The EGR system of a heavy-duty vehicle makes a big difference in its performance and safety. In this guide, discover how these systems work. Then, learn how to make your truck's EGR work for you.
What Is An EGR System?
EGRs are pollution-reduction systems in diesel vehicles. Specifically, they reduce the amount of nitrogen oxides, a byproduct of diesel combustion, that are released into the air.
Nitrogen oxides are particularly dangerous pollutants: they damage the human respiratory system, cause smog and acid rain, and are potent greenhouse gases contributing to climate change.
As Motor Trend senior editor Jason Gonderman explains:
“When Tier 2 emissions standards went into effect in 2004, diesel engine manufacturers were tasked with finding a way to lower nitrogen oxide (NOx) output. Their solution was found in the use of exhaust gas recirculation (EGR).”
“The basic function of an EGR system is to route a portion of spent exhaust gases back into the engine's intake tract. This oxygen-deprived air is used to limit peak in-cylinder combustion temperatures, which in turn lowers the engine's NOx production.”
Today, EGRs are standard systems in all trucks and heavy-duty vehicles. They're also mandatory components to comply with EPA standards.
How Does An EGR System Work?
ERGs have been at least partially effective. Emissions have decreased by 82% since 2005, with 3 million fewer tons of NOx released. However, this isn’t quite enough of a reduction to reach goals for human and environmental health. So, in recent diesel vehicles, the EGR works in tandem with three other parts / systems:
The Diesel Particulate Filter (DPF)
The Selective Catalytic Reduction (SCR) system
The Diesel Oxidation Catalyst (DOC)
These pollution-reduction systems are controlled by the Engine Control Unit (ECU). An ECU is also called an Engine Control Module (ECM) or a Powertrain Control Module (PCM).
Regardless of the acronym used, the unit is a monitoring and regulating device composed of a computerized “brain” and sensors.
To learn more about the role DPFs and DOCs play in anti-pollution systems, read, “Diesel Oxidation Catalysts & Diesel Particulate Filters.”
To troubleshoot potential problems with these parts, check out our quick guides by automotive pros: “Common Diesel Particulate Filter Problems and What to Do About Them,” and “5 Reasons for Diesel Oxidation Catalyst Failure.”
Basic Components of an EGR System
A truck’s EGR system is composed of six parts: the EGR valve, the intake throttle valve, an EGR cooler, a turbocharger, and an actuation turbine.
Optionally, the system might also include a cooler bypass, secondary valves and turbines, intercoolers, and an exhaust throttle. These parts are integrated with the ECU and its sensors.
EGR Valve
The EGR valve directs byproduct gases, collected in the exhaust manifold from each of the engine's cylinders, into the EGR system.
Older systems used analog, pneumatic EGR valves, which directed air using a mechanical pressure-response mechanism. While inexpensive and streamlined, these valves are prone to leaking or getting stuck in the “open” position, making them less than ideal.
Vacuum-controlled EGR valves are a step up. These semi-automatic components utilize the vacuum pressure created by the intake stroke of the engine’s pistons, directing exhaust gases at regular, predetermined intervals. Intervals are determined either by the engine's speed and load, or by solenoid valves.
These EGR valves are less likely to leak or get stuck than pneumatic valves. But they are prone to clogs, as their shape makes it easy for soot to build up inside. They're also less precise than electronically controlled valves.
Electronically-controlled valves are the most widely used valves in EGR systems today. Their integration with an ECU allows for more precise, responsive valve positioning and direction. The precision allows for a design that reduces soot buildup without reducing its effectiveness, resulting in a component that requires far less frequent cleaning than previous iterations.
Intake Throttle Valve
Electronic EGR valves are typically combined with intake valves further along the circuit in the system. Intake valves create vacuum pressure by locking out external air from the system. This helps boost the force pushing the gases through the system.
EGR Cooler
The EGR cooler is a reverse heat-exchanger. It’s a metal tube divided by plates into chambers, receiving hot exhaust gases in one chamber and coolant fluid in the other. The exhaust gases cool due to indirect exposure to the coolant in the neighboring chamber.
This increases the density of those gases. It also (somewhat) alters the relative composition and ratios of gaseous compounds within the exhaust gas as a whole. Both of these effects lower the gas's combustion temperature.
Thus, when the gas is recycled into the engine cylinders as fuel, the engine can burn that fuel in a way that generates fewer NOx byproducts. As a result, each cycle generates fewer NOx emissions than the one before it, while simultaneously increasing the lifespan of a gallon of fuel itself—directly increasing the truck's fuel efficiency.
