Diesel EGR problems

Exhaust Gas Recirculation (EGR) valves cause more than their fair share of problems.

Problems with the EGR system, like so many vehicle problems, are often caused by a lack of maintenance: diesel owners typically expect high mileages and low maintenance bills. Because owners expect their diesels to go for huge mileages without much maintenance, they don’t pay much attention to their diesel engines until something goes wrong. By then it may be too late. Also, many diesel owners resent paying to have their engine’s EGR valve replaced at a fairly early age (it can be expensive). However, faulty EGR valves can sometimes be fixed by simply cleaning them. Moreover, many EGR problems can be avoided by simply ensuring that the vehicle is maintained regularly and gets a decent blat down a highway at least once a month, with plenty of accelerating and decelerating (pottering around town at low speeds is especially bad for EGR valves).

Many owners have tried to remove the EGR assembly for this reason, but there are lots of fishhooks in the process. The first one is that the removal of the EGR assembly will confuse the ECU and trigger a ‘check engine’ light. There are, however, proper removal kits for those who wish to take this path. This removal kit has blanking plates for the manifold, together with a custom computer chip that fools the ECU into thinking that nothing’s changed.

There are lots of reasons for removing the EGR, and lots of reasons for not removing the EGR.

The Exhaust Gas Recirculation (EGR) valve is an anti-pollution device, aimed primarily at reducing the amount of nitrogen oxide (NOx) spewing out of the vehicle’s exhaust pipe. The engine produces nitrogen as part of the combustion process. As the temperatures inside the engine increase, this nitrogen and the oxygen in the engine’s combustion chamber can chemically combine to form nitrogen oxides. NOx reacts with sunlight to cause smog.

Exhaust Gas Recirculation works by recirculating a controlled portion of an engine’s exhaust fumes back through the engine. The EGR valve is the tap that turns the flow of exhaust gases on and off, hopefully at appropriate times.

These recycled, carbon dioxide-rich exhaust fumes cool the combustion chambers within the engine. This cooling effect lowers the combustion chamber temperature. These recycled exhaust fumes also displace some of the oxygen that would otherwise be drawn into the engine from the surrounding atmosphere. Because the combustion chamber is cooler and because there’s less oxygen, less NOx is produced.

Also, because the combustion chambers are cooler, the engine as a whole runs cooler. This is particularly important to the engine’s turbocharger: turbochargers (and the oil that they need for their lubrication) don’t last long if the engine is overheating.

Because the EGR’s recycled exhaust gas displaces some of the air within the combustion chamber, the engine’s efficiency is reduced. For this reason, the EGR system doesn’t operate during times of heavy acceleration. It also doesn’t operate during idle, because the presence of exhaust gases at idle tends to cause uncontrollable rough running.

EGR systems operate primarily when the vehicle is cruising under light load. Because there is less demand on the engine when the vehicle is cruising under light load, the engine can afford to temporarily lose some capacity. Also, because of the cooling effect of the EGR system, the engine can run cooler when the vehicle is cruising under light load. Because the engine is cooler, the engine’s lubricating oil is cooler as well. Cool oil lasts longer and works better than oil that has been continuously overheated. Therefore, the EGR system is important for the cool running of the engine and also in the protection of the engine oil from continuous heat.

In the real world, EGR valves often don’t work very well.

What goes wrong is this: the exhaust gases from the vehicle’s engine contain much more than just carbon dioxide: they also contain dozens of chemical by-products, left behind after the fuel was burnt inside the engine. One of these by-products is a fine dust, known as particulates. This dust is mostly unburnt carbon fuel. The more efficiently the diesel engine burns its fuel, the less carbon dust is produced. However, no diesel engine works with anything like complete efficiency and, over time, the carbon dust inside the exhaust gases begins to clog up the EGR system, reducing the movement of the EGR valve. This clogged EGR valve causes the engine to run badly due to an imbalance in the fuel/air mixture. Because the engine is running badly, the fuel is not being consumed efficiently. Because the fuel is not being consumed efficiently, more carbon dust is produced. Some of this carbon dust within the exhaust gets recycled by the EGR system, clogging the EGR valve still further. The more the EGR gets clogged, the more carbon dust is produced. The more carbon dust is produced, the more the EGR valve clogs. This viscious circle continues until the EGR valve jams completely open or closed.

If the jammed EGR valve is not repaired or replaced quickly, all sorts of problems may occur.

Four of the most common are rough running, high fuel consumption, turbocharger failure and, sometimes, total engine failure. Here’s how it works:

When the engine is cruising, the EGR valve is supposed to open to allow the carbon dioxide in the exhaust to cool the combustion chamber. However, if the EGR valve jams open, exhaust fumes are being fed into the engine at all times. This can mean poor acceleration and rough running, because the excess exhaust fumes are depriving the engine of oxygen. Because there is insufficient oxygen, there’s too much fuel, so the unburnt fuel starts spewing out of the exhaust as black smoke, especially when the engine is at idle. Fuel consumption is likely to rise substantially. Also, due to a lack of oxygen (which is needed to complete the combustion process), the engine sometimes starts to misfire, sometimes seriously (this misfiring may produce an unusual metallic rattle or knocking when the engine is under load). If the engine is left in this condition for too long, the engine life will be shortened considerably. In the worst case this problem may cause melted pistons and therefore engine failure. The valves in the cylinder head may also begin to stick due to being heavily coated with carbon.

However, other really nasty problems occur when the EGR valve jams closed: without the cooling effect of the exhaust gases, the engine starts to overheat. Over time, this overheating causes the engine oil that feeds the turbocharger to break down. When the engine oil starts to break down, the bearings in the turbocharger fail. When the bearings fail, the turbocharger will fail, and replacement won’t be cheap. If the turbocharger is replaced without solving the EGR problem, the next turbocharger will also fail before too long.

It gets worse: when the engine overheats, more lubricating oil can enter the combustion chamber and the engine can start running on lubricating oil even if the diesel fuel supply is cut off. The engine may then begin to run on its own engine oil instead of diesel fuel. The driver may be unable to switch off the engine except by stalling it. Eventually, the engine sucks away all the lubricating oil and the engine disintegrates.

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