White smoke from the exhaust: do I need to take it to the garage?

If your car is producing white smoke from the exhaust, pay attention: depending on whether it has a petrol or diesel engine, you could be facing a costly fault to fix.

Recognising a possible problem with your car’s engine, or anticipating its appearance and diagnosing a fault through the colour of the smoke coming out of the exhaust, can save you a lot of headaches. And one of the most common situations, and one that generates the most doubts, is when the car emits white smoke from the exhaust.

Why might my car be emitting white smoke? Can I keep driving the vehicle? When should I worry or go to the garage? In this case, to carry out a correct diagnosis and assess the probable causes, we need to distinguish whether we are talking about a car with a petrol engine or a diesel one.

White smoke from the exhaust in petrol engines

If, when starting your car from cold, especially in winter, you see a little white smoke coming from the exhaust, there is no need to panic, as long as it disappears once the car reaches its optimal operating temperature. It is due to the water vapour generated during combustion, which means that when the engine is switched off some water remains in the exhaust. That water is what causes the smoke when starting from cold, and it disappears shortly after the engine starts.

But if the white smoke is dense and persistent, and it also occurs while driving with the engine already at temperature, then you should be on your guard. In this case it is a sign that coolant is entering the cylinders. Additional symptoms alongside the white smoke are a drop in the coolant level in the expansion tank, as well as a characteristic smell.

Possible causes:

• The head gasket is damaged
• The cylinder head is warped
• There is a crack in the block

In any case, it is advisable to visit the garage. Replacing a head gasket can cost around 800-1,000 euros. But driving the car in this state will end up warping the cylinder head and will force a larger intervention to plane it and eliminate the deformation, multiplying the cost of the fault by up to three.

White smoke from the exhaust in diesel engines

In the case of diesel engines, persistent white smoke with both a cold and a warm engine could also be due to a head gasket problem. However, here the most common situation is white smoke caused by problems related to the injection system. Especially if the smoke is very dense on a cold start but disperses as soon as the engine reaches its proper temperature, and is accompanied by an irregular idle or engine knocking.

Possible causes:

• Faulty glow plugs
• Faults in the fuel pump or the common rail
• Faults in the injectors

In these cases, the white smoke occurs because if the fault causes the fuel to be injected at the wrong time (too early or too late), the fuel does not burn completely under compression, and the unburnt part evaporates.

It is advisable to go to the garage, since a fault in an injector, for example, can get worse and cause that injector to eventually break completely and inject a jet of diesel when it should not, which would cause serious damage to the piston. It can also allow fuel to leak past the piston rings, which ends up in the engine oil.

Related information of interest

–The 10 most expensive faults to repair
–Diesel particulate filter: follow these tips to avoid faults
–The most common failures at the MOT