If the gasoline engine does not start, it can be very difficult to find the cause. Although often simple breakdowns occur, which any non-specialist can fix. To simplify repairs, it is desirable to understand what a gasoline engine needs to start successfully. Knowing these conditions, the problem can be searched for by the elimination method. It is much easier and more effective than acting at random. This article on the ABS website describes the most likely causes, where and how to look for and fix them.

Conditions for a successful start of a gasoline engine

There are many types of internal combustion engines that run on gasoline. The most difficult is the four-stroke injection. Therefore, in order to make the material as complete as possible, the problem will be considered on its example. If your two-stroke or carburetor engine does not start, then this material will definitely help you solve the problem.

To start a gasoline engine, you must:

  1. Pressurize the fuel rail. This is done by the fuel pump, which is activated after the key is turned to the "ignition on" position. The pump turns on for two to three seconds to pump gasoline from the fuel tank to the fuel rail, which is located directly on the engine. After turning the key to the “start” position, the pump starts to work constantly, and the starter also turns on.
  2. Intake stroke. The piston moves from TDC to BDC. The cylinder is under negative pressure. At the beginning of the stroke or a little earlier, the intake valve opens. Air from the street enters the cylinder and swirls.
  3. Compression stroke. The intake valve closes and the piston moves from BDC to TDC. The air in the cylinder is compressed, heated (due to pressure).
  4. Fuel injection. Occurs at the end of the compression stroke. The fuel is atomized and fed by a nozzle either directly into the combustion chamber or into the pre-chamber. Once in the cylinder, the fuel mist is intensively mixed with air.
  5. Ignition. Before the piston passes TDC, a spark jumps between the electrodes of the spark plug. The mixture ignites.
  6. Working move. The ignited air-fuel mixture burns intensively, releasing a huge amount of thermal energy. Due to this, high pressure arises, which “presses” on the piston, “forcing” it to move from TDC to BDC, and perform useful work (rotate the crankshaft, which drives the rest of the vehicle).
  7. Release of the fulfilled gases. The piston by inertia after the working stroke begins to move from BDC to TDC. At the very beginning of the stroke (or a little earlier), the exhaust valve opens, through which the combustion products are pushed out by the piston into the exhaust system and further into the atmosphere.
Operating cycle of a four-stroke gasoline engine
Operating cycle of a four-stroke gasoline engine

Carbureted engines. There are no nozzles, and the finished air-fuel mixture enters the cylinders from the carburetor. This is a complex and high-precision device in which gasoline and air are mixed in exact proportions. The carburetor also doses the amount of air-fuel mixture that enters the cylinders on the intake strokes. In injection engines, the air damper and the electronics that control the operation of the nozzles are responsible for this.

Why can't a gasoline engine start?

All the minor reasons due to which the gasoline engine may not start can be conditionally divided into four groups:

  1. Failures in the ignition system.
  2. Problems in the power system.
  3. Incorrect air supply.
  4. Insufficient compression.

The search for a specific reason due to which the gasoline engine does not want to start is best done in this sequence.

Ignition system

The ignition system is the only way that gasoline engines are globally different from diesel engines. Its task is to generate a spark in the combustion chamber in a timely manner so that the prepared air-fuel mixture ignites.

Gasoline engine ignition system
Gasoline engine ignition system

The main elements of the ignition system:

  1. Spark plug.
  2. High voltage wires.
  3. Ignition coil.

If the gasoline engine does not start, then you should always start with these elements.

Spark plug

Spark plugs may not work, or give the "wrong" spark. For example, it can be weak, untimely, slipping in the wrong place. In order for the spark to be of high quality and timely, the spark plugs must be in good condition, and the system must be properly configured. In addition, candles must be clean and dry. The electrodes must be intact, and between them the correct gap. Only under such conditions will electric discharges jump along the shortest path and give a powerful spark.

