A faulty fuel pump and iron particles blocking the filter regularly cause starting issues at low temperatures. This combination of problems restricts the fuel supply so much that the engine will not start, or only starts with great difficulty. The solution is to replace both the fuel pump and the filter. Read on to find out how to recognise these cold-start problems and what you can do about them.
Why cold starts are extra sensitive to fuel problems
With a cold engine, combustion is less efficient than at operating temperature. The engine management computer therefore adjusts the fuel mixture to help the engine fire up more quickly. If the fuel system is not working optimally, you will notice this immediately. The engine then needs more cranking time, runs roughly or even stalls shortly after starting.
This problem occurs regularly with the Dacia Logan MCV, and other models can also be affected. The cause is often a combination of a weakened fuel pump and contamination in the fuel system.
The fuel system consists of several components that all need to work together properly. The fuel pump provides sufficient pressure, the filter blocks contaminants, and the injectors atomise the fuel finely enough for good combustion. If one of these parts fails, you will mainly notice it in cold conditions.
Recognisable symptoms of a faulty fuel pump
A poorly functioning fuel pump gives off specific signals. The engine takes a long time to start, especially after standing still overnight. Some owners report that the car starts better if the ignition is switched on briefly without starting, so that the pump can build up pressure.
Other recognisable symptoms are:
- The engine fires but stalls again immediately
- Jerky running behaviour during the first few kilometres
- Loss of power when accelerating
- A humming or hissing noise from the fuel tank
- The engine idles unevenly
These complaints are exacerbated when the fuel pressure drops after switching off the engine. This may indicate a worn pump, but also leaking injectors or a faulty non-return valve in the system.
Iron particles in the fuel filter block the flow
Besides pump failure, contamination of the fuel filter is a very common cause of starting problems. Iron particles and other dirt can gradually clog the filter medium. At low temperatures the fuel is thicker and the flow is further restricted.
The result is that the engine does not get enough fuel while starting. You will notice this from longer cranking and jerky running in the first few minutes after start-up. In some cases the engine will not start at all.
Iron particles are often released due to wear of the fuel pump itself. As the pump ages, metal components shed small fragments that enter the filter via the fuel line. This makes the problem worse: the weak pump faces even more resistance due to the clogged filter.
What to do about cold-start issues caused by the fuel system
If you suspect that the fuel pump or filter is causing problems, checking the fuel pressure is the first step. A mechanic can use a pressure gauge to measure whether the pressure is sufficient and whether it remains stable after switching off.
If the pressure drops quickly, this indicates a fault in the system. In that case, these steps are needed:
- Replace the fuel pump if it no longer delivers sufficient pressure
- Install a new fuel filter to restore flow
- Check the injectors for leakage or poor atomisation
- Test the non-return valve to rule out pressure loss
It is sensible to replace pump and filter at the same time. A new pump can release extra particles during the first hours of operation, and an old filter will no longer trap these properly.
‘ A weak battery makes any fuel problem worse, because the starter motor and fuel pump do not receive enough voltage. ’
Preventive measures to avoid starting problems
Regular maintenance significantly reduces the risk of cold-start issues. Replace the fuel filter according to the manufacturer’s service schedule, usually every two years or after 30,000 to 40,000 kilometres. Use quality fuel and fill up at reliable fuel stations to limit contamination.
Also keep an eye on the condition of the battery. A weak battery makes any fuel problem worse, because the starter motor and fuel pump do not receive enough voltage. Make sure the battery terminals are clean and firmly attached.
With diesel engines, using winter diesel in cold periods is important. Summer diesel can become thicker in freezing conditions and cause filter problems due to paraffin formation. Keep the fuel tank sufficiently filled to prevent condensation.
Additional causes of cold-start problems
Although the fuel pump and filter are often the main culprits, other components can also contribute to starting issues. In petrol engines, spark plugs and ignition coils play an important role. Worn spark plugs weaken the ignition, making it harder for a cold engine to start.
In diesel engines, glow plugs are crucial for a smooth cold start. Faulty glow plugs mean the combustion chamber is not preheated sufficiently. The engine may then only fire after several attempts or remain completely silent.
In addition, unmetered air in the intake system can disturb the fuel mixture. A split hose or leaking gasket lets in unwanted air, making the mixture too lean for a good cold start.
Diagnosis and inspection of the fuel system
For a thorough diagnosis, it is wise to use a diagnostic tool to read out any fault codes from the engine management computer. Sometimes the engine warning light stays off even though there are stored errors.
A mechanic can then carry out targeted tests. In addition to the fuel pressure, the condition of the injectors is also assessed. Leaking injectors leave fuel in the combustion chamber, which can cause both starting problems and a petrol smell.
Also check the behaviour of the fuel pump when switching on the ignition. You should hear a brief humming sound when the pump builds up pressure. If this sound is absent or weak, the pump may be defective or not receiving enough voltage.
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Frequently asked questions
The most common symptoms are having to crank for a long time before the engine fires, and the engine running roughly or stalling again immediately after starting. Jerky running and hesitation during the first few minutes are also common, sometimes accompanied by a petrol smell due to an overly rich mixture. The engine often starts better if you first switch on the ignition for a few seconds (allowing the pump to build pressure), which indicates pressure loss in the fuel system.
Worn fuel pumps, clogged fuel filters and poorly performing injectors are often the cause of difficult cold starts. A weak pump or faulty non-return valve allows the fuel pressure to fall away after the car has been standing, so the engine receives too little fuel during the first revolutions. A clogged filter restricts fuel flow, especially when the fuel is thick (cold). Poorly atomising or leaking injectors disturb the mixture, making it harder for a cold engine to start and causing it to run unevenly.
Replace the fuel filter on time and use quality fuel, avoiding running the tank almost empty as a habit. Have the fuel pressure and injectors checked regularly so that leaks or poor atomisation are detected at an early stage. Make sure routine servicing (including spark plugs/glow plugs and battery check) is carried out according to schedule, because weak ignition and low voltage make fuel problems during cold starts worse. Make a winter check part of your routine so that any pressure-loss or flow problems at low temperatures are identified in good time.
Start by checking whether the car starts better if you switch the ignition on and off 2–3 times before cranking the engine, to allow the fuel pump to build up pressure. Then check whether the fuel filter has been replaced on time and replace it if in doubt. Next, have the fuel pressure measured, both while starting and after the car has been standing still for several hours, to trace pressure loss in the pump, pressure regulator or injectors. Finally, read out the fault codes to see whether there are any signs of problems with the injectors or fuel-pressure control.
Fuel system problems can cause fuel pressure that is too low or unstable and poor atomisation, so the engine is harder to start, runs roughly and delivers less power. Because the ECU enriches the mixture with a cold engine, blockages or leaks in the fuel system are particularly noticeable in cold conditions and more quickly lead to misfiring or stalling. A clogged filter, worn pump or poorly performing injectors disturb the air–fuel mixture, resulting in incomplete combustion and reduced engine performance. All of this increases fuel consumption, because relatively more fuel is needed to achieve the same performance, especially during cold starts.