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When a diesel engine does not start, there may actually be not so many reasons. There are only 7 main and most common ones. The more complex the engine is, the more malfunctions due to which these causes arise. The simplest example is a single-cylinder diesel walk-behind tractor - there can be only one and a half times more malfunctions than causes. Knowing them, you are 90% able to find, fix the problem, and successfully start the engine. The remaining 10% fall on the specifics of a particular engine, and therefore the likelihood of encountering them is not very high.

Even if you consider yourself not a beginner, and are well versed in diesel engines, do not be lazy to periodically return to the basics. Very often, when a problem arises, the brain flatly refuses to recognize elementary things, which can cause you to lose sight of the simplest malfunction. Therefore, if you can’t start a diesel engine in any way, brush up on the basics. This often helps.

Conditions for a successful start of a diesel engine

Single cylinder, four stroke diesel engine
Single cylinder, four stroke diesel engine

First, let's refresh our memory of what happens inside a diesel engine when we try to start it:

  1. Air inlet.
  2. Compression and injection.
  3. Working move.
  4. Release of the fulfilled gases.

Each of these points is called a stroke, and together they are one cycle of a four-stroke diesel engine. Let's take a look at the beats in more detail.

Diesel working cycle cycles
Diesel working cycle cycles

1.      Air inlet

Air is needed to ignite and burn diesel fuel. It is sucked into the combustion chamber due to the rarefaction created by the cylinder-piston group.

Hereinafter, CPG is a cylinder-piston group, which includes a cylinder (or cylinder liner), a piston and piston rings.

On the first stroke, the piston moves down, creating a vacuum in the cylinder. When he just starts to do this, the intake valve should open. Air, passing through the filter, is sucked into the cylinder, where it swirls intensively. The latter is necessary so that the diesel fuel injected later is immediately mixed with it, and an air-fuel mixture suitable for ignition and combustion is obtained.

The intake stroke ends when the piston reaches bottom dead center (BDC). At the same time, the intake valve should close completely.

2.      Compression and injection

Having passed after the first cycle BDC, the piston moves up. Since both valves must be closed, the previously drawn in air has nowhere to go but to be compressed. This leads to a sharp jump in pressure, due to which the temperature in the combustion chamber rises rapidly. The ignition temperature of the diesel-air-fuel mixture is 800°C . Accordingly, the combustion chamber must develop a pressure sufficient for such heating.

When the piston reaches TDC, the injector should fire. Diesel fuel flies out of it under high pressure. This should happen before the piston skips TDC, that is, with the so-called lead. This is needed for two reasons. Firstly, this way the nozzle will have time to do most of its work before the start of the next cycle. Secondly, diesel fuel should be in the combustion chamber at a time when the air in it is maximally compressed and heated.

With injection, too, everything is not so simple. In addition to the advance, a couple more conditions must be met. The main one is that diesel fuel should be injected in the form of a finely sprayed mist, and not sluggishly pour in trickles. This is necessary so that the fuel mixes better with air and quickly turns into flammable vapors.

This completes the compression and injection stroke. If conditions are favorable, when the piston approaches TDC, the fuel mixture ignites.

3.      Working stroke

The ignited air-fuel mixture burns, forming gases. The temperature rises sharply, the volume of gases increases, due to which the pressure in the combustion chamber increases. Solar oil gave its energy. This energy, in the form of pressure, presses on the piston and pushes it down. The engine starts. We hear a characteristic sound. If we don't hear it, the ignition hasn't happened.

4.      Exhaust gas outlet

Having converted the energy of the fuel into the movement of the crankshaft, the piston passes BDC. At this point, the exhaust valve should open. The piston moves up by inertia, forcing combustion products through the exhaust port and the muffler into the atmosphere. When the piston reaches TDC, the exhaust valve should close, and not open until the start of the next exhaust stroke.

On many diesel engines, at the end of this stroke, the intake valve is already “in a hurry” to open. This allows you to "ventilate" the combustion chamber and take in more air on the next intake stroke. It is for this reason that valve adjustment should be done on the compression stroke when both valves are exactly closed.

Diesel does not start: reasons

Now, if your walk-behind tractor or just a diesel engine does not start, take the 7 reasons discussed below, and exclude them in order.

