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Will a Diesel Engine Run Without Glow Plugs?

The diesel engine dates back to the 1890s, and glow plugs weren’t created until 1929. The modern glow plugs were developed in the 1970s and have evolved since then. However, even today, there are manufacturers like Cummins that don’t use glow plugs for their diesel engines (source). 

A diesel engine will run without glow plugs because it doesn’t require a continuous ignition source, unlike gasoline engines with spark plugs. However, sub-freezing conditions can affect a diesel engine’s combustion chamber temperatures, thus delaying the ignition. 

A diesel engine shouldn’t have any starting issues when the ambient temperatures are warm or hot. Mildly cooler temperatures don’t pose any problems as most diesel fuels don’t gelatinize until 15°F (-9.44°C). Read on to know if and how a diesel engine will start and run without glow plugs (source).

How Does a Diesel Run Without Glow Plugs?

Glow plugs aren’t an integral or inseparable component of a diesel engine. Unlike gasoline or petrol, a diesel is a compression engine. Gasoline engines need a spark plug to provide the ignition continuously. Thus, it’s a fundamental requirement to start and run a gasoline engine (source).

A diesel engine can run without glow plugs because it doesn’t need a spark plug. It uses the heat of the compressed air inside the combustion chamber to ignite. Diesel’s autoignition temperature is around 410°F (210°C). 

However, the temperatures required inside an engine’s combustion chamber vary depending on the type of diesel. 

Here are the autoignition temperature ranges of different types of diesel (source):

  • Diesel #1: 350 – 624°F (176.67 – 328.89°C)
  • Diesel #2: 490 – 545°F (254.44 – 285°C)
  • Diesel #4: 505°F (262.78°C)

The temperatures of compressed air inside the combustion chamber of a diesel engine with the standard 15:1 up to 20:1 compression ratios can be more than 932°F (500°C) at the piston’s top dead center position. These temperatures are more than sufficient to ignite the fuel.  

Since diesel engines have a compression ratio of 25:1, the temperatures of the air-fuel mixture in the chambers can get significantly hotter during continuous combustion. Such high temperatures can conveniently sustain a diesel engine’s combustion when you drive (source).

Diesel engines with high compression ratios can run without glow plugs, even in colder climates. Also, some diesel engines use grid or intake heaters instead of glow plugs, but frigid conditions may pose startup issues without any type of heat source to facilitate a faster ignition. However, the delayed or first ignition issue has solutions.  

Can You Drive a Diesel With a Bad Glow Plug? 

You can drive a diesel with a bad glow plug. Glow plugs assist diesel engines in starting by increasing the combustion chamber temperature for the air-fuel mixture to combust. However, the glow plugs have no role in running a diesel, unlike the spark plugs in gasoline engines. 

If you’re already driving a diesel, you may not know if a glow plug is bad or failing. Gasoline engines will stop abruptly if all the spark plugs fail at the same time. In contrast, you may discover a bad glow plug only when a diesel doesn’t start, or the initial ignition is delayed on a very cold day or night. Also, modern diesel engines using glow plugs don’t have only one. 

Therefore, you may not detect a bad glow plug until there are telltale signs, such as odd engine noises, poor performance, and other evident symptoms. A bad spark plug in a gasoline engine will stop a cylinder. Thus, the engine will suffer damage despite the other cylinders functioning optimally. Such issues aren’t relevant for diesel engines and glow plugs. 

However, an unavoidable reality remains unaddressed. How will you run a diesel if it doesn’t start in the first place without glow plugs? Also, will any diesel start and run in extremely cold climates or sub-freezing conditions? Well, there are several caveats in this context. 

Will a Diesel Start Without Glow Plugs In Extreme Cold?

A diesel engine can have a delayed first ignition or may not start without glow plugs in extreme cold. Smaller diesel engines with low compression ratios may encounter such issues more than others. Also, the engine’s condition and diesel type influence the start. 

The components of an idle engine may be too cold for the compressed air-fuel mixture to attain the autoignition temperature. Also, you may have two concurrent issues at hand. First, the freezing ambient air and the cold engine components can become a collective heat sink. 

The heat sink issue is more common in diesel engines that have been parked or unused for a while in extreme cold. Thus, the cold air intake won’t heat up as quickly as a warm setting, despite getting compressed optimally inside the combustion chamber. Also, the cold engine parts will absorb some heat from the air-fuel mixture and combustion chamber.

The second issue pertains to the type of diesel you have. Winterized diesel or additives that prevent the standard and premium fuels from gelatinizing are not as susceptible to extreme cold as the regular variants. Gelatinized diesel doesn’t ignite as readily as the fuel in its normal state. 

Also, the cloudy and gel-like diesel may not flow as smoothly into the combustion chamber. The compressed air may gradually get hotter, but the diesel may still resist auto-ignition unless the temperatures are higher. Essentially, the diesel needs to revert to its normal liquid condition. Or, the compressed air-fuel mixture inside the combustion chamber must get hotter than usual. 

These practical challenges are almost inevitable in cold climates. Thus, many of the original equipment manufacturers and brands of cars, vans, and trucks use glow plugs, grid or intake heaters, and other such solutions for their diesel engines. 

How To Start a Diesel Engine With Bad Glow Plugs

You may start a diesel engine with bad glow plugs by cranking or using a starter system, be it electric, hydraulic, or air. Also, consider a block heater, winterized diesel, or special additives. Replace any clogged filters, change the engine oil, and keep your vehicle in a warm place. 

