Kubota GL6000 Lowboy Diesel Won’t Start: Diagnostic Guide

Understanding the Problem

A diesel engine that cranks normally but refuses to start is telling you one of two things: either fuel isn’t reaching the combustion chamber, or the combustion chamber isn’t hot enough to ignite it. The Kubota GL6000 Lowboy Diesel is a robust workhorse, but it’s also sensitive to fuel quality and glow plug condition—especially in cold weather or after sitting idle. The good news is that most no-start conditions on this model are preventable and fixable without major teardown.

At-a-Glance: Most Likely Causes

Diagnostic Walkthrough

Work through these steps in order. Most no-starts are solved by the time you reach step 4.

1. Check fuel level and tank condition. Open the fuel cap and listen for sloshing. If the tank is empty or nearly empty, fill it with fresh diesel. If you haven’t run the engine in months, the fuel may have degraded or accumulated water. Drain the tank and refill with quality diesel fuel. This is the cheapest fix and rules out the most obvious culprit.
2. Inspect the fuel filter for water or sediment. Locate the fuel filter bowl (usually a clear plastic or metal canister on the fuel line between the tank and engine). If it’s dark, cloudy, or contains visible sediment or water droplets, the filter is clogged. Loosen the drain plug at the bottom of the filter bowl and let fuel drain into a container. If water pours out, you’ve found your problem. Replace the fuel filter element and bleed the system (see step 5).
3. Turn on the glow plugs and listen for the relay click. Turn the ignition key to the “on” position (do not crank yet). You should hear a distinct click or hum from the glow plug relay, usually located near the battery or under the dashboard. The glow plugs will warm for 3–10 seconds, then the relay will release. If you hear nothing, the relay or glow plug circuit is dead. Check the battery voltage with a multimeter—it should read 12V or higher. If the battery is weak (below 11V), charge it fully and retry. If the battery is good but you hear no relay click, the relay or glow plugs are likely faulty.
4. Check for fuel at the injection pump inlet. Locate the fuel line running from the filter to the injection pump. Carefully loosen the banjo bolt or fuel line fitting at the pump inlet (have a rag ready—fuel will drip). Turn the ignition on and manually operate the fuel lift pump priming lever (if equipped) or crank the engine briefly. Fuel should flow out of the loosened fitting. If no fuel appears, the lift pump is not priming and needs replacement or the fuel line is kinked/blocked. If fuel flows freely, tighten the fitting and move to step 5.
5. Bleed air from the fuel system. Air in the fuel lines prevents the injectors from firing. Locate the bleed screw on top of the fuel filter housing or on the injection pump. Loosen it by one-quarter turn. Operate the manual priming lever on the lift pump (or crank the engine gently in short bursts) until fuel flows from the bleed screw without bubbles. Tighten the bleed screw. Repeat this process at the bleed screw on the injection pump if one exists. This step is essential after any fuel filter change or fuel system work.
6. Verify glow plug operation with a test light or multimeter. If the relay clicked in step 3 but the engine still won’t start, test the glow plugs themselves. Disconnect the glow plug electrical connector (usually a flat spade terminal). Set a multimeter to resistance mode and touch the probe to the glow plug terminal. A good glow plug reads 0.5–2 ohms; a failed plug reads open circuit (infinite resistance). If one or more plugs are open, they need replacement. Replace all glow plugs at once to ensure even heat distribution.
7. Check fuel viscosity in cold weather. If the outside temperature is below 32°F and the engine cranks but won’t catch, the diesel fuel may have thickened. Diesel fuel can gel in freezing temperatures without a winter additive. Move the generator to a warm location (garage, shed) and allow it to sit for 30 minutes. If it starts, add a diesel fuel conditioner or winter additive to the tank and refill with winter-grade diesel. If it still won’t start after warming, the problem is not fuel viscosity.
8. Perform a compression check (advanced). If you have a diesel compression tester, remove all glow plugs and install the tester in one cylinder. Crank the engine 5–6 times and note the pressure. Healthy compression on a GL6000 is typically 350+ psi. Low compression indicates internal engine wear or a timing issue, which requires professional service. Normal compression rules out major internal problems and points to fuel delivery or glow plug issues.

Parts You May Need

• Fuel filter element (primary and secondary if equipped)
• Glow plugs (set of 4 or more, depending on your model year)
• Glow plug relay (if testing shows it’s defective)
• Fuel lift pump (if priming fails)
• Diesel fuel conditioner or winter additive
• Fresh diesel fuel (for tank flush if needed)
• Fuel line clamps and banjo bolts (assorted kit)

When to Call a Pro

Stop troubleshooting and contact a Kubota dealer or certified diesel technician if:

• You’ve bled the fuel system, replaced the fuel filter, and verified glow plug operation, but the engine still won’t start.
• The compression test shows pressure below 300 psi, indicating possible internal engine damage or timing misalignment.
• The fuel lift pump is not priming even after you’ve confirmed fuel is in the tank and the filter is clear.
• The injection pump is suspected of being out of time or failed (this requires specialized equipment and expertise).
• You’re uncomfortable working with fuel systems or electrical components.

Frequently Asked Questions

Can I start a GL6000 without glow plugs in warm weather?

Diesel engines require heat to ignite fuel. Glow plugs provide that heat, especially during cold starts. In warm weather (above 70°F), some diesels will fire without glow plugs, but it’s unreliable and stresses the engine. Never intentionally disable or remove glow plugs; they’re essential to the engine’s design.

How often should I change the fuel filter on my GL6000?

Kubota recommends changing the fuel filter every 500 operating hours or annually, whichever comes first. If your generator sits unused for long periods or you suspect fuel contamination, change it more frequently. A clogged filter is one of the most common causes of no-start conditions.

What’s the difference between air in the fuel system and a clogged filter?

Air in the fuel system prevents the injectors from atomizing fuel properly; the engine may crank and briefly catch but won’t sustain combustion. A clogged filter starves the injectors of fuel entirely, so the engine cranks but never fires. Both are fixed by bleeding the system and replacing the filter if needed.

Why won’t my GL6000 start after sitting for six months?

Diesel fuel degrades over time, especially in warm or humid conditions. Bacteria and water accumulate in the tank, clogging the filter. The glow plug relay may also weaken if the battery has discharged. Always drain old fuel and refill with fresh diesel before long storage. Use a fuel stabilizer if you plan to leave the generator idle for more than a month.

Disclaimer

This article provides general troubleshooting guidance based on common diesel engine principles and Kubota factory service information. Always consult your GL6000 owner’s manual and shop manual for model-specific procedures, torque specifications, and safety precautions. If you are unsure about any step, contact a Kubota dealer or qualified diesel technician. Improper fuel system work can damage the engine or create a fire hazard.

Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.