An engine that cranks longer than usual, catches and stalls, or idles rough for the first few seconds on a cold morning is a problem we see every winter. It is important to distinguish this from a car that will not start at all, because the causes and approach are different. Here we are talking about an engine that eventually fires up, but takes too long or runs poorly until it warms up.
The causes vary between petrol and diesel engines, and there is also a specific situation with LPG-equipped cars. Let us start with the shared culprits that apply regardless of fuel type.
Common Causes Regardless of Fuel Type
The battery is the single most common reason for difficult cold starts. At sub-zero temperatures, a battery can lose 40-50% of its capacity, especially older units. The result is a slow, sluggish starter that cannot reach proper cranking speed. If you hear the starter dragging, sounding dull and weak, the battery is the first thing to check. Three to five years of service in our climate usually means replacement time, particularly if the battery barely survived the last winter. You can read more about why batteries drain while the car sits in our parasitic draw guide.
A worn starter motor can produce similar symptoms. It works, but it lacks the power to turn the engine quickly enough, especially on a cold block. This is more common on larger-displacement engines and on diesels that need higher cranking speeds for compression ignition.
Ground connections and cables are silent culprits. Corroded or loose terminals on the battery, engine block or body create resistance that reduces the power reaching the starter. Cleaning the contacts costs almost nothing, yet the effect can be dramatic.
Oil viscosity matters in winter. If the engine has oil that is too thick for low temperatures, the resistance during cranking is higher and the starter needs more power. Check the specification for your engine, because the difference between 5W-30 and 10W-40 at minus fifteen degrees is enormous.
Petrol Engines, Specific Causes
On petrol cars, the most common culprit for hard cold starting is the coolant temperature sensor. This sensor tells the engine computer how cold the engine is, and the computer adjusts the fuel mixture accordingly. When the sensor sends an incorrect value, for example signalling that the engine is warm when it is actually freezing outside, the computer prepares too lean a mixture and the engine struggles to catch. This is a classic problem we see regularly.
Fuel pressure bleed-down overnight is another frequent cause. A faulty check valve in the fuel pump or a leaking injector allows pressure to drop while the car sits. In the morning, instead of having fuel ready under pressure, the pump must first build it back up. The engine cranks longer, sometimes catches and stalls, then finally starts on the second or third attempt.
Spark plugs and ignition coils directly affect spark quality. Worn plugs produce a weak arc that might be sufficient on a warm engine, but on a cold start the problem is immediately apparent. Standard plugs should be replaced every 30,000-40,000 km, iridium plugs every 80,000-100,000 km, depending on the manufacturer.
A dirty throttle body or idle air control valve restricts airflow at low RPM. On a cold start, when the engine needs slightly more air and fuel, a clogged throttle can cause the engine to idle rough or stall.
Diesel Engines, Specific Causes
On diesels, glow plugs are the first suspects. A diesel engine ignites fuel through compression, and glow plugs heat the combustion chamber so fuel ignites more easily when cold. When one or more glow plugs fail, the engine cranks longer, runs rough for the first few seconds and produces white smoke. Often only one or two out of four will fail, so the engine still starts, just harder. Many drivers ignore this until a severe frost hits. We wrote about this in detail in our glow plug guide.
Compression on high-mileage engines may be low. Since diesel relies on compression for ignition, even a modest drop in cylinder pressure means harder cold starts. This is especially pronounced on engines above 250,000-300,000 km that have not had a major overhaul.
Paraffin in winter diesel is a real problem. Standard diesel starts to crystallise at temperatures below minus five to minus ten degrees, and paraffin particles clog the fuel filter. A winter additive or winter-grade diesel solves this, but many drivers do not think about it until it happens.
Air ingress into the fuel system is an insidious issue. A loose fitting on the low-pressure side, a cracked hose, or worn seals on the fuel filter allow air to enter while the car sits. In the morning, the injectors receive a mix of fuel and air instead of clean fuel, and the engine cranks for a long time until the system bleeds itself. If you notice your engine smoking on cold start, that can be a related symptom.
LPG Cars, Why the Gas Is Not the Problem
This is something we explain to LPG car owners almost every week. A car with an LPG system always starts on petrol, then switches to gas once the engine warms up. So if your car struggles to start in the cold, the LPG system has nothing to do with it.
In our workshop we see this pattern constantly: the owner drives on gas all day, and the petrol side is neglected for years. The spark plugs are old, the fuel pump has weakened because it only runs for the first thirty seconds each day, and the petrol in the tank is stale because so little is used. Then winter comes and the car is hard to start. The owner assumes it is a gas problem, but the real issue is that the petrol side has been completely ignored.
Our advice for every LPG car: check the spark plugs once a year, let the engine run on petrol occasionally so the pump does not seize, and never drive with an empty petrol tank because a pump running dry wears out quickly.
What to Check and When to Book a Diagnostic
Start with the basics: how old is the battery, when were the spark plugs or glow plugs last replaced, and are you using the correct oil viscosity for winter. These are things you can check yourself or at least have the information ready when you visit the workshop.
If all of the above is in order and the engine still struggles on cold mornings, then a diagnostic scan is needed to read fault codes from the engine computer and determine whether the problem is a sensor, fuel pressure, compression, or something else. In our workshop we resolve this quickly because difficult cold starting is one of the issues we deal with most often. Get in touch to arrange an appointment.