Although white smoke can billow from an exhaust stack in warm weather, the problem is more prevalent when you start a cold engine in sub-zero weather. Look around the truck stop this morning. The plumes surround you.

This sight occurs because the engine’s combustion chamber is too cold to allow proper combustion, and the partially burned fuel is exhausted from the stack in the form of white, vaporous smoke. The main reason for the white color is the way re-condensed droplets of unburned or partially burned fuel scatter light.

It’s fairly normal for this to appear and clear up within several minutes, although the length of time will be dictated by the engine design itself.

Mechanically governed and controlled engines tend to need more time to clear white smoke than their electronically controlled counterparts that enjoy tighter combustion controls, with sensors feeding data to the Electronic Control Module.

But it’s not something to ignore. Excessive periods of white smoke can be caused by poor-quality fuel with a low cetane rating, low inlet air temperatures, low coolant temperatures, low idle speeds, low compression ratios, retarded injection timing, and incorrect or failed components.

To limit the smoke, you need to select a fuel with a minimum cetane rating of 40 when operating in ambient temperatures above 0 Celsius (32 Fahrenheit). When operating at conditions below those temperatures, select a fuel with a minimum cetane rating of 45.

If you’re in doubt about the grade of fuel that you’re using, temporarily disconnect the fuel suction/supply line and submerge it into a large can of what you know to be good quality, clean fuel.

If the white smoke disappears when you start the engine, then the smoke is being caused by poor fuel.

You can also use a fuel hydrometer to confirm the API rating of the fuel in the tanks. With electronic engines, check the fuel supply for high or low pressure, combustion gas/air in the fuel, and poor fuel quality.

The lower the ambient air temperature drops below freezing, the lower your cylinder compression pressure and temperature will be. That’s why it is much harder for the injected fuel to vaporize and initiate combustion in the winter.

On electronic engines, the Intake manifold air temperature sensor may read higher than the ambient temperature at idle because of the intake manifold’s heat soak, which limits the flow of air to the sensor and leaves the device detecting heat that is conducted from the manifold.

Drive the vehicle to see if the additional flow of air eliminates the problem.

Low coolant temperatures will also cause white smoke, and can come from extended idling times in sub-zero ambient temperatures. Typically, you should maintain a minimum of 71 C (160 F) at the radiator top-tank.

White smoke should clean up when coolant temperatures rise to between 60 and 71 C (140 and 160 F). But if coolant temperatures remain below 60 C (140 F) when an engine is idling, or drop to 60 C (140 F) or lower during the engine’s cool-down, there may be a problem with the thermostat.

It could be restricted, stuck, have an incorrect temperature range or leaking seals. If it’s vented, replace it with a fully blocking model.

You could also have problems with a leaking radiator baffle, or coolant flowing in the wrong direction because of a poorly located fill line or the angle made by the coolant bypass tube as it enters the coolant inlet connection.

For that matter, ensure that the thermatic/viscous fan is not running continually, and check the engine’s low-idle speed to ensure that it is not falling below the minimum recommended rpm.

Low compression in one or more cylinders can also cause white smoke.

The most accurate way to determine if this is a problem is to use a digital point-and-shoot pyrometer (an exhaust temperature diagnostic tool) to determine if one or more of the cylinders are running cold. Typically, cylinder exhaust temperatures should not vary more than 10 C (50 F).

Simply refer to service literature for the typical exhaust temperatures for your make and model of engine.

If you don’t have such a device in your toolbox, place a small amount of high-temperature grease on the exhaust manifold opposite each cylinder, run the engine and note if the globs melt at equal times.

This will provide you with a general indication of which cylinders may be running cold.

Incorrect injector timing can also contribute to white smoke – along with poor combustion and the poor distribution/atomization of fuel.

Incorrectly set valve clearances, or worn valve operating mechanisms can contribute to white smoke because of the resulting drop in delivered fuel, or the variations between valve and injector timing.

Manually check for signs of loose or tight valves or injectors. A loose valve could indicate a bent pushrod or tube, while a loose injector could be caused by a cracked injector cup or spray-tip, or a failed plunger. But be sure to check the static injector timing if doubts still persist.

Heavy carbon deposits within the combustion chamber – and particularly on the injector tip – can deflect the spray of fuel from its normal path.

In addition, the dribbling of raw fuel into the combustion chamber will cause incomplete combustion and exhaust smoke.

A running compression check or a cylinder leak-down check can determine if the cylinder compression is within specifications. Carefully note whether each cylinder’s pressure is within the allowable tolerance between cylinders.

A leaking intake valve can be determined by listening for a pop back into the intake manifold, while a popping sound at the exhaust side usually confirms a leaking exhaust valve, or possibly a bent valve stem/head.

You can isolate the cylinder that is causing white smoke by removing the exhaust manifold and running the engine for a very short time, to allow you to see which cylinder is emanating the white smoke.

A worn camshaft lobe(s), or the cam follower rollers for a specific cylinder, can cause problems with the valve or timing of the fuel injector.

As a final step, don’t discount coolant leaking into the cylinders through either a leaking jacket water aftercooler, leaking cylinder head gasket or cracked cylinder head(s). n

– Bob Brady is the president of HiTech Consulting in Burnaby, B.C.