Smoke Signals: What exhaust smoke tells you.

Smoke Signals: What exhaust smoke tells you.

March 19, 2020 0 By Ray Bohacz

When an engine, either gas or diesel, is running correctly and has no mechanical issues, there is nothing visible but the heat from the exhaust pipe. That is the way it is supposed to be, but we all know that is often not the case.

Visible exhaust smoke needs to be appropriately interpreted. It can be deceiving; what may look like a major repair is not, while a serious condition may present as if it is nothing much.

Due to the unique combustion characteristics of diesel and gasoline, the color of the exhaust smoke will be explored individually, even though there is some overlap.  

Gas engine exhaust smoke

White smoke

Other than during warm-up on a cold day, which would be condensation and a temperature inversion (this applies to diesel also), this color smoke is indicative of coolant getting into the combustion chamber. Depending on the intensity of the smoke and when it occurs, it will lead to the problem area.

White smoke is traditionally accompanied by an odor from the engine coolant, which is a mix of glycols being burned. Most coolants will produce a slightly sweet aroma when exposed to combustion. If the amount of coolant being introduced is minimal, the odor may not be present.

To get a better handle on this problem, look at the engine with an open mind. Do not automatically think head gasket or cracked cylinder head, though it may end up being that. Coolant can enter the induction path via an intake manifold gasket seal or water-cooled turbo center section. On a gas engine from a throttle-body or under the carburetor heater that uses engine coolant.

I have also seen some engines have a plugged or poorly functioning breather/PCV circuit.  This has allowed so much condensation to build up in the crankcase that moisture is pulled past the rings, and the engine occasionally emits white smoke.

Black smoke

To most people, this is a rich condition, but in engineering, it is identified as oxygen-deficient. There is not enough oxygen present to burn all the fuel introduced.

This is important since you need to not only look for what may be causing excess fueling but for what may be limiting airflow. A restricted intake path will starve the engine for oxygen and, depending on the fuel delivery method, may cause black smoke. On carburetor equipped engines, look for proper choke plate movement and full opening.

Black smoke during warm-up (older engine)

Many farms still have older carburetor equipped engines in use, especially in trucks. With the introduction of the automatic choke in the early 1960s, the choke pull-off or sometimes called a choke break came into existence. The purpose of this part is to pull the choke butterfly open via engine vacuum against choke spring pressure. This allows for the proper amount of oxygen to burn the rich cold start mixture.

If the choke pull-off fails or the diaphragm grows old and expands, the mechanism will not be able to pull open the choke plate the required amount. Then the engine will load up, blow black smoke, and possibly stall. Once it warms up, it will run fine.

Blue smoke or blue/white smoke on start-up

Blue smoke is caused by engine oil being introduced into the combustion event. If the amount of oil is not too significant, then the smoke may appear to be blue/white. Keep in mind that the blue color is only a tinge, and it will not be Kansas summer sky blue!

Again, do not necessarily think piston rings. This can be caused by the following:  fuel diluted engine oil, too low a viscosity oil, intake manifold gasket, turbocharger seal, automatic transmission with a vacuum modulator that is leaking, wrong/defective PCV/breather circuit or, overfill of the crankcase.

If the engine only emits blue smoke on start-up and then is clean, that is historically valve stem seals and possibly guides (not likely). If the seal is bad when the engine is shut off, the hot, thin oil leaks between the valve stem and the guide and drips on the piston crown. When the engine is started, the oil burns off, and then the smoke stops.

Almost every engine can have the valve stem seals changed with only removing the valve covers and rocker arms. The cylinder heads do not need to come off.

I have also seen bad valve stem seals cause blue smoke during an extended idle condition such as a long traffic light. Then as the engine speed increases, the oil is burned off, and the exhaust cleans up.

Diesel smoke analysis

Black smoke

This can also be a grey/black color, and it represents an insufficient amount of combustion air. Look for anything that can limit airflow and, if possible, perform an intake manometer (water meter) test. Typical values should be 15 inches H2O for naturally aspirated engines. Readings of 25 inches H2O will be found on turbocharged engines.

Black smoke can also be the result of an exhaust system restriction, uneven cylinder-to-cylinder fuel distribution (injector, nozzle, injection pump, and piping) or fuel related.

Old diesel fuel will have some of the volatile fractions evaporate and will not burn correctly.

White smoke

Diesel has more things that can cause white smoke than a gas engine. They are cylinder misfire, low compression in a cylinder(s), extremely low cetane number fuel, air entering the high-pressure injection pump circuit, coolant leak into the cylinder, excessively cold engine coolant for the operating state due to a stuck open thermostat.

Another cause for white spoke that changes color to greyer and is joined by an acidic odor is burn–off or regeneration of the diesel particulate filter (DPF) with a newer engine.

In theory, during regeneration, there should be no smoke of any color. Still, depending on the condition, the process can begin with the generation of white smoke from the condensation in the unit as it comes up to burn-off temperature.

The system, when functioning correctly, will generate no visible smoke. Still, an engine with more use on it or one that has been poorly maintained can experience bouts of excessive smoke during the early stages of regeneration.

Keep in mind that poor quality diesel fuel may result in inefficient combustion. Any engine oil that is getting into the combustion event will impact the regeneration process and smoke formation. 

Blue smoke

As with a gas engine, this is the result of engine oil entering the combustion process. Look for any region that has engine oil and a path to the induction system. Common areas external to the engine are the center section bearing on the turbocharger and the crankcase ventilation system.

A high hour/mileage engine can have also collected oil film that became liquid in the intercooler. It then feeds it into the engine under high boost conditions and creating blue exhaust smoke.

Though none of us want to see any smoke from the exhaust, quick response to this condition often will lead to less downtime and a reduction in the cost of repair. Ignore it, and you will not have to search for the problem…. it will eventually become very apparent and expensive.