Diesel engine turning over but not starting

Imagine this: you turn the ignition key or press the start button in your car, you hear the familiar cranking, and then . . . that's it. The cranking continues, but the usual sound of the engine roaring to life doesn't happen. If this happens to you, chances are it's not the starter. Instead, it's likely because your engine isn't getting at least one of the four major components it needs to start: fuel, air, spark, or compression. On this page, we'll take a look at all four of these things, as well as these things could cause the engine not to start.

4. Air

For combustion to take place, there needs to be a precise mixture of fuel and air. While it's not incredibly common, a lack of air may be causing the engine not to start. This could be due to an overly clogged engine air filter, which can often be fixed simply by replacing it. There are some other possibilities, as well, including a vacuum leak.

3. Fuel

Much more likely than the engine not getting enough air is the engine not getting enough fuel. This may mean that there's not any fuel in the tank, in which case filling it up again may do the trick. However, if the lack of fuel is due to a leaky or damaged gas tank, you should instead have the vehicle towed. But an empty tank isn't the only reason an engine might not be getting the fuel it needs. The fuel filter may be clogged, or the fuel pump might have sustained some damage. In these cases, the fuel wouldn't be able to get from the tank to the combustion chambers. Or, you may have clogged fuel injectors.

2. Spark

If your car has a gas engine, a spark is what causes the fuel and air mixture to combust. This spark is, fittingly, provided by spark plugs. When a spark plug fires, a small spark is created in the gap between the electrodes. Each spark plug fires at least hundreds of times a minute, so a plug that isn't working is a big problem. If your engine isn't getting the spark it needs, it could be that one or more spark plugs need to be changed. As time goes by, the space between the electrodes may become imprecise, making it necessary for the plug to be replaced. However, there are problems other than the spark plugs themselves that could cause them not to fire, so components like the distributor may be to blame.

1. Compression

Even though gas engines don't use compression to actually ignite the fuel like diesel engines do, compression is still a vital part of the process. If one of the cylinders in the engine has low compression, the engine may not be able to start up. This could be caused by a loose timing chain, bad head gasket, stuck valve, or bad piston rings. Whatever the cause, it's best to have it looked at by an expert.

Anyone who owns a vehicle has likely experienced the frustrating problem of a car that cranks but doesn’t start, even after repeatedly turning the key in the ignition.

However, don’t let desperation keep you from logically figuring out why your vehicle cranks but won’t start normally.

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Related: 4 Reasons a Starter Spins But Doesn’t Engage

Table of Contents

• Reasons a Car Cranks But Won’t Turn Over
  • #1 – Spark Problems
  • #2 – Lack of Fuel Flow
  • #3 – Low compression
  • #4 – Power Supply Problems
• Troubleshooting Advice

Reasons a Car Cranks But Won’t Turn Over

Cranking the car is engaging the starter to energize the engine. The starter causes the flywheel to turn which rotates the crankshaft when all is working correctly. Sometimes this process is interrupted when there is a hitch in the system, and the car engine won’t continue to run after it “turns over” or cranks. 

For the engine to start normally, it requires sufficient fuel pressure, an appropriately timed spark and normal compression. When it doesn’t start, the problem usually lies with one of these systems though the starter system can also be the culprit. Below are some common causes of an engine that cranks but doesn’t start and some troubleshooting tips to identify the cause.

See Also: What To Do If Your Car Shuts Off While Driving

#1 – Spark Problems

Lack of spark may occur due to a damaged ignition module, a faulty crank position sensor, a flooded engine (sometimes happens in older or high-mileage cars), bad spark plugs, or a problem in the ignition circuit, such as the wiring, security system (the fuel flow may have been shut off to prevent theft or the chip in the key could be faulty), or a malfunctioning ignition switch.

An inappropriately timed spark can occur if there’s an issue with the timing system. This can be difficult to diagnose, but a timing light is a useful tool to check that all the cylinders are firing exactly when they are supposed to.