Turbocharger & Actuation Turbine
The turbocharger and actuation turbine function as a unit. These ECU-controlled components speed the flow of air and gases through the ECR, activating a compressor to push more air into the engine's cylinders when needed. It helps mitigate the risk of sub-optimal engine performance in situations demanding high torque—an otherwise fairly common side effect of EGR processes. Fortunately, with these components, it's a side effect that heavy-duty trucks can avoid.
Optional: Intercoolers, Cooler Bypass Valves, & Exhaust Throttle Valves
The three optional components offer considerable benefits to the EGR systems of heavy-duty vehicles.
Intercooler
Intercoolers are essentially additional cooling chambers. They cool gases further into the EGR circuit (after bypassing the initial cooler), or they cool gases a second time—reducing them to an even lower temperature.
Cooler Bypass Valve
Cooler bypass valves are ECU-controlled components that let exhaust gas circulate directly into the engine, skipping the cooler, when the engine is cold.
Without a bypass valve, the oxidation catalyst would take longer to warm up (thanks to its prolonged interaction with cooled exhaust gases). The longer the oxidation catalyst stays cool, the more the truck offgasses dangerous hydrocarbon (HC) and carbon monoxide (CO) emissions.
Exhaust Throttle Valve
An exhaust throttle valve works somewhat similarly to an intake throttle valve, creating vacuum pressure. But, rather than push gases into the EGR cooler, it typically interfaces with a DPF. It controls oxygen intake and directs residual gases to the combustion chamber.
As one study in the International Journal of Engine Research explains, “Exhaust throttling reduce[s] the intake air mass flow rate and increase[s] piston pumping,” making the throttling process “more effective for regenerating a diesel particulate filter at a high temperature than intake throttling.”
In essence, exhaust throttles reduce pollutants indirectly by facilitating superheated DPF cleansing—a process that destroys particulate pollutants and keeps the filter functional.
Integrated ECU Components
The ECU is a small, automated computer. It controls the EGR system in response to data input from its sensors. The input information runs through a relatively simple algorithm, which then outputs instructions to automatically reposition valves and adjust internal mechanisms to affect gas flow, pressure, and temperature.
Placed throughout and adjacent to the EGR system, the sensors monitor and quantify:
Exhaust gas temperature
Engine temperature
Differential pressure
Relative levels of different gases and vapors within the system
Valve positions
Engine speed
Engine load
Fuel flow rate
The ECU’s “brain” is housed in a specialized compartment. It's designed to protect the control module from vibrations and environmental elements, without blocking the sensors’ transmissions.
The compartment may be in the engine near the battery, adjacent to the engine's firewall, under passenger seats, behind interior trim kick panels, or under the dashboard in the cab.
Recirculation Process (Summary)
In essence, the EGR is a circuit made of several parts. It pumps most of a truck's exhaust gases into the intake valve, pushing them through the circuit.
The EGR cools most of the exhaust gases as they flow through the circuit, lowering combustion temperatures. This ultimately reduces NOx formation.
The circuitous pump-and-flow process is “recirculation.”
If the system incorporates an exhaust throttle valve, the recirculation process also streamlines DPF regeneration—a second anti-pollution mechanism.
4 Types of EGR Systems: Explained
Not all EGR systems are identical. A modern heavy-duty truck likely has one of three types of EGR systems, while a truck a few decades old might have a fourth type.
Each modern system has distinct benefits and drawbacks.
Passive EGR (Discontinued)
Passive EGR systems are no longer used, though you may still find them in much older truck models. They were discontinued in 2004. Many companies had already begun phasing them out in 1999-2000.
These systems used pneumatic mechanisms to direct the flow of exhaust gases. Rather than incorporate active cooling components, passive systems simply created low-pressure zones to guide gases through a series of filters and chambers.
While these systems were better than nothing, they couldn't reduce NOx emissions enough to meet modern regulatory standards.
High-Pressure EGR
High-pressure (HP) EGR systems are more common in older diesel engines. HP-EGR systems run with an inlet pressure of ~3.5 bar, over an internal section pressure averaging ~1.5 bar.
These systems intake exhaust gas exclusively from the exhaust manifold. Typically, they operate with recirculation rates of about 35% to 40%.
The inlet within an HP-EGR reaches incredibly high temperatures, often reaching 950°C (1742°F).
Benefits
HP-EGRs offer several benefits, particularly when contrasted with low-pressure EGRs. High-pressure EGR systems:
Respond faster to changes in engine load, temperature, or speed
Grant significant transient control
Reduce NOx emissions more dramatically (by lowering combustion temperatures to a greater degree)
Achieve higher recirculation rates
Drawbacks
HP-EGRs perform worse than their low-pressure counterparts by some metrics. Drawbacks include:
Uneven gas recirculation across cylinders
Greater volume of soot deposits in intake manifold
Higher risk of valve clogs
Higher overall operating temperature
What makes a low-pressure EGR so different?
