It is possible to start a warm diesel engine without using a glow plug, or in warm climates, thanks to the relatively high temperature of the intake air and the relatively low ignition temperature of diesel.
However, this isn’t enough to ensure proper ignition of the injected fuel in all temperatures, which results in increased exhaust emissions.
A glow plug solves this issue. It is an electrical heating device located in each cylinder of the diesel engine that ensures reliable starts in all weather conditions.
What Is A Glow Plug?
In a diesel engine, a glow plug (also spelled glowplug) is a heating device used to aid the starting of the engine in cold weather. This device is a pencil-shaped piece of metal with an electric heating element at the tip.
A glow plug system consists of either a single glow plug in the inlet manifold or one glow plug per cylinder. In older systems, the driver is required to manually activate the glow plug system and wait approximately 20 seconds before starting the engine.
Newer systems automatically activate the glow plug(s) before the engine is started and have a quicker warm-up time.
What Does A Glow Plug Do?
A glow plug is a heating element that heats incoming fuel and air to encourage efficient fuel combustion in a diesel engine. The fuel combustion starts your engine so that you can drive.
Each cylinder of your engine has one glow plug, either in the pre-chamber (pre-chambered engines) or in the combustion chamber (direct-injection engines).
Consequently, glow plugs are used to make starting a diesel engine in cold weather conditions much easier by preheating the air in the combustion chamber, allowing it to reach its ignition temperature more quickly.
Glow plugs are activated automatically when you turn on the ignition and typically only remain active for a few seconds. In most cases, modern glow plugs are made from ceramic or metal and are electrically powered.
How Does A Glow Plug Work?
Unlike a spark plug that is working continuously while you drive, a glow plug is only needed during the ignition process. The glow plug works by electrifying the heating element so it heats up and emits visible light (hence the name).
Intake air is compressed before the fuel injector spray directs fuel onto the hot tip of the glow plug during the injection of the fuel. The injected fuel mixes with the compressed air, evaporates and starts combustion almost simultaneously, even when the engine is cold.
How Many Glow Plugs Are In a Diesel Engine?
There are two different types of glow plugs. One is the type that goes in the cylinder, the other is a type that goes in the manifold. For in-cylinder types of glow plugs, there is a plug in each cylinder.
For the in-manifold type, there’s only one glow plug that serves all the cylinders. So the number of glow plugs in a diesel engine will vary by the number of cylinders, or it will just be one Thermostart (in-manifold) glow plug.
For example, an eight-cylinder engine using in-cylinder plugs would have eight plugs.
Types Of Glow Plug
A few types of glow plugs are available in the market, catering to specific engine requirements and conditions.
Pressure Sensor Glow Plugs
Pressure Sensor Glow Plugs (PSG) are “smart” plugs with a sensor in the tip that measures combustion chamber pressure.
This information is sent to the Engine Control Unit (ECU) to adjust the heating pattern accordingly. They provide precise control for optimizing engine performance.
Ceramic Glow Plugs
Designed for extreme temperatures, Ceramic Glow Plugs (CGP) can reach up to 1500°C in under three seconds. This rapid heating helps start engines quickly.
It’s beneficial in low-compression diesel engines facing Environmental Protection Agency (EPA) restrictions.
High-Tech Steel Glow Plugs
They come in two kinds: GE and GN. GN plugs are modern three-phase glow plugs that heat up quickly, remain hot during ignition, and cool down after three minutes. The plugs are long, pencil-like.
Older diesel engines used GE-type plugs, which could take up to 30 seconds to heat, but modern high-tech steel allows them to heat in just seven seconds. GE plugs are typically shorter and more similar in size to spark plugs.
What Are The Symptoms Of A Bad Glow Plug?
The engine warning light on the dashboard is often the first clue that something is wrong. Vehicles displaying this light should be taken to an authorized workshop for proper diagnosis and service as soon as possible.
A number of faults can cause the engine warning light to illuminate – below, we outline the clues that point to faulty glow plugs.
