Diesel Won't Start in the Cold: Glow Plugs, Fuel and Battery

Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent

Why cold weather hits diesels harder

A petrol engine uses a spark to light the fuel. A diesel has no spark. It squeezes air so hard that it gets hot enough to ignite the fuel on its own. On a frosty morning in Aylesham, cold metal and cold air make that much harder.

If the glow plugs, battery or fuel supply is weak, the engine turns over, maybe coughs white smoke, but does not catch.

The three usual causes

1. Glow plugs or the glow plug control unit

Each cylinder has a glow plug that heats up before and during starting. They wear out slowly, and one failed plug often goes unnoticed until the first frost. Signs include:

• Long cranking on cold mornings but normal starts once the car is warm
• Rough running and a misfire for the first minute after starting
• Extra white smoke from the exhaust on a cold start
• The glow plug warning light flashing while you drive

If that light is flashing, read our guide to the glow plug light flashing on a diesel, as it can point to plugs, wiring or a wider engine fault.

2. A battery that cannot cope

Diesels have high compression, so the starter motor works harder than on a petrol car. Cold also reduces the power a battery can deliver, while the glow plugs draw a lot of current at the same moment. Slow, laboured cranking is the giveaway. Our article on battery, starter motor or alternator problems explains how to tell them apart.

3. Fuel supply and waxing

Diesel contains natural waxes that can thicken in very cold weather. Forecourt diesel sold in the UK in winter is blended to resist this, but an old fuel filter loaded with water and wax can still restrict flow. Air leaking into the fuel lines through a perished seal often shows up as a car that starts, runs briefly, then dies. Our guide to diesel fuel filter replacement covers why this part matters so much, and there is more on cold fuel in our post on diesel fuel waxing in winter.

What you can safely try yourself

1. Turn the key to the position where the dashboard lights come on and wait for the coil-shaped glow plug light to go out. On a very cold morning, you can switch off and repeat this once more before cranking.
2. Turn off the heater blower, heated screen, lights and radio so all the battery power goes to starting.
3. Crank for up to about 10 seconds, then rest for 30 seconds or so before trying again.
4. After three or four failed attempts, stop. More cranking will flatten the battery and can damage the starter motor.

Avoid easy start spray. On modern diesels it can cause uncontrolled combustion that damages pistons and glow plugs.

When to stop and get help

Call for help rather than keep trying if you notice any of these:

• A strong smell of diesel or a visible fuel leak under the car
• The engine turns over very slowly or just clicks
• The engine management or glow plug light stays on after starting
• Heavy smoke that does not clear within a minute or two

How to prevent cold start trouble

• Change the fuel filter at the interval in your service schedule, and before winter if it is close to due.
• Have the battery tested in autumn, particularly if it is more than four or five years old.
• Ask for the glow plugs to be tested at your next service if starts are getting longer.
• Give the car a proper run of 20 minutes or more each week if most journeys are short village trips.

How we can help

We come to your driveway or workplace across Aylesham, Canterbury, Dover, Deal and nearby villages. We read the fault codes with a diagnostic code reader, test battery and charging voltage, check glow plug operation and look at the fuel filter and lines for air leaks. If it needs a fuel filter or a service, we can usually do that at the car. Labour is £30 to £50 per hour depending on the job, and we always quote before work starts. Send your reg on WhatsApp 07563 350335 or use our contact page.

Frequently asked questions

Why does my diesel start fine in summer but not on frosty mornings?

Diesel engines rely on heat from compression to ignite the fuel, and cold metal steals that heat. Worn glow plugs, a weak battery or thick fuel make little difference in summer but can stop the engine firing in frost.

How many times should I try to start a cold diesel?

Wait for the glow plug light to go out, then crank for no more than about 10 seconds at a time with a short rest between attempts. After three or four failed tries, stop, because you risk flattening the battery and overheating the starter.

Can a blocked fuel filter stop a diesel starting in winter?

Yes. Water and wax collect in an old filter and can restrict fuel flow once the temperature drops. Changing the filter at the right interval is one of the cheapest ways to avoid cold start trouble.

Should I use easy start spray on my diesel?

We do not recommend it. On many modern diesels with glow plugs it can cause violent combustion and engine damage, so it is better to find the real cause.