1.      Lack of air

Dirty filter causes lack of air
Dirty filter causes lack of air

In order for diesel fuel to ignite and burn out qualitatively, giving up all its energy, more than just air is needed. It should be enough. Air in an elementary diesel enters the combustion chamber through an air filter. Accordingly, if it is clogged with dust, problems arise. Most often, due to a clogged air filter, at first there are problems not with starting the engine, but with its power. He doesn't develop it. It takes less air to start than to run under load. Therefore, in order for the diesel not to start due to this reason, the filter must be literally clogged.

To eliminate this cause, it is enough to temporarily remove the air filter from the engine. If the problem went away without it, and the diesel engine started up, replace, clean, blow, rinse the filter element. If the engine still does not start, do not rush to install the filter in its place. Perhaps you have several problems. The only exceptions can be cases if you installed a completely new filter element, or cleaned the old one so that you are sure of its throughput.

You can remove the filter and try to start the diesel engine only when dust and other debris have no chance to enter the engine. If it's windy outside or the garage is drafty, dust will rise from the floor and act as a harsh abrasive on the intake valve and cylinder liner.

2.      CPG does not give compression

Having eliminated the problem with the start cycle, we move on to compression. The most important thing here (but not the only one) is the serviceability of the cylinder-piston group. If the cylinder, piston and compression rings are worn or damaged, the inlet air will not be able to compress and heat up to the required condition. By the way, this is also affected by the speed at which the piston moves on the compression stroke. If it is not enough, the air, even with a fully serviceable engine, will have time to bleed through the gaps between the piston with rings and the cylinder (mainly through the gaps of the rings).

Looks nice, but the ring gaps...
Looks nice, but the ring gaps...

There are several ways to eliminate this cause. The most faithful and reliable is the measurement of compression using a compression gauge. If we consider as an example the simplest diesel walk-behind tractor, then its engine should have a compression of 20 to 28 atmospheric parrots. If the compression gauge shows less than 18, the diesel engine can start only by a miracle. In other engines, the working compression may vary slightly, but on average for diesel engines, you can focus on these numbers.

The second way to determine the compression of a motoblock diesel engine is very inaccurate, but it allows you to navigate if there is no compression gauge at hand. It is performed as follows. First, the engine oil filler plug is turned out to eliminate the force that “holds” the piston when moving up due to the vacuum created in the crankcase.

Next, the engine is slowly turned by hand with the decompressor closed, and its resistance is estimated. If we talk about walk-behind tractors, then with good compression it is quite difficult to turn it smoothly on the compression stroke. This can be done only when the handle is very strongly and for a long time, giving time for the air from the combustion chamber to leak through the gaps of the rings into the crankcase. If your compression stroke skips with little resistance, there is clearly a problem with compression. You need a compression gauge.

To restore compression, if the CPG is to blame, depending on the degree of wear (or breakdown), you will have to:

  1. Replace rings.
  2. Replace piston.
  3. Ream the sleeve and install the repair piston kit.

Sometimes it is enough to replace the rings, however, most often you have to change them along with the piston, and with heavy wear, the third option is inevitable.

3.      The valves do not hold the compression created by the CPG

Simultaneously with working out the previous reason, it is important to remember that in a diesel engine, compression depends not only on the CPG, but also on the valves. Therefore, without having worked out this reason, do not rush to climb into the piston. The task of the valves at the time of starting the diesel engine is very simple - they must be completely closed and the air compressed by the piston must not leak through them.

Intake stroke - on compression and power stroke, both valves are closed
Intake stroke - on compression and power stroke, both valves are closed

There are five main malfunctions due to which the valves do not cope with their work:

  1. Clearances incorrectly adjusted. Especially if they are small or absent altogether. In such cases, on the compression stroke, the rocker arms press on the valves, due to which they remain slightly ajar. Adjust gaps. For motoblock engines, instructions with detailed explanations are available on the Auto without Service Station website.
  2. Valves are not lapped. Valves with their working chamfer must fit precisely against the working chamfers of the cylinder head. If there are scratches, bumps, soot, etc., the compression will run either to the air filter or to the muffler. Lapping of not very worn valves takes just a few minutes, and the effect of it is sometimes colossal.
  3. Bushings worn out. The “legs” of the valves move back and forth in the bushings. When they wear out, or the guides of the valves themselves, a slight misalignment may occur, due to which the working chamfers of the valves and the cylinder heads do not line up when closed. Especially often this problem is observed on the intake valve, as it is subject to rapid wear due to the poor quality of the air filter.
  4. Weakened springs. Valves are pressed when closed by springs. If they are weakened, then the closure is not sharp and tight enough.
  5. Burnout. Burnt valves are usually visible to the naked eye, but you can only look at them after removing the cylinder head.