Gasoline engine spark plugs
Gasoline engine spark plugs

Unscrew and inspect the spark plugs visually. Melted or otherwise damaged, replace. Dirty - clean. Wet - dry. Clearances - if possible, check and correct. Check the performance of the spark plugs. To do this, connect the removed spark plug to a high-voltage wire and attach it with a thread to the metal surface of the engine. Scroll the crankshaft. If the spark is clearly visible, and slips gently between the electrodes, it is working.

High voltage wires

They connect the coil to the spark plugs. May be torn or broken. Not subject to repair. How to check? Take a known good spark plug and repeat the test above. If there is a spark, the wire is working. Similarly, check all high voltage wires.

High-voltage wires of the ignition system of a gasoline engine
High-voltage wires of the ignition system of a gasoline engine

Attention! Candles can be interchanged, but high-voltage wires should never be mixed up. Therefore, if you disconnected them for inspection or replacement, mark the ignition coil outlets in accordance with the cylinder numbers.

Ignition coil

Responsible for the timely supply of high voltage to the candles. Most often it is not repaired. The need for replacement is usually indicated by the fact that obviously good spark plugs and wires do not spark.

Gasoline engine ignition coil
Gasoline engine ignition coil

Fuel system

An air-fuel mixture must be supplied to the cylinders of a gasoline engine. Quality. On time and in the right amount. For this, the following conditions must be met:

  1. There is gasoline in the gas tank. It is checked elementary - according to the pointer on the instrument panel or visually (if possible). If you are not sure that there is gasoline, refuel a little.
  2. The pump is pumping. In injection engines, the pump should turn on briefly when the ignition key is turned. It can be heard. If there is no sound, check the relay, the pump itself, as well as automatic systems that turn off the pump forcibly in case of an accident.
  3. Gasoline flows freely through the fuel filters. Make sure the filters are clean and free of water and ice. Change filters regularly.
  4. Air enters the cylinders. This item is considered separately.
  5. Fuel in the required volume is supplied by the nozzle into the combustion chamber. Carburetor engines are considered separately. As for the injectors, you can check them at a special stand.
  6. It sprays well. It also depends on the injectors. Their prevention must be performed every 20,000 kilometers.
  7. The injection is on time. The electronic control unit is responsible for this. Problems can be found by reading errors with special diagnostic equipment.
Fuel injection system of a gasoline engine
Fuel injection system of a gasoline engine

Check all items carefully.


If a gasoline engine with a carburetor does not start, then here are its main malfunctions:

  • The carburetor fills with fuel. The float system does not work.
  • Fuel does not enter the mixing chamber. Fuel jets clogged.
  • Too rich air-fuel mixture. Too much fuel and not enough air.
  • Lean air-fuel mixture. Here, on the contrary, there is little fuel, but a lot of air.

To repair the carburetor, it is usually enough to thoroughly rinse it and blow it with air.

Incorrect air supply

To start a gasoline engine, air must be supplied to the cylinders. In the correct ratio to fuel. The main problem due to which there is not enough air is a dirty air filter. To eliminate this cause, try starting the engine without a filter. If the attempt is successful, clean or replace the filter element.

Air purification filter
Air purification filter

Compression in cylinders

Compression is not as important for starting a gasoline engine as it is for a diesel engine. However, this indicator also affects. Especially when the engine is cold. Use a compression gauge to check compression. At what compression does a gasoline engine not start? As practice shows, if the indicators are below the norm by 1...5 units, then the engine should start. If the compression is two times lower than normal (or more), the engine will not work.

The main reasons for low compression:

  1. Worn or broken pistons.
  2. Worn or broken compression rings.
  3. Scratches and other damage on cylinder liners.
  4. Engine head gasket burnt out.
  5. Deformation or destruction of the engine head.
  6. Worn or broken valves.
  7. Critical wear of the connecting rod bearings.
Checking compression in the cylinders of a gasoline engine
Checking compression in the cylinders of a gasoline engine

In total, if the gasoline engine does not start, it is necessary to act according to the following plan: ignition; fuel; air; compression.

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