Cold-weather hard starting on diesel engines is usually caused by weak glow plugs, fuel gelling, or a battery that can’t deliver enough cranking power for diesel compression.
If your Aurora AGI6500DE diesel portable generator cranks slowly or refuses to fire up when the temperature drops, you’re not alone. Diesel engines are notoriously finicky in cold weather because they rely on glow plugs to pre-heat the combustion chamber—and when those plugs fail or the battery weakens, starting becomes nearly impossible. The good news is that most cold-start failures can be diagnosed and fixed with basic tools and a methodical approach.
This guide walks you through the most common causes and shows you exactly what to check before you call a technician.
At-a-Glance: Most Likely Causes
| Cause | Likelihood | Typical Cost to Fix |
|---|---|---|
| Weak or dead battery | Very Common | $ (battery replacement) |
| Glow plug failure (one or more) | Very Common | $$ (glow plug set) |
| Fuel gelling in cold temperatures | Common | $ (fuel additive or drain) |
| Glow plug relay not energizing | Occasional | $$ (relay replacement) |
| Injector nozzle carbon buildup | Occasional | $$$ (injector service) |
Diagnostic Walkthrough: Step-by-Step
Work through these checks in order. Most are free or nearly free, and they’ll narrow down the problem quickly.
1. Check Battery Voltage and Condition
Diesel engines need robust cranking power. A weak battery is the fastest way to a no-start in cold weather. Use a digital multimeter set to DC voltage and measure the battery terminals with the engine off. A healthy 12V battery should read 12.6 volts or higher. If it reads below 12.0 volts, the battery is too weak to crank a diesel engine reliably.
Also inspect the battery terminals and cable connections for corrosion (white, blue, or green crusty buildup) or loose connections. Corroded terminals can drop voltage even if the battery itself is sound. Clean them with a wire brush and reconnect firmly.
If the battery is more than 3–4 years old and reads low, replace it. Cold weather drains battery capacity dramatically, and a marginal battery will fail in winter.
2. Verify Glow Plug Operation
Glow plugs are the diesel engine’s pre-heater. When you turn the key to the “on” position (before cranking), you should hear a faint clicking relay sound, and a glow plug indicator light should illuminate on the dashboard for 3–10 seconds. This tells you the glow plugs are warming up.
If there’s no clicking and no light, the glow plug relay may be dead. If the light comes on but the engine still won’t start, one or more glow plugs may have failed.
To test individual glow plugs, you’ll need to remove them (consult your owner’s manual for location). A working glow plug should glow orange-red when powered. If it doesn’t glow or glows dull, it’s failed and needs replacement. Most diesel generators have 4–6 glow plugs; if one has failed, the others are likely aging too, so consider replacing the full set.
3. Inspect Fuel for Gelling
Diesel fuel thickens and can gel in freezing temperatures, blocking fuel filters and lines. If the engine cranks normally but won’t fire, fuel starvation is a strong suspect.
Locate the fuel filter (check your manual for its location). If it’s visibly clogged or dark, fuel gelling is likely. You can also drain a small amount of fuel into a clear container and place it in a freezer for 15 minutes. If it becomes cloudy or thick, it’s gelling.
In cold climates, use winter-grade diesel fuel (marked as #1 diesel or #1-D) or add a diesel fuel additive designed to prevent gelling. If you suspect gelling has already occurred, drain the tank, replace the fuel filter, and refill with fresh winter-grade fuel or treated summer fuel.
4. Check Fuel Filter Condition
A clogged fuel filter starves the injectors of fuel and will prevent cold starts even if the glow plugs are perfect. If the fuel filter looks dark, wet, or hasn’t been changed in over a year, replace it. This is a cheap, easy fix that often solves hard-start problems.
5. Test the Glow Plug Relay
The glow plug relay energizes the plugs when you turn the key on. If it’s failed, the plugs won’t heat up at all. You should hear a distinct click when you turn the key to “on.” If there’s no click and no glow plug light on the dashboard, the relay is likely bad.
Consult your owner’s manual to locate the relay in the electrical panel. A multimeter set to resistance (ohms) can test it, but relay replacement is often simpler and cheaper than diagnosis. If you’re not comfortable testing, a technician can swap it in minutes.
6. Inspect Injector Nozzles for Carbon Buildup
Over time, injector nozzles accumulate carbon deposits that disrupt fuel spray patterns. This makes cold starts harder because the fuel doesn’t atomize properly. If the engine has high hours and you’ve ruled out battery, glow plugs, and fuel issues, carbon buildup is a possibility.
Removing and cleaning injectors requires specialized tools and knowledge. This is a job for a professional diesel technician. However, you can try running a quality diesel fuel system cleaner additive through a full tank to dissolve light deposits before resorting to injector removal.
7. Warm the Engine Before Starting
As a temporary measure, you can warm the engine block with external heat (a heat lamp or warm water poured over the block—never boiling water, which can crack the block). This reduces the compression resistance and makes starting easier while you diagnose the underlying problem. This is not a permanent fix, but it helps confirm whether the issue is cold-related or mechanical.
Parts You May Need
- 12V battery (if weak or dead)
- Glow plug set (typically 4–6 plugs)
- Glow plug relay
- Fuel filter cartridge
- Diesel fuel additive (anti-gel)
- Wire brush (for battery terminal cleaning)
When to Call a Pro
Stop troubleshooting and contact a diesel technician if:
- The engine cranks normally but won’t fire, and you’ve confirmed the battery, glow plugs, and fuel are all good.
- You hear abnormal noises during cranking (grinding, knocking) that suggest mechanical damage.
- The glow plug relay clicks but the plugs won’t glow (possible wiring or control module issue).
- You suspect injector carbon buildup or need injector removal and cleaning.
- You’re uncomfortable testing electrical components or removing fuel system parts.
Frequently Asked Questions
Can I use summer diesel in winter if I add an anti-gel additive?
Yes, in most cases. A quality diesel anti-gel additive can lower the pour point of summer fuel by 10–20 degrees Fahrenheit. However, in extreme cold (below −10°F), switching to winter-grade #1 diesel is safer and more reliable. Check your owner’s manual for the minimum operating temperature of your AGI6500DE.
How often should I replace glow plugs?
Glow plugs typically last 100,000+ engine hours under normal conditions, but cold climates and frequent cold starts accelerate wear. If you’re experiencing hard starts and the plugs are original to the unit, replacement is a worthwhile investment. Many technicians recommend replacing the full set at once rather than individual plugs.
Will a jump-start help if the battery is weak?
Temporarily, yes. A jump from another vehicle can provide enough cranking power to start the engine. However, this masks the underlying battery problem. If the battery won’t hold a charge or reads below 12.0 volts, replace it. A weak battery will fail again in cold weather and may damage the starter over time.
Is it normal for diesel engines to be harder to start in cold weather?
Yes, diesel engines are inherently harder to start in cold weather than gasoline engines because they rely on compression ignition rather than spark plugs. That’s why glow plugs exist. However, a properly maintained diesel with a strong battery and good glow plugs should still start reliably down to 0°F or lower. If your AGI6500DE struggles below 32°F, something is wrong.
Disclaimer
This article provides general troubleshooting guidance for cold-start issues on diesel engines. Always consult your Aurora AGI6500DE owner’s manual and follow the manufacturer’s recommended maintenance schedule and procedures for your specific model. If you’re unsure about any step, contact a qualified diesel technician or Aurora customer support at https://www.aurora.com/support/. Improper diagnosis or repair can damage your engine or create safety hazards.
Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.