Given that EGR valves exist solely to control the flow of mostly untreated exhaust gas into engines, it is hardly surprising that EGR valves fail on almost all applications as often as they do. Let us put this into some perspective-
The only reason why exhaust pipes/systems/tubes do not clog up quickly is that exhaust systems have a) relatively large bores/inside diameters, and b), exhaust systems are designed to take advantage of several laws of physics to drive the flow of exhaust gas through the system. Put differently, this means that instead of the engine’s cylinders pushing exhaust gas through the exhaust system until the gas leaves the tailpipe, modern exhaust manifolds are designed to accelerate each "plug" of exhaust gas as it exits a cylinder.
This process is known as “exhaust scavenging”, and it has the effect of a) somewhat compressing the exhaust gas that exits each cylinder, and b) launching each somewhat compressed plug of exhaust gas into the exhaust system proper at a velocity that is significantly higher than when the gas exited the cylinders.
Following the laws of physics, each plug of high-density exhaust gas creates an area of low pressure behind it as it travels along the exhaust system. The next plug of compressed exhaust gas follows the first plug but with a small area of lower pressure between them. This ensures that each low-pressure area “draws” the plug of high-density exhaust gas behind it as it moves along the exhaust system, and it is this phenomenon that produces the pulsing sensation one experiences when holding one's hand close to the end of the tailpipe while the engine is running.
In most OEM-designed exhaust systems, the process of scavenging exhaust gas from the engine is highly efficient but by way of contrast, EGR valves generally do not have any of the advantages of a properly designed exhaust system.
In fact, in terms of flow dynamics, the design of most EGR valves actively prevents the free flow of exhaust gas through them. Small orifices, acute changes in the direction of flow, low exhaust flow rates, and the composition of exhaust gas all actively prevent EGR valves from being efficient, even in EGR valve designs that are labelled as high-efficiency valves. With that said, let us look at the main reasons why EGR valves and by extension, EGR systems, fail-
Excessive carbon deposits
Of course, the above is not the same as saying that all, or even most EGR valves do not do what they are designed to do- far from it. As a general rule, all EGR valves perform as expected when they are new, but carbon deposits begin to accumulate inside the valve within about a year or so of use. Depending on the valve’s design, carbon and soot deposits typically begin to accumulate around the points where the exhaust gas’ direction of flow changes, on the inside surfaces of orifices, or on the valve pintle.
Since the deposits that settle at these points represent obstructions in the exhaust gas flow path, oil, soot, and fuel residues settle on the obstructions (existing deposits), and over time, carbon deposits may reach the point where exhaust gas can no longer flow through the valve freely- if at all.
The same thing often happens in EGR systems in which exhaust gas travels through long feed pipes on its way to the EGR valve. Since the exhaust gas cools down during its movement through a feed pipe, volatile constituents of exhaust gas, with vaporized oil being a prime example, condense and settle on the inside surfaces of the feed pipe. Over time, the oil residue becomes carbonized, and over longer periods, the accumulated carbon begins to impede the flow of exhaust gas through the feed pipe. This process is continuous, and at some point, the flow of exhaust gas through the feed pipe is cut off altogether.
Although there is no single period that applies to all EGR vales and feed pipes, most EGR valves in use today will provide relatively trouble-free service for about four years or so. However, bear in mind that factors such as fuel type and quality, the predominant use of the vehicle, the type of oil used in the engine, and the overall mechanical condition of the engine all play critical roles in the question of how long an EGR valve should last.
Since all of the aforementioned factors influence the composition of exhaust gas, the EGR valve on one engine could last far longer than four years, while the EGR valve on an identical engine in another vehicle may only last for a few months. This could be because a) the second engine may be burning excessive amounts of oil because it is in poor mechanical condition, or b) the second engine suffers from built-in factory defects; some Honda and KIA engines come to mind in this regard.
Clogged flow channels in cylinder heads
On some engines, exhaust gas is introduced into the cylinders through small-diameter channels that originate at the EGR valve. Given the nature of exhaust gas, it will be readily understood that these small channels will clog up with carbon relatively quickly and easily.
In fact, on some German-made vehicles that are notorious for their high rates of oil consumption, the EGR flow channels in cylinder heads typically become clogged in well under two years after first use, and sometimes, in less than a year. Regardless of how soon it happens, though, clogged flow channels in a cylinder head effectively disable the entire EGR system since exhaust gas cannot flow into the cylinders.
In terms of practicalities, there are no known ways of opening or clearing clogged flow channels in cylinder heads without removing the affected cylinder head(s), but there are no guarantees that clogged flow channels can always be cleared, regardless of the methods employed. In some cases, though, and provided that the carbon deposits in the flow channels have not carbonized fully, it might be possible to clear the channels by soaking the entire cylinder head in a suitable solvent for a few (to several) days, but again, there are no guarantees that this will always be successful.
Unfortunately, if the deposits are fully carbonised in the flow channels, the only remedy for the problem is the replacement of the affected cylinder head.
Control system defects
Control system failures can take many forms, but the most common such failures could include one or more of the following-
Typical wiring issues include damaged, burnt, shorted, disconnected, or corroded wiring and/or electrical connectors anywhere in the wiring that is associated with the control and/or monitoring of any component(s) of the larger EGR system.
This simply means that the EGR valve does not respond to commands from the ECU (Engine Control Unit), or the EGR valve does not respond appropriately to commands from the ECU.
As a rule of thumb, such failures are caused by excessive carbon deposits inside EGR valves that prevent the valve’s pintle from moving freely or causing the valve to be stuck in a fully or partially open or closed position. Note that these conditions will set trouble codes relating to excessive or insufficient exhaust gas flows.