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What is it, how does it work, why does it get angry in used cars

The term “EGR valve” has probably been heard by everyone whose used diesel engine stopped pulling or did not start. Whoever picked up a repaired car with an invoice for twenty-five thousand crowns from the service center must have thought to himself, what kind of damned stupidity is this, which only costs money and is of no use. After all, everywhere on the Internet, various tricksters offer the possibility of blinding the EGR, so that there is peace of mind.

But EGR (exhaust gas recirculation) is not in the car like some kind of filter or so-called. “whore”. The so-called The EGR valve is a necessary part of modern diesels (and many gasoline engines) in order for the car to meet strict emission standards.

You probably know that emission standards monitor several items. In addition to carbon monoxide, also unburned hydrocarbons, solid particles and, from the Euro 3 emission standard, also nitrogen oxides (0.5 g NOx per km for diesels and 0.15 g NOx per km for gasoline engines, today 0.08 g/km NOx for diesel and 0.06 g/km NOx for gasoline). And the EGR valve is meant to reduce nitrogen oxides.

Why are nitrogen oxides formed in the engine? They are formed at high temperatures during combustion by oxidation of atmospheric nitrogen, as air contains 21% oxygen and 78% nitrogen. Under ideal condition, it would look like this in the engine: 2 C8H18 + 25 O2 = 16 CO2 + 18 H2O + energy. Nitrogen would then be expelled again. But combustion is never absolutely perfect. In addition to undesirable substances in the fuel, the engines do not work in a completely ideal mode anywhere, and often too lean or too rich a mixture is burned. And precisely the burning of a lean mixture with an excess of air leads to a higher formation of nitrogen oxides. Nitrogen binds to free oxygen faster than carbon.

Photo: Petr Dorotovič

This is how it looks in a lot of used diesels. An example of a clogged valve from a second-generation Octavia with a TDI engine after an indicated 380,000 km, but it will probably have more mileage. The reason for such deposits is primarily imperfect combustion combined with already poor engine condition (oil consumption, etc.). Not even chiptuning will help.

This happens mainly in diesel engines, which in principle do not need a throttle and work with excess air. Of course, some petrol engines can also work with excess air. The drive unit can work in the so-called mode when a lower performance is required during steady driving. lean mixtures. We wrote about it, for example, in the article about the BMW N53 engine, where the necessary reservoir catalysts for nitrogen oxides cause quite a lot of trouble.

EGR valves began to spread already in the 70s of the last century with the first emission regulations, because carbon dioxide had not yet been addressed (the demonization of this completely self-evident product of any energy conversion in nature is still ongoing today). They began to deal with truly harmful substances, i.e. unburned hydrocarbons and nitrogen oxides.

The principle of the EGR circuit is relatively simple. It is supposed to reduce the burning temperature and at the same time reduce the excess of oxygen, thus reducing the conditions suitable for the formation of nitrogen oxides. This can be achieved by collecting the necessary amount of exhaust gases from the exhaust and sending them back to the intake and to the combustion chamber. And that’s what the EGR valve is for.

Internal vs. external EGR

Now you’re probably thinking that there doesn’t have to be some naughty trick in the engine for that, and that it can be achieved just by simply tuning the internal combustion engine. And you are partly right. Thanks to variable valve timing systems and suitable distribution timing, it is possible to extend the opening of the exhaust valve (or, more often, to speed up the opening of the intake valve) and use this overlap to suck part of the exhaust gas into the cylinder without having to take a detour to the intake.

But this only works to a certain extent and only for engines that do not produce excessive amounts of NOxi.e. in principle for petrol engines and without direct injection. But some engines with it also managed it. For example, the 1.2 TSI EA 111 engine (e.g. in Škoda Rapid) was able to do it, which did without an EGR valve, and thanks to this, it does not suffer from excessive carbon fouling. Most modern gasoline engines with direct injection must have an EGR valve anyway, because this solution with the so-called “internal” EGR is not sufficient. And that’s why a detour route is used, i.e. through the aforementioned valve. And this is already called “external EGR”.

How the EGR valve works

In the case of external EGR, it is a device in the form of a valve or slide valve for regulating the passage of exhaust gas back into the intake in an arbitrarily set ratio (from 0 theoretically to 100%). At first, a mechanically controlled valve controlled by negative pressure was used, today a combination with electronic control, or exclusively electronic control, is used more often. It is more efficient and accurate.

Demands for an even more accurate or faster EGR function then forced the manufacturer to proceed with additional functions, i.e. primarily active cooling of exhaust gases using an EGR cooler. By cooling the flue gases, their volume can be increased, similarly to how the volume of intake air can be increased in turbo engines using a compressed air intercooler. The EGR cooler is connected directly to the engine cooling circuit.

In turbocharged engines, the EGR circuit is also distinguished according to whether it is high-pressure (exhaust gas returns to the intake before the turbocharger) or low-pressure (exhaust gas returns after the turbo and catalyst with particle filter). The motor then regulates which circuit will be used as needed. Most often, when the load is low, the flue gases go through the high-pressure path, and when the load is high, the low-pressure path.