Low-Pressure EGR
Low-pressure EGRs are more recent inventions. They're used in most modern heavy-duty diesel engines.
Unlike their high-pressure counterparts, LP-EGRs draw in exhaust gas from a point further into the circuit. Typically, these systems intake gases after they’ve passed through the turbocharger / turbine, and—frequently—through the DPF as well.
LP-EGRs operate with a very low inlet pressure, typically ranging from 0.01 bar - 0.03 bar (i.e. 10 - 30 mbar).
The inlet within LP-EGRs reaches 800°C (1472 °F).
Benefits
Low-pressure systems improve longevity and, often, decrease total emissions. Key benefits include:
Reduces risk of overheating by lowering total thermal stress on the system
Improves turbocharger efficiency
Reduces NOx emissions more dramatically than HP-EGR if engine loads are high
Can reduce the frequency of necessary DPF regeneration by reducing total soot production
Drawbacks
There are a few "cons" to LP-EGRs, particularly when compared to high-pressure alternatives. For example:
Slower response to engine changes
Lower recirculation rate (roughly 27%, on average)
Increased risk of condensation and oil contamination
Often requires more powerful intercooler
Can increase risk of DPF clogs by inadvertently disrupting DPF regeneration
The lower gas temperature can hinder the fuel oxidation necessary for combustion-regeneration
In order to balance the benefits and drawbacks of both EGR types,many modern trucks incorporate a hybrid (dual loop) recirculation system.
Hybrid EGR (Dual Loop)
Hybrid systems offer the benefits of both types. They incorporate two intersecting loops. The ECU switches from one EGR to the other, balancing performance needs with fuel efficiency. The control unit accounts for variables like engine load, external and temperature, speed, and soot density when switching between systems.
Studies of dual-loop and hybrid EGR systems show clear gains in efficiency, emissions reduction, and performance.
Benefits
The benefits of dual loop / hybrid EGRs are clear and quantifiable.
According to comprehensive comparison studies (one published in Fuel and another in Diesel Engine: Combustion, Emissions and Condition Monitoring) HP-EGRs win on several fronts. Compared to high- and low-pressure alternatives, dual loop systems:
Increase thermal efficiency by 3.4% − 7.3%
Reduce gas consumption by 5.82% − 8.83%
Increase fuel saving rates by 4.02% − 6.91% (for heavy-duty trucks)
Improve brake thermal efficiency by 12%-22%
Offer the greatest reduction of NOx emissions in several circumstances
In studies of heavy-duty trucks with medium loads, hybrid EGR systems reduced NOx emissions by 50%
HP-EGR systems reduced NOx emissions of HDVs with similar loads by 26.8%- 41%
Hybrid EGRs can often also reduce NOx emissions without sacrificing as much torque as HP-EGRs.
Drawbacks
Hybrid EGRs are the best option by almost all metrics. The primary drawback is cost.
These dual loop systems are more complex, and they involve more recently invented components. As a result, installation and maintenance can take more time than they would for either alternative option.
The system also costs more overall, both to install and to repair if any components break.
Benefits of EGR Systems
The key benefit of integrating an EGR system into your truck is the increased ease of compliance with emissions regulations. Meeting EPA standards by other means is challenging. These systems are the most straightforward paths to staying legal and avoiding environmental fines.
Within that benefit is a bigger picture “pro”: improved air quality. With a functional EGR system, your truck will produce far less NOx, making the air a little bit cleaner for everyone.
It's a central pillar of an effective, overarching emissions control strategy. That said, for best results, ERGs should coordinate with SCR systems.
EGR System Trouble: Signs & Symptoms
Like any automotive system, ERGs can wear down, take damage, and even fail completely. Stay alert for these signs of trouble:
Decreased fuel efficiency (increased consumption)
Rough (unstable) idle
Delayed or irregular acceleration response
Engine stalling out
Reduced power on inclines
Notable fuel smell (unburned diesel)
Black smoke from exhaust
Failed emissions test
Engine makes knocking or pinging sound
CEL (Check Engine Light) illumination
While EGR problems create a host of symptoms, there are fortunately only a few likely underlying causes.
EGR System Failure: Common Causes
Four culprits are to blame for almost all EGR issues.
The EGR Valve Is Stuck In One Position (Or, It Swings Too Readily)
When the EGR valve is the problem, you're most likely to experience a rough idle, some degree of power loss, and the illumination of the check engine light.
