ASCO Equipment — Blog

Published: September 8, 2026

When your diesel fights you on a cold morning

Few things slow a workday down like a diesel that won't fire on a cold morning. The engine cranks and cranks, coughs out white smoke, maybe catches and stumbles before it finally settles. On construction sites, farms, and standby generators across Canada, worn glow plugs are one of the most common — and most fixable — causes of hard cold starts.

With over 40 years working with Kubota, Perkins, and Yanmar engines, we've seen how a set of tired glow plugs turns a routine startup into a daily headache. The good news: glow plugs are inexpensive, and understanding how they work makes diagnosing a no-start far simpler. Here's what you need to know.

What glow plugs actually do

Unlike a gasoline engine, a diesel has no spark plugs. It ignites fuel purely by the heat of compression — air squeezed so hard it gets hot enough to light the injected diesel. That works beautifully once the engine is warm. But when the block and the incoming air are cold, compression alone may not create enough heat for reliable ignition.

That's where glow plugs come in. Each cylinder has a glow plug — a small heating element that warms the combustion chamber before and during startup. When you turn the key and wait for the glow indicator, the plugs are heating up to give that first bit of combustion the boost it needs. On many engines they also keep working for a short time after startup (afterglow) to smooth out the rough, smoky running you sometimes get in the first minute.

Signs your glow plugs are failing

Hard starting in cold weather. The classic symptom. If the engine starts fine when warm but fights you when cold, suspect the glow plugs first.

White smoke on startup. Unburned fuel from incomplete combustion shows up as white smoke that clears as the engine warms. Worn glow plugs are a frequent cause.

Rough running right after start. If one or more plugs have failed, those cylinders contribute poorly until the engine heats up, causing a rough, uneven idle for the first minute.

Extended cranking. Having to crank noticeably longer than usual before the engine catches, especially first thing in the morning.

Because glow plugs usually fail one at a time, symptoms often creep in gradually — the engine gets a little harder to start each cold week until it finally won’t go.

Why replace them as a set

Glow plugs share the same age, heat cycles, and hours. When one fails, the others are usually not far behind. Replacing all of them at once — rather than chasing one bad plug at a time — restores reliable cold starting and saves you from repeating the job in a few weeks. They’re inexpensive relative to the aggravation and lost time of a no-start, which makes a full set one of the best-value repairs on a diesel. Browse glow plugs for Kubota, Perkins, and Yanmar to get the right set for your engine.

Glow plugs are not always the whole story

Hard cold starts can have other causes, and it’s worth ruling them out so you fix the real problem:

Weak batteries or poor connections. Cold saps battery power, and diesels need strong cranking. Corroded terminals or a tired battery can mimic a glow-plug problem.

Fuel issues. Diesel can gel in extreme cold, and air or water in the fuel system causes hard starts. Check the fuel system — filters, lift pump, and water separator — if starting problems persist.

Low compression. An engine with worn rings or valves loses the compression heat it needs to fire, especially when cold. This points to deeper wear.

Faulty glow relay or timer. If none of the plugs are heating, the fault may be in the relay or controller that powers them rather than the plugs themselves.

Keeping cold starts easy through winter

A few habits keep Canadian winter mornings from becoming a battle: replace glow plugs proactively before the cold season if starts are getting sluggish, keep batteries healthy and terminals clean, use winter-grade or treated fuel to prevent gelling, and let the glow cycle finish before cranking rather than rushing the key. On equipment that sits outside, a block heater takes a huge load off the glow plugs and the starter alike.

The bottom line

Glow plugs are small, cheap, and easy to overlook — right up until the morning your diesel won’t start. If your engine is fighting you in the cold, a fresh set is usually the fastest, most cost-effective fix, and doing all of them at once keeps you from repeating the job.

Not sure whether it’s the glow plugs, the batteries, or the fuel system? Call us at 647-350-1570 — with 40+ years behind us, we’ll help you pin down the cause and get the right parts the first time.

This article is general guidance. Always follow the maintenance schedule and specifications in your engine's operator's manual for your specific model.