The Effects of Exhaust Backpressure on Your Engine

Fuel, Emission and Carb

By Rodney (Mechanics)

Exhaust backpressure can be a reason for a lot of problems with the engine. If your catalytic converter is plugged, this condition will result in the inability of your engine to perform properly and your fuel economy will also drop. In fact, if the converter is completely plugged, there is a possibility that the engine will stall. This condition can also occur when a muffler, a resonator, or double-walled exhaust pipe has collapsed. These things will be able to restrict the flow of exhaust from the engine and will result in excessive backpressure in the system.

Backpressure Gauge

Common Symptoms of Excessive Exhaust Backpressure

The most common symptoms of excessive exhaust backpressure can be noticed from the lack of power from the engine, the fuel economy is bad, and the temperature of your engine gets too high. This is because the heat will be backed up and will not be able to escape the engine. Usually, about 1/3 of the heat that is being produced by the engine will escape from the tailpipe as waste heat. Therefore, if there is a condition in the engine that prevents the heat from escaping, the cooling system will overload and the engine will run a lot harder than usual. This can also occur more when you drive your vehicle at high speeds.

You may also notice that your engine will be strangled after you turn it on for about a minute or two. This is a symptom of a complete blockage in the exhaust system. In some extreme cases, the buildup of excess pressure can also lead to a blown-out converter shell or the connector of the pipes. Normally, the symptoms will make it easier for you to diagnose if there is excessive backpressure on your exhaust system. But, some cases will still require you to measure the exact backpressure number that occurs in your engine.

Measuring the Exhaust Backpressure

Measuring the exact number of exhaust backpressure will require you to have a pressure gauge that can read between 0 to 15 PSI. You might also be able to get a test kit to measure the exhaust backpressure which will cost you around $60. To get a more accurate reading on the pressure, you can also use a digital manometer that will be able to display readings for pressure in your engine using different units of measurement. Normally, this type will be able to measure pressure between 0 to 15 PSI and cost normally around $170.

Getting the exact measurement for the exhaust backpressure will also require a complicated method since there is no easy way to get into the exhaust system. If your vehicle is equipped with an air pump in the engine, you might be able to get into the exhaust system from the valve for an air pump check. You can do this by installing a pressure gauge after disconnecting the check valve. But, you can only get a correct reading if you can connect the exhaust system with the check valve ahead of the converter. The reading will not be reliable and accurate if you can only connect the air pump plumping to the converter.

Another method to measure the backpressure reading is by first removing the oxygen sensors from their locations on the exhaust manifold. Then, you can connect the pressure gauge onto a hose fitting. The uninstallation process of the oxygen sensors on newer vehicles will be relatively easier than on older vehicles. Besides, there is a possibility of a damaged condition on the sensors if you do this on older vehicles.

If those methods are unsuccessful, you can also try to make a small hole on the pipe of your exhaust system. The hole should be located ahead of the converter. You can then connect manometer or your pressure gauge to the system. While this method is easier compared to uninstalling the oxygen sensor, this method will also require you to plug the hole after you are done measuring the backpressure.

If your vehicle is still using diesel engines, it might also be equipped with a backpressure sensor that is located in the exhaust system. This component will be able to measure the reading for the exhaust backpressure without any additional tool. You might be able to check the reading on the sensor PID and get the actual value for the measurement.

Installed Backpressure Gauge

The Optimal Number for Exhaust Backpressure

When the engine is idling, the optimal number for backpressure should be less than 1.5 PSI which is equal to 10 kPa. But, this number will be different from one vehicle to the others and it will depend solely on the design of the exhaust system. There are also some components that might contribute to the different readings for the backpressure of your vehicles, such as the muffler or the resonator. Some vehicles might show readings of more than 2.75 PSI. But, 1.5 PSI is the normal reading number for most vehicles.

If the converter or the muffler is partially restricted, the flow of the exhaust might also be restricted but will not cause problems when the engine is idling. The problem usually arises when you drive the vehicle at higher speeds. You can test this condition by revving and holding the engine speed at around 2000 RPM. If the rating shows around 3 PSI or less, this means that the engine is in a good condition. You should also know that there are vehicles that will show higher readings with no problems, but the normal number should be around 3 PSI or less.

When you are holding the engine speed at around 2000 RPM, you also need to pay attention to the backpressure reading. If the reading remains at the same level, there is probably no restriction in the exhaust system. You can suspect a blockage if the reading gradually increases over time, which means that there is a buildup of backpressure in the system.

You can also try holding the engine speed at around 4000 RPM. The reading for the backpressure will usually get a lot higher to around 4 to 8 PSI. Similar to the case above, you can usually notice an abnormal restriction in the system if the reading continues to climb significantly when you hold a steady RPM.

Inspecting the Exhaust System

If the backpressure reading is higher than the normal number, this usually means that there is something that restricts the flow of exhaust from the engine tailpipe. The converter is the most common reason for this condition, but restrictions can also happen on the mufflers or the resonators. The collapse of double-walled exhaust pipes might also be able to cause blockage on the system.

Plugged Converters Are The Most Common Cause Of High Backpressure

The high backpressure number is commonly cost by a plugged converter

You can inspect the condition of your exhaust system to find any signs of damage if the engine is experiencing exhaust blockage. The damage conditions might occur on the pipes along with corrosion on some components. You can also try to shake the converter to see if there are any loose components inside.

After that, you can try to disconnect the exhaust pipe located behind the converter and then see if the reading number is getting lower. Normally, the reading will get down a little bit when the converter is disconnected from the exhaust system aft. But, if the reading stays intact, you can suspect that there is a plugged converter inside the engine. Another possibility is the collapse of connection between the converter and exhaust manifold that can cause a buildup of backpressure.

If the backpressure reading drops significantly after disconnecting the converter from the exhaust system, then your converter is actually in a good condition. The blockage that causes a high backpressure number might be located somewhere else, such as on the resonator, the muffler, or the tailpipe.

Checking the Backpressure Using Intake Vacuum

You can also try to inspect the condition of the backpressure by checking the intake vacuum from the engine. This is an easier way compared to uninstalling the oxygen sensor because you just need to hook up the vacuum gauge to the exhaust system. The result from using the vacuum gauge will be presented in inches of Hg.

You can start checking the vacuum reading from the intake manifold after turning the engine off and disabling the EGR valve. Disabling the EGR valve can be done by disconnecting its connection to the solenoids. You can then connect the vacuum gauge to a source of vacuum on the throttle body or the intake manifold. Proceed by starting the engine and inspecting the vacuum reading when the engine is in idling condition.

If you see that the vacuum reading shows a lower number than normal, then there might be an exhaust restriction inside the engine. Another sign of pressure buildup is when the reading number continues to drop even when you let the engine idle.

The vacuum reading number will usually drop slightly if you increase the engine speed. But, if the vacuum reading drops suddenly, there might be a blockage problem on the exhaust system. The reading might also indicate a periodic blockage if the vacuum gauge is showing erratic readings.

How to Reduce Exhaust Backpressure

You can increase the engine performance and improve your fuel economy if you can reduce the exhaust backpressure. Eliminating restrictions in the exhaust system will allow a better flow of exhaust to escape from the engine. One easy way to do this is to replace the stock muffler with one that has a low restriction performance. You can also replace the whole exhaust system from your engine with one that has a free-flowing feature.