Most of the time, your car’s exhaust is semi-translucent with perhaps whisps of white. If you suddenly have black smoke coming from the exhaust, it’s likely a sign of a very serious problem.
This is usually a sign that the fuel/air mixture is too rich due to either too much fuel or not enough air being introduced to the combustion chamber. The underlying fault here is often either a leaking fuel injector, a faulty fuel pressure regulator, or a bad air filter. Though there are a few other options to be concerned about as well.
To help you dial in what’s causing black smoke to come out of your exhaust, we will have to investigate the most common causes and what can be done about them.
What Color Should Exhaust Smoke Be?
During warm weather, car exhaust is almost completely translucent with just the slightest whisps of gray. In the deep cold weather of winter, the exhaust becomes a little more opaque but is still mostly white to very pale gray.
What Does Black Smoke Mean?
Black smoke coming from the exhaust is typically related to an overly rich fuel/air mixture with far too much fuel entering the combustion chamber. Many times it’s a problem with the fuel injectors and/or the car’s air intake system.
Top 5 Reasons Causing Black Smoke from Exhaust
Several mechanical faults can cause black smoke to come from the exhaust. While the most common culprit is leaking or stuck-open fuel injectors, air system issues and a bad fuel pressure regulator are also some of the stronger possibilities.
1. Leaking Fuel Injectors
Leaking fuel injectors that are stuck open is one of the most common causes of black smoke coming from a car’s exhaust. Oftentimes the electromagnetic solenoid is stuck open or a fuel system clock is preventing the fuel injector solenoid from closing when it’s supposed to.
The end result is a stream of fuel into one or more cylinders that causes the engine to run excessively rich, leading to black smoke from the exhaust.
Sometimes when the check engine light comes on due to a leaking or stuck-open fuel injector, the ECU will throw code P0172, which indicates that bank 1 of the engine has a rich fuel mixture, or it might throw P0175 to indicate Bank 2 has a rich fuel mixture.
How to Fix Leaking Fuel Injectors
A leaking fuel injector usually needs to be replaced rather than cleaned. Sometimes you can get lucky, and a small piece of debris in the fuel system causes the solenoid to stick open, and a cleaning will release it.
Though most of the time, one or more fuel injectors need to be replaced. Your mechanic will rightly advise you to have them all replaced simultaneously, saving you labor costs in the long run.
The average cost to have a mechanic replace your leaking fuel injectors ranges from $350 to $800.
The cost to have a single malfunctioning fuel injector replaced runs between $75 to $120.
2. An Air Intake Problem
Black smoke can also come from the exhaust due to a clog in the air intake or a bad mass airflow sensor that allows the engine to run too rich with fuel. If it’s a physical obstruction, you’re usually looking at a severely dirty air filter or something like an unforeseen mouse nest that badly impeds the air. The ECU tries to allow more air into the combustion chamber, but it’s simply not available.
Sometimes a clog or physical malfunction with a car’s intake problem will throw a code P2195 which indicates a functional problem with the way the air and fuel feed into the engine or the way your engine interprets the oxygen reading.
If the ECU throws a code 1101 it means that there is a fault with the Mass Air Flow (MAF) sensor system.
How to Fix an Air Intake Problem
If it’s a physical clog causing black smoke from the exhaust, then you’ll need to remove the obstruction. Even if it wasn’t an air filter issue, replacing the air filter is still wise. Then reset the codes and test drive the car to see if black smoke is still coming from the exhaust.
The cost to replace a severely clogged air filter costs around $15 to $25 for the filter itself.
If your ECU threw a code 1101 indicating a problem with the MAF sensor is causing black smoke from the exhaust, then you need to clean or replace the sensor. This is the sort of thing you can do on your own. Generally, a MAF sensor in this state won’t be improved by cleaning and you’ll need to replace it.
The cost for the new MAF sensor is usually less than $100 and will take you less than a half hour to replace it on your own.
3. A Bad Fuel Pressure Regulator
A leaky or faulty fuel pressure regulator can cause the engine to run excessively rich, leading to black smoke from the exhaust. Most modern fuel pressure regulators have some type of internal diaphragm that helps maintain consistent fuel pressure for the demand dictated by the car’s ECU. This often relies on a vacuum system or some type of electronic regulator.
A lot of fuel pressure regulators are mounted to the fuel rail of the engine. Though some are integrated into the fuel pump. If your car’s fuel pressure regulator is in the fuel pump assembly, and you have a badly clogged fuel filter, or fuel pump issue, it can also affect the regulator, causing a very rich mixture in the cylinders.
Signs of a Bad Fuel Pressure Regulator
If you get a check engine light with black smoke coming from the exhaust, the ECU might throw a code P0089 which indicates a possible issue with the fuel pressure. A mechanic can also perform a fuel pressure test to confirm the problem.
How to Fix a Bad Fuel Pressure Regulator
If your car has a fuel pressure regulator that’s integrated with the fuel pump, the wise move is to pay a professional mechanic to replace it. Chances are good the fuel pump also has a problem as well.
