Types of smoke from the exhaust on startup. Causes and solutions

Smoke coming from the exhaust on startup, especially if the engine is cold can mean a potential problem that is need to be checked no matter the color of smoke.

Common causes of smoke are usually worn valve seals, stuck pistons rings, when using a wrong oil viscosity, or simply low-quality engine oil. In diesel engines smoke from the exhaust can be due to bad glow plugs, and problems with the fuel system, especially the high-pressure pump. These causes are usually accelerated by using low-quality diesel fuel.

Why does smoke comes out from the exhaust?

To better understand what can cause this problem, you need to know what is the smoke color, which can be white, gray, or dark blue.

Worn valve seals

The role of the valve seals is to keep the oil away from the cylinders, but when they are worn out, a small amount of oil may leak into the combustion chamber.

Therefore, when the engine is cold, the gaps in the engine are smaller, and, after starting the engine, a very small amount of oil gets in the cylinders, and after the engine gets warm, the gaps increase thus the oil stops from entering the cylinders. That’s why you will often see blue smoke on startup and after a few minutes, it does not come out anymore.

Another case with valve worn seals is that some engines are designed in such a way that a small amount of oil can enter the cylinders when the machine is idle. So, when starting, this oil is immediately burned, and after a few minutes, cars no longer eliminate smoke.

Piston rings overlap

Another common cause for the smoke on engine startup is overlapping piston rings. Because of this, you will often see gray and white smoke coming out from the exhaust.

Oil can get into the cylinders also because of worn piston rings. This happens often when the engine is cold. After warming up, the piston rings are running properly, but this can pose a serious threat to the engine, so you’ll need to clean it from soot and carbon deposits to resolve the problem.

Removing the engine and checking the piston rings is a good solution here. However, before doing this, it’s best that you check the engine compression first. You can also use a temporary solution like additives that can help you, but this a short-term solution.

Using an incorrect oil for your engine

Here I’m talking about the engine with high mileage. In the car user manual, the manufacturer often says that you can use different types of viscosities for your engine. The possibilities for this to happen reduces drastically when the engine has higher mileage and the parts wear out.

If you use thinner oil, it will leak into the cylinders until the engine warms up and the clearances increase. The chances for this to happen with a thicker oil are very low.

Also, pay attention to the oil quality. Smoke from the exhaust can be present even if you selected the viscosity correctly, but because the oil is low-quality, you can be in trouble.

In very rare cases, drivers saw that after changing the oil filter, the smoke stopped coming out from the exhaust, which means that the oil filter was fake or bad for the car model.

Worn or damaged cylinder head gasket

Usually, the coolant enters the cylinders through a damaged cylinder head gasket. When you start the car, you will often see white smoke from the exhaust which means that coolant is present in the cylinders.

The problem can also be that the cylinder head is not tightened sufficiently, the white smoke stops after warming up due to the expansion of the metal and the restoration of a tight fit of surfaces.

Failing engine sensors

The electronic Control Unit (ECU) is responsible for the composition of the fuel mixture. It depends on readings from various sensors, including coolant temperature sensor and intake air temperature sensors. 

Therefore, on startup, it is possible for the ECU to use a re-enriched fuel mixture, the result will be black smoke coming out of the exhaust. After the engine heats up, the fuel mixture becomes leaner and everything comes back to normal.

Condensation in the exhaust

Condensation is not a problem for the engine. In winter. After the engine has cooled down, condensation forms on the walls of the exhaust system. When the engine starts in the morning after it sat some time in cold temperatures, the exhaust gases heat up this condensation and turns it into steam. 

Therefore, after starting, it takes a few minutes for the steam to evaporate from the exhaust system. Evaporation time will depend on outside temperature, engine size, and exhaust system design. So, you don’t have to worry about if you see this.

Smoke from the exhaust on startup in diesel engines

Diesel engines have more causes why you see smoke from the exhaust when starting the car, especially when the engine is cold.

Bad glow plugs. If the diesel glow plugs are failing then, when you start a cold engine, you will notice black smoke from the exhaust because, in the cylinders, the fuel is not burned completely. This is however temporary because when the engine gets warmer, black smoke will not be present anymore.

Bad injectors. If the diesel engine injectors are not functioning properly, this will lead to incomplete fuel combustion that can be caused by a failing injector nozzle or poor pressure when spraying fuel from the injector. Like the above cause, this can happen when the engine is cold and you will see black smoke coming out from the exhaust, and after the engine heats up, the smoke will disappear.

A bad PCV valve (crankcase ventilation system). Because of a failing PCV valve engine draws up the oil and burns it along with fuel. As a result, you’ll see black or dark blue smoke until the engine warms up enough.

How to diagnose and stop smoke from the exhaust

If, for whatever reason, you see black, grey, blue or white smoke coming out from the exhaust, then you will need to perform the below checks:

  • Check the valve seals. This is a common cause of why smoke comes out from the exhaust on startup, especially if the engine is cold. When you begin to perform these checks, you can start with the valve seals. Again using a high-quality engine oil will prolong the life of the valve seals.
  • Check your oil level and condition in the engine. If you see an increase in the oil’s volume, then most likely coolant got into it. So if you see noticeable changes in their levels, you’ll need to perform additional checks in a car workshop to see if valve seals, piston rings, or the cylinder head gasket have problems.
  • Check the oil quality and viscosity. Low-quality oil and its wrong viscosity are bad for the engine, especially if the engine has high mileage. If you used an incorrect oil viscosity for your engine, then it makes sense to replace it with a thicker one if the engine is more worn out, or with the proper viscosity by checking your car user manual.
  • Check the engine compression and the piston rings. If you see black or gray smoke on engine startup especially if the engine is cold, this is a good reason to check the compression and the condition of the piston rings. If the compression is low, you need to find out the causes. Cleaning the piston and rings from carbon buildups often solves the problem. If you notice a consistently high-oil consumption then it makes sense to change the piston rings.
  • Check the engine sensors with a scan tool to see if you have a failing one

Additional checks for diesel engines

  • Check the condition of the injectors. Bad diesel injectors are common cause why you will see black smoke from exhaust. So it’s better to check them out.
  • Check the high-pressure pump.
  • Check the EGR. Another common cause why diesel engines have smoke on startup. Cleaning often is a solution, but if the EGR does not function properly after that, change it.

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