To determine if there is a problem with the spark, visually check the distributor cap (if your vehicle has one) and the spark plug wires, as these may degrade with age. A spark tester should be used to check for a proper arc from each spark plug wire or coil.

If you suspect the engine may be flooded after repeated attempts to start the car, remove the spark plugs and let them dry, then replace them and try again.

Related: Symptoms of a Hydrolocked Engine

#2 – Lack of Fuel Flow

Problems with fuel flow may be due to a damaged fuse for the fuel pump, a bad fuel pump, contaminated or incorrect fuel in the tank, a faulty or clogged fuel filter or injector, or simply an empty fuel tank (the fuel gauge is not always accurate). 

Having the appropriate fuel pressure is important for your car engine to start or run, especially for fuel-injected engines. Listen to hear the fuel pump buzz for a few seconds as you turn the ignition to the “on” position.

If no buzz is heard from inside the car or back by the fuel tank, the pump may be dead and fuel is not reaching the engine at all.

Note that some fuel pumps only flow while the car is cranking so some cars don’t have an audible buzz. Consult the internet or your owner’s manual for more information for your specific model.

If you do hear the buzz of the fuel pump, you can try putting a flathead screwdriver on top of each injector (with the handle next to your ear) while the car is cranking. If the injectors are firing, you will hear a faint ticking sound from each injector, transmitted by the shaft of the screwdriver.

Some cars have a safety feature called an inertia switch which shuts off the flow of fuel automatically after an impact. If your vehicle recently sustained an impact, check your owner’s manual to see if this feature is present in your vehicle and learn how to manually switch it to allow fuel to flow again.

Related: What to Do if Your Car Breaks Down

#3 – Low compression

Each cylinder needs compression for correct engine function. The compression ratio compares the maximum cylinder volume to the minimum cylinder volume during each stroke of the piston. If one or more cylinders have low compression, air from the combustion cycle leaks past the piston rings which limits the amount of work that cylinder can do to spin the crankshaft.

Compression problems can be caused by a broken or loose timing belt or chain or a snapped overhead camshaft. An overheated engine is another severe problem that can prevent your car from starting. 

Try using a compression gauge or tester to see if you have a problem with the compression in your car. If so, a leak down test is a secondary test to check for leaks in the cylinder. A professional mechanic can perform these tests and examine the cylinders if you don’t feel comfortable checking yourself.

#4 – Power Supply Problems

Another possible problem is a weak starter motor that uses a lot of amps to crank the engine, then doesn’t have much juice left to switch on the fuel injectors and ignition system. In this case, you will probably notice that the starter makes an unusual noise when you attempt to crank the engine or it doesn’t turn over at all. 

Weak or corroded battery cables or a dying battery can contribute to the problem as well. Check the voltage of the battery with a multimeter while cranking the engine. It should show over 10 volts. 

Check for blown fuses by visually removing and inspecting the wiring of each fuse when the car is shut off. If they appear to be in good condition, put them back in then try turning the car ignition into the “on” position and using a test light to check each fuse for electrical current flow. Replace any damaged fuses with new ones from an auto supply store.

Related: 7 Reasons a Car Starts Then Immediately Dies

Troubleshooting Advice

If the engine cranks but won’t start, turn the car off and remove the air inlet tube attached to the throttle body. Then spray a small quantity of starting fluid into the engine after gently pushing the throttle open. With that done, try cranking the engine once again. 

If the engine starts but dies after a few seconds, this means that it has no fuel but the spark and compression are okay. However, if the engine does not start, it almost certainly lacks spark.

Avoid repeatedly cranking the car engine to try to make it start, as this can wear out the starter or drain the battery.

If you must try multiple times, wait several minutes for every 15 seconds of cranking performed to allow the starter to cool down. It shouldn’t take more than a couple seconds per attempt to know if you’ve resolved the issue.

Checking sensors and actuators for problems is critical, as modern cars have a variety of electrical components that can cause failure in the engine starting process.

The best way to do this is to check the car computer for codes (faults in the electrical system) with a scan tool that can be found at most auto supply stores. Most of these problems will also cause the check engine light to illuminate, but not all of them. 

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