Diesel Won’t Start When Cold – How to Fix?

Petrol engines start with the assistance of a spark from a spark plug to ignite the fuel. Whereas diesels start using the compression of the fuel mixture. However, diesels don’t like starting when cold. Hence diesel engines are fitted with glow plugs that heat the air fuel mix to aid starting when cold.

If a diesel engine is running well and starts easily when the engine is warm, but won’t start when cold, (or takes ages to get it started when cold). It could be an issue with the glow plugs, the glow plug system (glow plug module, relay, or voltage supply).

Topics:

List of checks that can be carried out:

With a Multimeter:

Without a Multimeter:

This page contains all the detail on glow plugs fault finding, their fault symptoms, testing and repair.. If you just want to view the videos, they can be found on this page ‘Diesel Won’t Start When Cold – The Video’s‘. – Opens in a new tab

This article is for guidance only…


Diesel Cold Starting – Fault Symptoms

  • The engine turns over at a reasonable speed (it’s not slow cranking), but the engine won’t start?
  • The engine starts, but only after some time (and multiple attempts)?
  • White smoke is seen from the exhaust (unburnt fuel).
  • When the engine is warm, it starts easily.

You may see white smoke from the exhaust whilst trying to start the engine and for a short while when the engine starts (unburnt fuel). The video below shows an example of what happens:

Video – Diesel Won’t start when cold?

Diesel Cold Starting – The Causes

If you see white smoke coming from the exhaust (unburnt fuel), after several starting attempts…

  • Some, or all of the glow plugs may have failed?
  • There maybe no voltage supplied to the glow plugs?
  • The glow plugs may switch off too soon. The timer (or control module) could be defective?
Glow Plugs
Glow Plugs

Glow Plug Fault Finding?

Diesel Cold Starting – Things to Check

  • The glow plug relay:
  • The glow plug timer (or control module):
  • The glow plugs:

Note:

The glow plug relay is sometimes a separate item, but it can also be incorporated inside the glow plug module?

What Should I Check First (three easy steps)?

For the initial testing, you will need a voltmeter, or a simple method of checking for a voltage. However, it’s not essential. If you don’t have a voltmeter, just go straight to the section on Testing Glow Plugs Without a Voltmeter (Multimeter)…

* If you are unsure about working on vehicle electrics, seek the advice of an electrical engineer *

  1. Is there a voltage at the glow plugs?
  2. How long is the voltage applied to the glow plugs?
  3. Check the glow plugs?

1. Is there a voltage at the glow plugs?

If there is a voltage supply to the glow plugs, then we know the glow plug relay and glow plug control module are probably working (also see 2. below).

2. How long is the voltage applied to the glow plugs?

There is a timer that applies a voltage to the glow plugs for a set amount of time. Therefore you need to check how long the supply to the glow plugs is switched on.

Glow Plug Supply Voltage Measurement
Glow Plug Supply Voltage Measurement

3. Check the glow plugs?

If the correct voltage is applied at the glow plugs (and for the correct amount of time), then the actual glow plugs will need to be checked.


Checking the Glow Plugs With or Without a Multimeter (Test Meter):

A Glow Plug
A Glow Plug

It’s easy to check the glow plugs, the glow plug timer and relay with a voltmeter. If you don’t have a voltmeter, you can still check the glow plugs, the glow plug timer and relay with a car battery and a jump lead (see the two options below).

Short video on how to remove glow plugs:

Video – Removing Glow Plugs + Where are they Fitted

With a Multimeter (Voltmeter):

There isn’t much you can check on the relay and glow plug control module, other than voltages. Therefore, all you need is a simple voltage meter, voltage tester (or you could use a vehicle lightbulb and some wire). If you have replacements, you can always swap the units over to check their operation?

Please refer to the vehicle manual, as each vehicle may be slightly different.

  1. Checking the Glow Plug Supply Voltage (the Glow Plug Relay):
  2. Checking the Glow Plug Timer (Control Module):
  3. Checking the Actual Glow Plugs (Resistance):

Notes:

  • To turn on the ignition (without starting the engine).
  • The ignition may need to be turned on and off when testing each glow plug, as the timer will operate (the supply may be switched off after approx 10 to 20 seconds depending on the vehicle).
  • To save time, tasks 1 & 2 can be done together…

1. Checking the Glow Plug Supply Voltage (and the Glow Plug Relay):

Remove (unclip) each of the glow plugs connectors and check the voltage to each of the glow plugs when the ignition is switched on. See the video at the end of 2. ‘Testing the Supply Voltage & Glow Plug Timer‘.

