Pushing starting a diesel engine can be possible in some situations, but it depends on several factors. Diesel engines require compression heat to start, which is generated when the pistons compress the air in the cylinders. This compression allows the diesel fuel to ignite when injected. When push starting, the engine must be rotated fast enough by manually pushing the vehicle to build up sufficient compression heat for ignition. However, there are some key considerations.
The Basic Process of Push Starting a Diesel
Here is the basic process for push starting a diesel vehicle:
- Make sure the parking brake is disengaged and the transmission is in neutral.
- Sit in the driver’s seat and steer the vehicle while it is being pushed.
- Push the vehicle up to at least 5 mph (8 km/h) to achieve enough engine speed for compression.
- As the vehicle is being pushed and the engine speed increases, turn the key to the start position (do not hold it there) which will activate the starter motor momentarily.
- Once started, quickly put the transmission into gear as the engine will immediately begin slowing down when not under load.
- If the engine doesn’t start after a few attempts, you likely need a faster push speed or there may be other issues preventing starting.
Factors That Affect Push Starting Viability
There are several important factors that determine if push starting a diesel engine is possible:
- Engine compression: Diesels require high compression ratios to generate enough heat for ignition. Older or high-mileage engines may have worn piston rings or leaking valves that reduce compression, making push starting difficult.
- Engine size: Large diesel engines, such as those found in trucks, require a great deal of torque to rotate the crankshaft fast enough. Pushing a heavy truck fast enough can be extremely difficult.
- Vehicle weight: Heavier vehicles like loaded trucks are much harder to get rolling fast enough to start the engine.
- Transmission type: Manual transmissions can be push started more easily than automatics. Automatics have torque converters that can prevent the engine speed from increasing enough.
- Battery condition: The battery must be adequate to activate the starter when the key is turned during push starting. Weak batteries may not engage the starter for long enough.
- Cold weather: Extremely cold temperatures make ignition more difficult. The engine needs higher compression and starter speed to start in very cold climates.
- Fuel filters: Clogged fuel filters can restrict fuel flow, causing hard starting. It’s important to change filters per the maintenance schedule.
- Mechanical issues: Problems like low compression, faulty injectors, bad sensors or other issues can prevent starting normally and may also interfere with push starting.
When Push Starting is Not Recommended
There are some situations when push starting a diesel is not recommended or may cause damage:
- Trying to push start an engine that has already overheated. This can cause further engine damage.
- Attempting to push start an engine with known mechanical problems. This risks exacerbating damage.
- Push starting engines with computer-controlled fuel injection. The injection timing can be thrown off while push starting damaging the engine.
- Trying to push start a diesel in cold weather without glow plug pre-heating. This can cause cylinder or piston damage.
- Pushing automatic transmission vehicles too fast in neutral. Excessive speeds can damage the transmission.
- Repeated failed attempts at push starting. This indicates other problems needing to be addressed first.
In general, push starting diesel engines should be avoided unless necessary due to an emergency situation. Diagnosing and repairing the root cause of starting difficulties is recommended over resorting to push starting.
Tips for Increasing Push Start Success
If you must attempt to push start a diesel, here are some tips to improve your chances of success:
- Check glow plug operation and allow time for pre-heating before attempting to start.
- Add diesel fuel injector cleaner or other fuel treatments to help ignition.
- Use the vehicle’s momentum downhill to pick up as much speed as possible while pushing.
- Have the largest, strongest person available push the vehicle if manual force is used.
- Check the battery and jumper cables are in good condition to properly energize the starter.
- Make sure the engine is free of obstructions and rotating smoothly before attempting to start.
- Consider using another vehicle to push or pull the disabled vehicle to start. This can make building momentum easier compared to manual pushing.
When to Avoid Push Starting a Diesel
There are some situations when you should avoid attempting to push start a diesel vehicle entirely. This includes:
- The check engine light is on indicating possible engine computer or sensor issues. Push starting could worsen these.
- The starter does not engage at all when the key is turned. The starter must operate for push starting success.
- There are known major engine issues such as very low compression or injector problems. Push starting with these issues risks significant engine damage.
- There is no safe location to get the vehicle up to speed for starting. Pushing along busy roads is extremely dangerous.
- Inclement weather makes pushing slippery and unsafe such as rain, ice or snow conditions.
Rather than risking injury or engine damage, consider calling for a tow or roadside assistance if push starting seems unlikely to succeed or cannot be done safely.
How Fast Does a Diesel Need to Be Pushed to Start?
The minimum speed required to push start a diesel engine depends on the size of the engine and compression ratio. Some general speed guidelines include:
- Smaller diesel car engines – 5-10 mph (8-16 km/h)
- Full-sized diesel pickup trucks – 10-15 mph (16-24 km/h)
- Large diesel engines (semis, generators, etc) – 15-20+ mph (24-32+ km/h)
The key is to rotate the crankshaft fast enough to build adequate compression heat for ignition when the starter is engaged. This requires getting the vehicle rolling at a brisk pace before disengaging the clutch on manual models or shifting to neutral on automatics.
Can You Push Start an Automatic Diesel?
Push starting an automatic diesel vehicle is more challenging but still possible in some conditions. The issues with automatic transmissions include:
- Torque converters allow the engine to freewheel when in neutral, making building speed more difficult.
- Automatics have more parasitic drivetrain losses slowing momentum.
- Transmissions may prevent starting the engine if shift lever is not in ‘Park’ on some models.
However, with a large enough vehicle for momentum and some special techniques, automatic diesels can be push started. This may include:
- Releasing the parking brake while the transmission is in ‘Neutral’ during pushing.
- Lightly revving the starter without engaging the transmission when up to speed to prime the fuel system.
- Putting transmission back into gear once the engine catches and maintains speed.
While not ideal, with sufficient speed and starter engagement, the engine can start compression ignition even with the torque converter slipping. But additional speed and force will definitely be required compared to a manual transmission.
Push starting a diesel is possible in some situations when compression and starter speed are adequate. However, it depends greatly on engine size, vehicle weight, transmission type, battery condition and other factors. Smaller, lighter diesels are most viable, but even larger vehicles can be push started with enough speed and force. Following proper precautions and techniques will improve success if a push start is absolutely necessary. But avoiding damage or safety risks should always be the priority over resorting to push starting a stalled diesel vehicle.