In total, compression problems can occur both due to the CPG and due to valves, that do not hold the air compressed by the piston.

4.      Delayed fuel injection

Injection timing is very important for a diesel
Injection timing is very important for a diesel

Fuel injection is timely, early or late. In order for a diesel engine to start normally and deliver its rated power, diesel fuel must be supplied to the combustion chamber with the so-called advance. That is, before the piston reaches TDC on the compression stroke. For different engines, the injection timing is different, but on average it is in the range from 13° to 22°.

A diesel engine can successfully start and work somehow even with a later or earlier injection than it should be. For example, the engine of a walk-behind tractor could be operated quite tolerably when the injection was advanced by 32°. And this, by the way, is a third of the piston stroke on the compression stroke.

If the injection is too early, then there can be two problems. Firstly, when starting, diesel fuel will ignite ahead of time, and you (or the starter) will not be able to overpower the sharply increasing resistance of expanding gases. Often this effect is felt on the handle in the form of a kickback. Too late injection is also bad, and does not favor the successful launch of a diesel engine. The very first sign of this malfunction is black smoke from the exhaust pipe and a pungent smell of smoldering (and not burning normally) diesel fuel.

Blot the paper with diesel fuel and set it on fire with a match, observing fire safety measures. Assess the color and smell of the smoke. If your exhaust is the same, then most likely the injection is too late. It is important to understand that the cause of black smelly smoke can be not only at the moment of injection.

5.      Poor fuel injection

This is what a good shot looks like
This is what a good shot looks like

High-quality diesel fuel injection is when diesel fuel is sprayed by a nozzle from all provided holes in the form of short-term sharp fog torches. For this to happen, both the nozzle itself and the injection pump (high pressure fuel pump) must be in good working order. Diagnostics of fuel equipment is a whole science, and it will not be considered within the framework of this material.

Since we are considering problems with starting a diesel engine using a walk-behind tractor as an example, here is an elementary way for you to assess the quality of injection. Remove the injector from the engine and connect it to the injection pump "outside". Lock the decompressor, open the fuel cock, move the engine stop knob to the start position, and turn on the “throttle to the fullest”. Crank the engine over slowly with a manual starter (not electric) and watch the injector fire. If she quickly sprays the fog with a characteristic clacking sound, this is the norm. If diesel fuel flows in streams and drips, the nozzle does not do its job.

If you perform this test using a starter, you may get a false opinion about the performance of a faulty injector. You will see fog it can only be created by high pressure fuel pumps.

6.      Poor fuel quality

Bad diesel fuel = difficult diesel start
Bad diesel fuel = difficult diesel start

Despite the fact that diesel engines are less demanding on fuel than gasoline engines, frankly bad diesel fuel will not burn. Also, a frequent problem is water in the fuel, which has the ability to settle at the bottom of the gas tank and be the first to penetrate the injection pump, nozzle and combustion chamber.

To eliminate this problem, completely drain the fuel, bleed the system, fill in a known normal diesel fuel and try to start the diesel.

At the same time, check whether fuel is flowing normally into the high-pressure fuel pump. Perhaps your filter is clogged, and the engine simply does not have enough diesel fuel.

7.      Clogged exhaust

Soot in muffler
Soot in muffler

The last step of the working cycle of the diesel engine remains - the release of exhaust gases. If the exhaust system is clogged with soot, the piston will not be able to push all the smoke out of the cylinder, and it will remain in it for the next stroke. This will lead to the fact that less air will enter the cylinder during the intake stroke, which means that there will not be enough oxygen for the normal ignition of diesel fuel.

This problem can be eliminated in two steps. First, if everything is in order except for the exhaust pipe, when you try to start, you will hear detonation of the air-fuel mixture (at least one). If this sound is not present, most likely, the reason is not in the exhaust. Secondly, remove the exhaust pipe and try to start the diesel. If it starts, immediately stop it and solve the problem with the muffler.

VIDEO: diesel engine wouldn't start


If you have carefully worked out all the reasons described above, and you can confidently say that the diesel engine does not start for some other unknown reason, contact a more qualified specialist than you.