The advantage of the low-pressure circuit is that the flue gas collected after the particle filter is clean and does not clog the valve. But if the particulate filter stopped performing its function, the valve would be destroyed. Therefore, for example, with new TDI engines of the EA 288 series, it is not a good idea to remove or puncture the particle filter over time. With the relatively reasonable prices of branded DPFs, such interventions are really pointless.

Why the EGR valve is angry

Now it is clear to you that where there is exhaust fumes, there is dirt, soot, plague and death. And the vast majority of defects in the EGR circuit are related to this, especially the valve as such. But the EGR may not be the only one to blame, because of course the condition of the engine, the quality of the combustion and the overall condition of other peripherals have an effect on its proper functioning.

Photo: Petr Dorotovič

If the engine is working properly, the EGR valve can be fine and fully functional even after a long run (even 250,000 km). In most cases, it is sufficient to clean it as a preventive measure. Petr Dorotovič, a specialist in BMW cars, shows the EGR from the six-cylinder diesel M57 just after a high run-in

The most common problem is that the EGR valve simply becomes clogged with carbon, which paralyzes its operation. But this is not primarily the fault of the valve as such, but of the processes in the engine. If there is an excess of fuel in the engine (for example, after chiptuning), or if it is already burning some oil, there is of course more “grease” that then sticks to the exposed places in the EGR circuit.

Photo: Petr Dorotovič

Not only the EGR valve suffers from clogging with soot, but of course the entire intake. “Here is an example of the intake of a BMW N57 engine after about 130,000 km. Rather than a design error, it is rather an inappropriate driving mode with a high proportion of urban short routes,” Petr Dorotovič describes one of the main reasons for the accumulation of carbon in the intake

Deposits then limit or completely disable the function of the valve, but of course its control can also fail, be it a vacuum valve or an electronic regulation. If the valve gets stuck in the “closed” position, you won’t be able to tell from the car’s function at all. In the newer ones, the warning light already lights up, but in older cars, nothing may happen at all. The car starts, drives normally… It just has higher emissions. This is often the solution of various handymen. The valve closes, the car continues to drive, but it certainly does not meet the emission standard according to which it was homologated.

But more often it happens that the valve cannot be fully closed and its regulation does not work. An open valve makes starting difficult, or makes it completely impossible. When the car starts and the engine is kept running, a typical manifestation of a defective or clogged EGR is a loss of power when driving at higher speeds in the revolutions. Modern cars then go into emergency mode with limited power.

How to solve it?

Sometimes the valve can simply be cleaned. Some services even recommend it as prevention for problematic engines (2.0 TDCI, 2.0 CRDI). Various preparations help with the dissolution of deposits (there are many sprays and solvents for this purpose), but sometimes even this is not enough and it is necessary to switch to mechanical removal of deposits. You just need to scrape off the “coal” and then thoroughly clean it chemically. But carbon does not settle only on the valve, but in the entire intake manifold, even on the intake valves. This is a more demanding cleaning process that only great enthusiasts and do-it-yourselfers can handle at home.

Photo: Petr Dorotovič

“Diagnostics for the B47 engine wrote that the position was not reached. The cause of the valve sticking was the penetration of exhaust fumes around the valve stem into the EGR transmission housing. The whole thing was cleaned and now the valve works and doesn’t rub,” shows an example of a non-functioning EGR valve Dorotovič on a relatively new and modern engine

Photo: Petr Dorotovič

That you yourself clean the valve on a new car as a preventive measure? It just won’t work. “With the help of diagnostics, the EGR must be set to the service position, otherwise it will not work, or it will be destroyed immediately. It is no longer possible to simply remove the valve, clean it and put it back in like with the old diesel,” points out Dorotovič. This is not just BMW’s achievement, but the majority of cars of almost all brands already have it

However, if one of the mechanical or electronic components of the EGR valve control fails, the only option is to replace the valve (if it is not possible to repair or replace a separate defective part). But the practice is that the entire valve is simply changed. However, the exact cause of the defect should be revealed by thorough diagnostics, because the defect may well be on one of the sensors. And if the engine works badly, takes oil and burns poorly, you will change the EGR brakes again.

Of course, you can also encounter a leaking EGR cooler, which must then be changed. This, of course, increases the cost of the service. While a new EGR valve can usually be purchased within the units of thousands, the entire system with a cooler costs easily 10 to 20 thousand crowns. Of course, it depends on the brand. The exact price of repairs with the EGR circuit cannot be said from the side. It depends on the scope of the work, somewhere you may pay a little for parts, but then you have to pay for hours of work, elsewhere it is the other way around.

In conclusion, we remind you that even an elaborate EGR circuit is no longer sufficient for today’s emission standards, and therefore diesels must use selective catalytic reduction with the injection of technical urea, i.e. AdBlue. But it has an advantage (if we leave out all the risks and disadvantages). The EGR circuit does not have to be exploited as much anymore, and in some models with AdBlue it appears that the former problems with EGR have disappeared for good. But AdBlue will bring new ones, so don’t worry, the services will not lose their work (and all the money).