What are Starting Fluids?
Generally speaking, a starting fluid is a volatile, highly flammable compound typically packaged in aerosol form that is sprayed into the air filter or intake of an engine in order to aid in starting under certain conditions. Diethyl ether is the most common fluid used as it atomizes rapidly and combusts easily, thus the term "ether" has become an umbrella term for starting fluids. Starting fluids/ether are typically used in cold weather on engines that are difficult to start or lack a choke system and thus struggle to start initially. It can also be used as a diagnostic tool to confirm ignition system function in spark-ignition engines, although this practice is not generally favored to more standardized troubleshooting techniques.
Can Starting Fluids be Used in Diesel Engines?
Starting Fluids & Ether in Compression Ignition Engines
Ether, or any starting fluid for that matter, should never be used in a diesel engine unless explicitly indicated as acceptable by the engine manufacturer. There's a variety of commercially available products advertised as universally safe for diesel engines; always consult your owners manual and/or diesel supplement guide for your vehicle to confirm the engine manufacturer's recommendations.
The sticker attached to the air box of a 1996 Dodge Ram clearly indicates the dangers of using starting fluid
Recall that the diesel cycle relies on fuel being introduced into the combustion chamber at the very moment that combustion is desired; fuel is not compressed in the cylinder as it is in other forms of internal combustion engines. Additionally, diesel engines utilize a relatively high compression ratio which ensures the air temperature is adequate enough to permit auto-ignition at the precise moment that an injection event occurs. There is therefore no way to control the timing of combustion of any fuel contained in the cylinder as the piston travels from BDC to TDC and auto-ignition will occur at the point that the air temperature inside the cylinder reaches the fuel’s flash point (note that although this is an oversimplified statement that negates other variables, it forms a basis for the thermodynamic activities inside the combustion chamber). Furthermore, the flash point of diethyl ether is slightly lower than that of diesel fuel, making it easier to ignite.
In a 4 cycle diesel engine, air is drawn into the cylinder during the intake stroke and compressed during the compression stroke. The compression stroke starts with the piston located at bottom dead center (BDC), its lower limit of travel. As the piston travels upwards the volume of the cylinder decreases while the temperature and pressure of the air charge increases proportionally. By the time the piston reaches top dead center (TDC), its upper limit of travel, the air temperature exceeds that of the auto-ignition temperature of diesel fuel. Thus, fuel injected into the cylinder at this time spontaneously ignites and the piston begins to travel downwards into the power stroke. Note that there is no spark plug or ignition mechanism as there is in a gasoline engine.
By way of design, a diesel engine therefore has no control over the combustion of any fuel being compressed with its air charge. As soon as the mixture reaches its flash point, it will ignite spontaneously. In some instances, the mixture may not ignite until diesel fuel is injected into the cylinder and obviously no damage will be incurred. However, should the mixture ignite before the end of the compression stroke, the piston and related gear train components will have to absorb the preemptive combustion event, which is known as detonation. Such conditions can result in catastrophic engine failures – broken pistons, wrist pins, connecting rods, cracked engine blocks, blown head gaskets, etc.
Starting Fluid/Ether in Diesel Engines with Glow Plugs
Ether/starting fluid is also not compatible with any diesel engine that features glow plugs or any form of intake heater (grid heaters, intake air heaters, etc). The glowing red hot tip of a glow plug provides more than enough heat to ignite starting fluid mixtures. As in the aforementioned case, there is no way to control the combustion of this fuel as it enters the cylinder and detonation is likely to occur.
Modern diesel engines are designed for all weather operation, and thus it should NEVER be necessary to use starting fluid in the first place. If an engine will not start, there is an obvious malfunction or fault in one or more systems. In engines that are so equipped, a properly functioning glow plug system is necessary for easy starting in cold conditions. Fuel system maintenance is also important for starting ease in cold weather; low fuel pressure may hinder combustion in cold conditions due to poor atomization. Most, if not all diesel engines are equipped with block heater provisions, which allow the engine oil and/or engine coolant to be warmed prior to starting. These should be used to your advantage as they not only promote easier cold starts, but also protect your engine by reducing wet stacking and fuel dilution during the engine’s warm-up cycle.
Exceptions - When Starting Fluid is Acceptable in Diesel Engines
A starting fluid can be used in a diesel engine when, and only when the manufacturer explicitly states that it is safe to use. You'll find that the only engines in which starting fluid is deemed acceptable (or is even required as a cold starting aid) are those produced scores ago. These engines do not utilize glow plugs and have relatively low compression ratios, thus the mechanics described above do not necessary apply in their entirety. The practice of using starting fluids to start a diesel engine in cold weather was abandoned decades ago, and modern engines are highly likely to be damaged due to detonation.
Put down the starting fluid and don’t ignore the underlying reasons why an engine is difficult to start in cold weather. Always use the engine block heater per the manufacturers recommendations and properly service/repair your engine for easy starting in any temperature. Starting fluid is strictly prohibited in all Power Stroke, 5.9L/6.7L Cummins, 6.6L Duramax, International IDI, and 6.2L/6.5L GM/Detroit diesel engines as indicated in their respective owner’s manuals.