by A van that starts perfectly at lunchtime and refuses on a frosty morning is a specific diagnostic case, and a more tractable one than a general no-start. The temperature dependence itself narrows the possibilities considerably, because only a handful of things behave differently when cold.
If your van will not start at all regardless of temperature, work through our general no-start diagnostic sequence instead. This guide is for the temperature-dependent case.
Why cold makes the difference
Four things change when the engine is cold, and each maps to a group of causes.
- Compression heat is lost faster to cold cylinder walls, so the air may not reach ignition temperature. This is why preheating exists and why marginal compression shows up cold first.
- Battery capacity falls in low temperatures, while the engine is simultaneously harder to turn because the oil is thicker.
- Diesel thickens, and in severe cold wax crystals can restrict the filter.
- Oil viscosity rises, increasing drag on the starter and slowing cranking speed, which itself reduces the heat generated by compression.
Safety notes
Allow the engine to cool before working near the exhaust, turbo or manifold, as these retain heat long after the engine is switched off. Never work under a van supported only by a jack; use rated axle stands on firm level ground with the wheels chocked.
Keep hands, tools and loose clothing clear of belts, pulleys and cooling fans whenever the engine may run. A cooling fan can start without warning.
Read the symptom pattern
Cranks at normal speed but will not fire, or fires reluctantly
Points at preheating or compression. Glow plugs are the first suspect.
Cranks slowly, labouring
Points at the battery, the starter circuit or oil viscosity rather than the fuel side. See battery and charging fault testing.
Fires, runs roughly for a minute, then smooths out
Classic partial glow plug failure. Some cylinders are firing and others are not until the engine warms.
White smoke on cold start that clears as it warms
Unburned fuel from cylinders not igniting properly. Consistent with failed glow plugs or low compression.
Needs several attempts, worse after standing overnight, better after a short run
Suggests loss of fuel prime, air ingress into the fuel system, or a leaking injector allowing rail pressure to bleed off.
Only fails below a certain temperature, fine above it
Consider fuel waxing, particularly with older fuel in the tank. See diesel waxing in cold weather.
Diagnostic sequence
1. Battery and cranking speed
Check: Battery voltage during cranking, and whether the engine turns at a healthy speed.
Result and next action: Slow cranking reduces compression heat and can prevent starting on its own. Fix this before anything else, because a weak battery also undermines the glow plug system, which draws significant current.
2. Glow plug system
Check: Test each plug for resistance and, better, current draw. Check the relay and its supply.
Result and next action: This is the most common cause of temperature-dependent starting. The control module, relay, wiring and power supply can also fail. See glow plug faults.
3. Preheat behaviour
Check: Does the glow plug light illuminate and go out normally? Try waiting for it and, in very cold conditions, cycling the ignition two or three times before cranking.
Result and next action: If multiple preheat cycles make the difference, the preheating is marginal, which points at plugs or the control module.
4. Fuel supply and prime
Check: Whether the pump primes on ignition, filter condition, water in the separator, and any sign of air in clear lines.
Next action: A van that starts on the second or third attempt after standing may be losing prime. See fuel filters and water in diesel.
5. Rail pressure while cranking
Check: Actual rail pressure on live data during a cold crank.
Result and next action: Insufficient pressure prevents starting regardless of preheating. This also reveals excessive injector return, which bleeds pressure away. See diesel injector problems.
6. Compression
Check: A compression test, ideally comparing cylinders.
Result and next action: This is the underlying variable behind most stubborn cold start complaints on higher mileage engines. Compression that is adequate warm may be marginal cold. Low compression across all cylinders can also indicate timing having slipped.
What not to do
- Do not use easy-start or ether-based products. They are widely sold and widely misused. On an engine with glow plugs they can cause serious damage, including cracked pistons, and they can mask the real fault.
- Do not crank continuously. Long cranking flattens the battery, overheats the starter and can fill a cylinder with unburned fuel, washing oil from the bore.
- Do not add petrol to thin the fuel. Damaging on a common rail system.
- Do not ignore it because it starts eventually. Hard cold starting puts unburned fuel into the bores and the DPF, and the underlying cause rarely improves.
Practical measures while you diagnose
These help a marginal van without fixing anything, and are reasonable stopgaps rather than solutions.
- Cycle the preheat two or three times before cranking in very cold weather.
- Park under cover or out of the wind.
- Keep the battery in good condition and consider a maintenance charger if the van stands.
- Keep the fuel tank fuller in winter.
- Use a block heater if one is fitted or can be fitted.
Indicative UK costs
Broad context only. A battery is typically in the low hundreds fitted. Glow plugs are inexpensive parts with labour that varies sharply depending on access and seizure risk. A glow plug relay is usually modest. Injector or compression-related work runs into the hundreds or more. A diagnostic session is generally worth paying for before replacing parts. Get quotes.
Common mistakes
- Replacing glow plugs without testing. A module, relay or supply fault can cause similar symptoms.
- Ignoring cranking speed. A slow crank causes cold start failure on its own.
- Using starting aids. Genuine damage risk on glow plug engines.
- Blaming the fuel without checking the filter or water separator.
- Assuming it is fine because it starts after several attempts. It is telling you something and will get worse.
When to stop and get help
Get help if cold starting is deteriorating, if it needs prolonged cranking, or if you suspect compression problems. Stop cranking and seek help if the engine turns unusually freely or the note changes, which can indicate a timing failure. Also seek help before winter if the van already struggles in mild conditions, since it will not improve.
Prevention
Have the battery and charging system tested before winter. Replace glow plugs at reasonable intervals rather than waiting for failure. Change the fuel filter on schedule and drain the water separator. Keep the tank fuller in winter. Address hard starting when you first notice it rather than after it has become a no-start.
Frequently asked questions
Why does it start fine in summer?
Because the deficiency only matters when cold. Failed glow plugs, marginal compression and a weak battery can all be invisible in mild weather.
Is cycling the ignition several times bad for it?
Cycling preheat a couple of times in very cold weather is a recognised technique and is not harmful. Prolonged cranking is the thing to avoid.
Can I use easy-start?
We would advise against it on any engine with glow plugs. It carries a real risk of engine damage and masks the underlying fault.
Why does it smoke white on cold start?
Unburned fuel from cylinders not igniting properly, typically failed glow plugs or low compression. Brief white vapour on a damp cold morning that clears quickly is just condensation.
This guide is general information for UK van owners, not vehicle-specific repair instructions. Procedures, tolerances and component layouts vary by engine code and model year. Confirm any figures against your VIN and manufacturer workshop information, and see our Disclaimer before carrying out any work.
Before buying glow plugs
Record whether the starter turns slowly or at normal speed, the outside temperature, and whether the same fault occurs with the engine warm. Slow cranking calls for battery, connection and starter tests before a glow-plug diagnosis. Normal cranking with poor cold combustion needs checks of glow control, fuel supply and cylinder condition.
Do not connect an unknown glow plug directly to a 12V battery. Modern plugs can operate below battery voltage and can be damaged by that test. Use the manufacturer test method and correct meter or diagnostic equipment. Seized or fragile plugs should be removed by a specialist with the correct procedure.