Measuring the Glow Plug Voltage
Measuring the Glow Plug Voltage

2. Checking the Glow Plug Timer (Control Module):

Check how long the glow plug timer (control module) voltage is supplied to each of the glow plugs. It should be for around 10 to 20 seconds dependant on the vehicle.

Other things to check:

  • If fitted, check that there are no blown fuses in the circuit.
  • Sometimes vehicles are fitted with more than one identical relay (fitted for other tasks). If so, you could use the identical relay to test the circuit. Make sure it’s an identical relay, otherwise there is risk of damage (there should be a part number printed on the relay, so you can check).
  • The glow plug relay is sometimes included inside the glow plug module?

Short video on testing the supply voltage and glow plug timer:

Video – Measure the glow plug supply voltage and test the timer operation

3. Checking the Glow Plug Resistance:

With a voltmeter (multimeter), we can check the resistance of the glow plugs. This will tell us if they are:

  • Working (low resistance).
  • Not working very well (elevated resistance).
  • Or have failed (high resistance).

The glow plugs can be tested either when fitted, or removed. However, a less accurate reading may be obtained if testing whilst still fitted (additional resistance in the circuit).

At the end of section 3. is a table of ‘Typical Glow Plug Resistance Values‘, and whether they pass or fail.

3.1. Testing the Resistance (with the glow plugs Installed):

We can measure the glow plug resistance whilst they’re still installed. This may only give an indication of their condition, as there may be additional resistance in the circuit. However, it can prove that there is a circuit and that corrosion is not a factor in the glow plug failing to work.

Short video on how to measure the glow plug resistance (when fitted):

Video – How to Measure the resistance (when fitted).

3.2. Testing the Resistance (with the glow plugs removed):

What can we check..?

Testing Glow Plugs - Where to Connect a Multimeter (Voltmeter)
Testing Glow Plugs – Where to Connect a Multimeter (Voltmeter)

The condition of the glow plugs?

Carry out a visual check for general condition, such as corrosion, or any obvious damage etc.

The resistance of the glow plugs?

The resistance check is a more detailed check on the condition of the heater element. It can indicate if the glow plug is worn, or on the way out (see the table of Typical Glow Plug Resistance Values below).

Short video on how to measure the glow plug resistance (when removed):

Video – How to Measure the resistance (when removed).

The continuity?

A continuity check will check if the glow plug is open circuit (broken internally). This can be just a simple ‘buzzer’ test. If you don’t get a ‘buzz’, (open circuit) then it’s dead!

Short video on how to test the continuity of glow plugs:

Video – How to test glow plug continuity.

How to Test Glow Plugs From Start to Finish (With a Multimeter) – The Complete Guide

Video – How to test glow plugs with a multimeter – The complete guide.

3.3. Typical Glow Plug Resistance Values:

The table below shows typical glow plug resistance values and whether they would pass or fail. Note the difference in values between measurements taken when installed and removed. This could vary greatly if there is any corrosion present, or there is a ‘bad contact’ when the glow plugs are inserted.

Typical Glow Plug Resistance Values

Glow Plug Condition Resistance (Fitted) 1 Resistance (Removed) 1 Pass or Fail
1 Old 3.031 MΩ 2.181 MΩ
2 Old 16.24 KΩ 15.24 KΩ
3 Old 47.26 KΩ 45.77 KΩ
4 Old 211.2 Ω 208.9 Ω
5 New 1-5 Ω 2 1 Ω
6 New 1-5 Ω 2 1 Ω
Note: 1 The difference in resistance for the same glow plug (when fitted and removed)…
2 The resistance (fitted) may vary and is approximate…

Testing Glow Plugs Without a Voltmeter (Multimeter):

There are some easy checks you can carry out, using a:

  • Vehicle battery, or suitable earth (-ve).
  • A jump lead.
  • A piece of insulated wire.

The following tests can be carried out:

You can test the glow plugs when fitted or removed, as indicated below:

  1. Is there a Circuit (When Fitted)?
  2. Does the Glow Plug Work (When Removed)?
  3. Does the Relay and Timer Work?

Safety Precautions:

Do not short the battery connections, (or yourself across the battery, it could give you a shock!) i.e. Between the live (+ve) and the earth, or bodywork (-ve). 12 volts is not usually enough to kill you, but batteries larger than 12v may do? Hydrogen gas given off by the battery can ignite, causing an explosion. Sending bits of battery and acid everywhere…

1. Is there a Circuit (When Fitted)?

The glow plugs can be tested for a working circuit (when fitted), by using a wire from a live 12v (or 24v depending on the system) supply and touching the end of the exposed glow plug (with the connector removed). If a spark is present when the end of the glow plug is touched, we know current is passing through the glow plug back to the battery. This indicates the glow plug is not open circuit.

