASTM spec and destroy lubricity, leading to real mechanical damage.

If you suspect gasoline contamination, have the fuel tested for flash point or distillation curve. Bell Performance can help walk you through an easy preliminary check, and if contamination is confirmed, you will need to dispose of the fuel.

Confirm the Fuel’s Quality With Testing

If mechanical issues have been ruled out, it’s time to test the fuel itself. ASTM-standard lab tests can confirm if your fuel is “in spec” for:

• Combustion properties (cetane rating, flash point, distillation)

• Contaminants (water, sediment, microbes)

If results show the fuel is good, that confirms two things:

1. The fuel is not the problem.

2. You’ve just saved yourself from wasting thousands on a misdiagnosis.

Need a lab recommendation? Bell Performance works with proven fuel testing labs and can point you in the right direction.

Sampling the Right Way

Good testing starts with a good sample.

Contaminants tend to settle at the bottom of the tank. If you’re checking for start-up reliability, not sludge, take your sample from 3 to 12 inches above the tank bottom.

The industry standard tool is the Bacon Bomb sampler, which can pull consistent, reliable samples at the right depth. Bell Performance can advise on using one for your specific setup.

When to Call an Expert

If you’ve followed all the steps above and your generator still won’t run, the issue may be more complex—but you’ve already narrowed the field.

Now it’s time to consult someone who understands both the mechanical and fuel-related sides of generator systems. Bell Performance can help you identify the real problem, whether it’s simple or more involved.

Conclusion: Don't Let Assumptions Cost You

Generator no-start or no-run problems are frustrating, but assuming “the fuel is bad” without investigation is often a costly mistake. By working through the right diagnostic steps, you can avoid unnecessary repairs, prevent downtime, and focus on the actual cause.

Want more details?

Download our comprehensive guide to generator troubleshooting and learn how to diagnose your system with confidence.

And suppose you need more detailed assistance, whether to nail down a problem that hasn’t responded to simple troubleshooting, or maybe you need guidance on a qualified lab to send a fuel sample to. In that case, Bell Performance is always available to help guide you through the steps you need to take so you are confident that the solution you pursue is going to be the right one for you and your situation. Give us a call at (407) 831-5021.