These valves can get stuck due to a buildup of carbon (soot) gumming up the hinges. Poor quality fuel and manufacturing defects can also cause sticking. In these cases, the solution is often to simply remove and clean the EGR valve with an appropriate cleansing solution.
If the valve swings too freely, it may be due to wear and tear or vacuum line leaks. Wear and tear requires a precise inspection to assess. It's typically resolved by replacing worn components.
Leaks are often resolved by applying new gaskets, tightening attachments, and repairing or replacing the lines themselves.
An EGR Cooler Is Clogged
The EGR Cooler and DPF can both end up clogged, typically by soot accumulation or contamination by coolant. When these clogs are the problem, you’ll usually experience the check engine light switching on, a rough idle, fairly reduced engine performance, and noticeably increased exhaust smoke.
Clogs carry a significant risk of overheating. The solution is to locate and remove the clog.
An EGR Cooler Is Cracked
When the EGR cooler is cracked, it typically creates white or gray smoke fumes offgassing through the exhaust pipe, a noticeable coolant levels drop (without visible leaks), and, often, the illumination of the check engine light.
Cooler cracks are usually caused by thermal stress on the system (i.e. the EGR overheats). High pressure from heavy loads, poor coolant flow, corrosion or design flaws can also be contributing factors.
Cracked EGR coolers can't really be repaired. Replacement is necessary.
Sensors Are Malfunctioning
Just about all of the symptoms of EGR problems in general can indicate a problem with sensors. Sensor issues can be serious, as faulty readings may lead to poor responses by the control module, worsening EGR performance across the board.
Fortunately, the solution to sensor dysfunction is fairly straightforward, if not necessarily easy. Simply identify the problem sensor, then clean or replace it.
How To Diagnose EGR System Problems
Determining the root cause of an EGR problem requires a precise, thorough diagnostic process. First, engage in your own visual inspection of the exhaust system. Take out discrete components carefully, then look each part over for soot, leaks, and corrosion. Take note of anything you find.
Then, work with a professional to perform a thorough exhaust system diagnostic check. This involves using specialized diagnostic tools developed specifically for heavy-duty trucks.
At Point Spring, we use the OBD-II scanner to assess your truck's systems directly, gathering firsthand data. The patterns the scanner recognizes will be matched against diagnostic codes to hone in on the most likely fits.
The scanner works in conjunction with our onboard diagnostics (OBD) tools. Altogether, they empower our technicians to incorporate analysis of complex scan data, exhaust and gas sensor input, telematics, and predictive analytics.
How To Prevent EGR System Failures
While no actions can completely guarantee your EGR system won't fail, there are things you can do to improve its likelihood of success.
Use Low-Ash Engine Oil
Low-ash oil is higher quality, and it helps reduce soot buildup. Be sure to choose an engine oil compatible with your system.
Adhere To A Regular Maintenance Schedule
Abide by the maintenance schedule recommended in your truck’s owner's manual.
In general, plan to clean the soot from your EGR valve and neighboring components every 40,000-60,000 miles. Coolant typically needs to be changed out every 50,000 miles. For many models, valves need to be replaced every 80,000-100,000 miles.
That said, every model of truck is different. If your truck's manual makes recommendations that diverge from these, go with the manual.
Avoiding Prolonged Idling
Some amount of idling is inevitable. But, limit it absolutely as much as you can. Idling increases soot accumulation and tends to leave unburned fuel in the exhaust. The more you can avoid doing that, the better.
EGR System Failure: Repair or Replace?
When your EGR system is malfunctioning, you might only need to repair a few damaged components. Or, you might have to replace those parts entirely—or even replace your system as a whole.
Which choice is best depends on several factors. Ask yourself:
How much does it cost to repair the components vs replace them?
Would a full EGR system upgrade ultimately net a strong ROI in terms of improved fuel efficiency and safety?
How old is your HDV?
How hard is it to get the OEM components you need for a particular repair?
How severe or widespread is the damage?
To what extent can you address the underlying cause of the damage?
Discerning the most accurate answers to these questions can be challenging. When you're dealing with complex ERG issues, it's worthwhile to get the opinion and insights of a professional automotive technician. The guidance of certified masters can make these decisions a bit easier.
Maintain Your EGR System With Point Spring
EGR system maintenance is an underrated part of truck care. Keep up with it, and your HDV will thank you.
At Point Spring & Driveshaft Co., our technicians can help maintain, repair, or upgrade your EGR. Talk to us and we'll work with you to understand whatever your truck's problem might be. Solving those problems is our promise.