Notes:

  • Do not use a jump lead for this test! A jump lead is usually too big (and you can’t easily see what you’re doing).
  • You are less likely to make the wrong connection with a piece of wire, (i.e. Shorting in the wrong place).
  • Shorting can cause wire to melt (with the potential for a fire!).

2. Does the Glow Plug Work (Tested When Removed)?

When a glow plug is removed, we can carry out a visual check whilst testing to see if the glow plug works.

For safety reasons:

In this case, use a jump lead for testing, not a thin piece of wire…

  • The jump lead will hold the glow plug.
  • The jump lead has an insulated handle…

Note:

  • Shorting can cause wire to melt and it could cause a fire!
  • For negative earth vehicles, always connect the negative jump lead first (or vice versa).
Testing Glow Plugs - Where to Connect a Multimeter (Voltmeter)
Testing Glow Plugs – Where do I Connect to?

Two Methods:

a. Connect directly to the battery: (not recommended)
b. Connect to the actual glow plug supply:

a. Connect directly to the battery:

  1. Testing the glow plugs by connecting directly to the battery is not recommended. Batteries can give off hydrogen gas. The glow plug can provide a heat source for the hydrogen, causing an explosion.

b. Connect to the actual glow plug supply:

To save time, two test’s can be carried out the same time. Goto 3. Does the Relay and Timer Work? below for more detail.

  1. With the glow plug removed, connect the glow plug to one of the connectors.
  2. Connect a jump lead to the negative (-ve) of the battery and the other end of the jump lead to the negative (-ve) end of the glow plug.
  3. Make sure the glow plug is not near anything that could be affected by heat.
  4. Switch on the ignition. If you have a working glow plug, the heater element will start to glow (see the image below).
Testing Glow Plugs Removed from a Vehicle With a Jump Lead
Testing Glow Plugs Removed from a Vehicle With a Jump Lead

3. Does the Relay and Timer Work (Tested when Removed)?

This test is similar to ‘Connect to the actual glow plug supply‘ above, but has an additional test. You can time how long the glow plug works for, (before the timer switches the supply off).

  1. With the glow plug removed, connect the glow plug to one of the connectors.
  2. Connect a jump lead to the negative (-ve) of the battery and the other end of the jump lead to the negative (-ve) end of the glow plug.
  3. Make sure the glow plug is not near anything that could be affected by heat.
  4. Prepare to time how long the glow plug glows?
  5. Switch on the ignition.
  6. Watch to see if the glow plug starts to glow, whilst also timing the glow plug.
  7. How long did it glow for? It should be for around 10 to 20 seconds dependant on the vehicle.
  8. If it doesn’t glow, the glow plug is defective (check the connections?).

Short video on how to test glow plugs without a test meter:

Video – How to test glow plugs without a multimeter.

How to Test Glow Plugs From Start to Finish (With Jump Leads & Wire) – The Complete Guide

Video – How to test glow plugs without a multimeter – The complete guide.

Ordering Parts (Glow Plugs, Test Meter, Tools, etc):

The following table gives examples of the glow plugs, test equipment, tools, etc you can buy to fix your vehicle.

Ordering Parts – Examples of What You Can Buy:

Item Location Link
Glow Plugs UK https://ebay.us/dCtHEE
USA https://ebay.us/oSlLOK
Test Equipment UK https://ebay.us/Q1oFtk
USA https://ebay.us/qliseA
Glow Plug Removal Tools UK https://ebay.us/TbyHfH
USA https://ebay.us/cDeIxc
Jumper Cables UK https://ebay.us/eJmRCL
USA https://ebay.us/UWZUPz
Disclosure: The links in this table are “affiliate links” This means we may garner a small commission (at no cost to you) if you choose to make a purchase.
Thanks for your support.

Replacing Glow Plugs:

Is the same as for the removal, except…

  • It’s in reverse!
  • It may be a good idea to lubricate the threads to prevent corrosion (use some high temperature copper grease, or something similar).
  • Don’t overtighten. See the workshop manual for the correct torque. It may be printed on a replacement glow plug suppliers box (see the image below).
  • Check all the electrical connectors are properly connected and not just resting on top of the glow plugs (it’s easily done!).
The Manufacturer's Suggested Glow Plug Torque Value
The Manufacturer’s Suggested Glow Plug Torque Value

– click or tap the image to view full size –