#1. Hard starting.
A hard starting engine is the most common indicator of damaged glow plugs. In cold conditions, without correctly functioning glow plugs, the combustion chamber may not reach the right temperature to ignite the fuel and start the engine.
If there are no problems with the fuel or the battery, then glow plugs should be checked.
#2. Engine misfiring.
The combustion process in diesel engines requires a high rate of compression, fuel injected at the correct time and in the correct amount. This ignites the diesel fuel, resulting in combustion. Misfires can be caused by issues with fuel quality, compression and fuel injection.
The combustion chamber heat comes from compression pressure and glow plug heat. It needs to reach at least 450° F / 232° C for ignition to take place. If the glow plugs are faulty, the engine chamber will not reach the required temperatures.
#3. Rough idling.
If one or more glow plugs have burned out, are carbon fouled or damaged, the engine will become progressively harder to start as temperatures drop and will idle roughly and produce white smoke from the exhaust for several minutes once it finally starts.
If all the glow plugs are burned at their tips, check the fuel injection timing.
#4. Decreased fuel efficiency.
Lots of factors can affect fuel efficiency and glow plugs aren’t always the biggest impacting factor. But if you’re seeing some of these other signs and your fuel efficiency is a lot lower than usual, then the glow plugs may be to blame.
#5. White or Black Smoke in The Exhaust.
Both black and white smoke pouring out of the exhaust are not great signs. White smoke occurs when there isn’t enough heat in the engine to properly combust the fuel. Those unburnt fuel particles then exit the engine via the exhaust and smell like diesel fuel.
Black smoke is often a result of the air and fuel mix being off, typically with too much fuel. Glow plug issues make both of these issues worse.
How Much Does It Cost To Replace Glow Plugs?
The average cost for a Glow Plug Replacement is between $242 and $296. Labor costs are estimated between $142 and $180 while parts are priced between $100 and $117. This range does not include taxes and fees and does not factor in your unique location.
How long will glow plugs last? Glow plugs normally last for around 80,000 to 100,000 miles – although this can depend on make, quality, and driving style.
So, if you are wondering how long your new glow plugs will last, rest assured that they will be reliable for a while.
How Is A Glow Plug Replaced?
Locations for glow plugs are mostly simple to access, and replacement requires that the battery is disconnected, the glow plug is disconnected from an electrical connection, and unscrewed like a spark plug.
The threads in the cylinder head are inspected, cleaned, and, if specified, coated with an anti-seize compound. The old glow plug will be inspected for signs of the engine running poorly. The new glow plug is screwed in, then its connector is plugged in.
After the battery is reconnected, the glow plugs should be tested on a cold start, preferably after several hours of sitting.
FAQs.
What does a glow plug do?
A glow plug is a heating element that heats incoming fuel and air to encourage efficient fuel combustion in a diesel engine. The fuel combustion starts your engine so that you can drive.
What happens when a glow plug goes bad?
If one or more glow plugs have burned out, are carbon fouled or damaged, the engine will become progressively harder to start as temperatures drop and will idle roughly and produce white smoke from the exhaust for several minutes once it finally starts.
Will a diesel start without glow plugs?
No, glow plugs are needed to provide the initial heat for the diesel engine to start on a cold day. Other than this, they do not have any other function. Without them also they will start, if the temperature is warm outside. This is the indication of the diesel engine condition.
Can I drive with faulty glow plugs?
No, it is not safe to drive with a faulty glow plug. An improper combustion chamber temperature will lead to inadequate fuel ignition, resulting in the misfiring or rough running of your engine. This could potentially put you in danger if you are driving at high speeds or in hazardous conditions.
Is changing glow plugs a big job?
Cars may have become more complex over the years, yet changing your vehicle’s glow plugs is, in most cases, still a job that you can handle yourself. However, before jumping into the job, you want to make sure you have a Manual Torque Wrench, and the right glow plugs for your vehicle.