Crank the Diesel Engine Intermittently, but Swiftly

There’s no peer-reviewed scientific theory that can work for every type of diesel engine in all kinds of cold settings. Too many variables are at play. However, an old trial and error approach is cranking the diesel engine intermittently. You may crank once and wait for a few seconds before repeating. If you’re lucky, your diesel may start in the fourth, fifth, or sixth attempt.

The relatively young may not be familiar with the archaic practice of roll-starting a car engine. Also, the remedy is a nonstarter for larger or heavier vehicles, and the sheer effort calls for many helpful souls. However, in theory, you can roll-start a diesel engine with bad glow plugs.

Consider a Block Heater for Your Diesel Engine

You can use a block heater for a few hours before you intend to start your diesel engine. The time necessary to reach the desired temperature varies based on the climate, engine condition, and diesel type. Also, the state of the engine oil, battery, and other factors, such as filters, will influence how long you need to use the block heater. 4 to 6 hours may suffice in cold climates.  

You may opt for an aftermarket block heater if your diesel vehicle doesn’t have one already. Block or coolant heaters use different ways to hook up to a diesel engine, such as the freeze plugs, radiator hose, dipstick tube, and other access points, or the transmission in some cases (source).

Get an Air, Hydraulic, or Electric Starter

Bad glow plugs and cold may create hiccups when you try to start a diesel. On the one hand, you have no heat from the glow plug to assist the initial ignition. On the other hand, the battery in your vehicle may not be in its optimum state. Thus, starting or even cranking is a challenge. 

You can use an air, hydraulic, or electric starter to crank and start a diesel, even when you have bad glow plugs. Pneumatic or air starters have better power-to-weight ratios than most electric variants. Also, you may check out recoil or manual starters if you have a small diesel engine. 

Using a starter is a practical option, especially when you don’t have enough power in the battery of your vehicle. Repeated but failed cranking is a nonstarter when you have bad glow plugs and a depleted battery. Thus, a starter can literally be your savior when nothing else works (source).

Use Winterized Diesel or Cold-Climate Fuel Additives

Standard diesel becomes more viscous as the ambient temperatures drop. In sub-freezing conditions, diesel may gelatinize to an extent. Thus, it won’t flow as readily as regular diesel. Also, an engine needs the diesel in an aerosol form to ignite and combust in the air-fuel mixture. 

Like diesel, the lubricant or oil will also be more viscous in colder temperatures. Thus, there are multiple problems to deal with, especially if you have bad glow plugs. To prevent such problems, you can use winterized diesel. Or, you may use fuel additives specifically for the winters. 

Don’t use fuel additives for winterized diesel because that’ll change the composition, and you may have issues, such as clogged filters. Technically, you need the diesel to be primed before it goes through the filters. Hence, use either winterized fuel or additives in advance (source).

Use the Right Diesel Types for Your Climate and Vehicle

As already mentioned, Diesel #1 has an autoignition temperature range of 350 – 624°F (176.67 – 328.89°C). The self or autoignition temperature range of Diesel #2 is 490 – 545°F (254.44 – 285°C). Thus, Diesel #1 will ignite and combust quickly in winters or colder climates. 

Also, Diesel #1 is the premium fuel compared to Diesel #2. Check the blend at your local gas stations. If you’re using Diesel #2, consider switching to #1. Most blends are fine in moderate climates. However, extreme cold or sub-freezing conditions demand Diesel #1, as it’s thinner, less viscous, more resistant to gelatinization, and ignites or combusts at a lower temperature (source).

Replace Clogged Filters and Change the Engine Oil

Bad glow plugs with little juice in the battery, coupled with clogged filters and gelled engine oil, will compound your problems. No matter how tactfully and skillfully you try to crank your diesel, the engine may not respond unless you replace the clogged filters and change the lubricant. 

Ideally, you should opt for winterized diesel and weatherproof your diesel vehicle. In any extreme climate, preparation is the key to avoiding inevitable problems, such as bad glow plugs. Like the spark plugs in gasoline engines, you’ll have bad glow plugs sooner or later. 

Park the Diesel in a Heated Garage or Warm Place

Glow plugs exist because diesel engines don’t start instantly in cold climates after you turn the ignition key. Thus, the crux of all these problems isn’t the glow plugs but the sub-freezing temperatures. Like us, all diesel engines need some warmth on the coldest days and nights.  

An ideal remedy is a heated garage. Alternatively, brainstorm to find a way to keep the parking space somewhat warm, unless it’s outdoors under a mountain of snow. Otherwise, you have to use one of the other solutions, like an electric, air, or hydraulic starter for your hibernating diesel.

Use a Powerful Battery or Disable All Optional Accessories

You may find yourself in the midst of a cold wave, with an almost dead engine, nearly frozen diesel, and no other remedies in sight, such as starters, new filters, fresh lubricant, or winterized fuel. Hence, cranking may be your only option. However, you need a powerful battery for that. 

Always maintain the battery and have a spare on board if you expect issues with the glow plugs and your diesel engine. Turn off all the accessories you don’t need when the priority is to start the vehicle. Crank, let the engine warm up, crank, and you may succeed in starting your diesel.

Conclusion

Some say that the intake or grid heaters are better than glow plugs. The jury is still out if one diesel engine is better than another. However, when you have to run a diesel without glow plugs in sub-freezing conditions, you need a block heater, starter, winterized diesel, or fuel additives. 

Also, inspect the engine, filters, lubricant, and battery. Keep a powerful battery well-maintained. Use block heaters to prevent the diesel from gelling. Ensure the filters are unclogged, opt for premium winterized diesel, and crank-start when it’s too cold (source).