The cost to have a mechanic replace a bad fuel pressure regulator ranges between $300 to $500.
If your car has the fuel pressure regulator mounted to the fuel rail, and you consider yourself a competent DIY mechanic, you might be able to source a replacement regulator for $150 to $190. You can then replace it via the following steps.
4. A Bad EGR Valve
A bad EGR valve that’s stuck open due to heavy carbon deposits can also cause black smoke from the exhaust. This Exhaust Gas Recirculation valve helps send some engine emissions back to the combustion chamber before they’re redirected to the exhaust system. When it’s stuck open the engine runs inefficiently causing black smoke.
Signs of a Bad EGR Valve
If the check engine light comes on, the ECU might throw EGR valve codes such as P0400 for EGR flow malfunction. You might also get code P0401 for EGR insufficient flow detected or P0402 for EGR excessive flow detected.
How to Fix a Bad EGR Valve
If the black smoke from the exhaust is caught early, you might be able to fix the bad EGR valve by cleaning it with some simple throttle body cleaning spray. Most modern cars have an electronic control with a wiring harness for the EGR valve. So, you want to avoid corrosive cleaners at all costs.
You can then attempt to clean your EGR valve with the following steps.
If the EGR valve is badly caked with carbon deposits and/or you don’t have throttle body cleaner, you might need to soak it overnight in carb cleaner. Just make sure that the electronic connections on the EGR valve aren’t submerged at al
If the bad EGR valve is beyond cleaning or the car still has black smoke from the exhaust, then you might need to replace it completely.
The cost of a mechanic replacing a bad EGR valve ranges from around $250 to $350. However, most of this is the cost of the part. If you feel up to it, you might be able to replace the EGR valve yourself for around $180 to $250.
5. Damaged Piston Rings
Damaged piston rings can allow excess gas and oil into the combustion chamber causing blue or black smoke in the exhaust. Left unchecked bad piston rings can also damage the engine, and lead to severe misfires, which can also damage the exhaust system as well as the catalytic converter.
Signs of Bad Piston Rings
Often times a simple compression test will reveal whether or not bad piston rings are causing black smoke from the exhaust. This test calls for removing one of the spark plugs to insert a compression tester.
You’ll need someone else to write down the results of the compression tester as you try to start the car. Then compare the result to the compression stats listed in your vehicle’s repair guide.
You can buy an inexpensive compression tester online or at most auto parts stores.
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How to Fix Bad Piston Rings
If bad piston rings are causing black smoke from the exhaust you’ll need to replace them. This is a job that’s usually beyond what a DIY mechanic can do on their own. It requires some special tools and there’s a lot of labor time wrapped up in the process. At the same time, worn-out piston rings often cause problems with other engine components that need to be repaired by a professional mechanic.
The cost to have a mechanic replace worn-out piston rings typically runs from $1,500 to $2,800.
Frequently Asked Questions
Is Black Smoke from Exhaust Bad?
Black smoke from the exhaust is usually related to a major mechanical fault such as bad piston rings, a major clog in the air filter, or some other problem that is causing the engine to run rich. Left unchecked, these issues can cause dangerous misfires and other damage to the engine or the exhaust system.
What Makes Black Smoke From Exhaust When Starting?
A puff of black smoke from the exhaust when you start your car is usually related to too much fuel in the fuel/air mixture. This is often linked to an air intake problem or a stuck-open fuel injector that is letting too much fuel enter the combustion chamber. You should consider this an early sign of a problem that needs to be addressed.
What Makes Black Smoke From Exhaust When Accelerating?
Black smoke from the exhaust when you’re accelerating is usually related to the fuel injectors introducing too much fuel, or the air intake system not being able to deliver enough air to maintain the proper fuel/air mixture. This is also an early sign of a more serious problem that needs to be addressed.
Can You Drive with Black Smoke Coming from the Exhaust?
As long as the car isn’t misfiring, stalling, or leaking fuel out of the engine might be able to drive with black smoke coming from the exhaust to limp home, to the mechanic, or the auto parts store. Though it should sit from there until you can troubleshoot and fix the underlying problem.
Conclusion
Black smoke from the exhaust is typically a sign of the engine running far too rich due to either decreased airflow, or excessive fuel entering the combustion chamber. This is usually linked to an underlying problem like a stuck-open fuel injector, an air intake problem, or bad piston rings. Though a gunky EGR valve or a failure of the diaphragm inside the fuel pressure regulator could also cause black smoke from the exhaust.
These are all serious mechanical faults telling you not to drive the car much farther. Especially if the engine is misfiring or stalling unexpectedly.
If you’re lucky fixing the cause of black smoke from the exhaust can be as cheap and simple as cleaning the deposits off the EGR valve or replacing the fuel pressure regulator.
These are projects that the average DIY mechanic can do themselves for less than $100. More major mechanical problems that could cause black smoke from the exhaust like bad piston rings or fuel injectors that need to be completely replaced can cause you upwards of $350 to $800 or more.
Written By
Jason Farrell
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.