The Evil Of EGR On Diesel Engines


Your engine’s manifold absolute pressure (MAP) sensor is your little buddy. Buried somewhere in the stream of your charged intake air (usually in the manifold), it employs a little ceramic or silicone element that senses variations in manifold pressure. The pressure changes indicate engine load, boost, operational elevation, and other pressure variables your ECU needs to know about to get your diesel to run at peak performance.

But if your MAP sensor is your buddy, your exhaust gas recirculation (EGR) valve is the bully with the huge forehead who stole your lunch money and pants’d you in front of the cheerleaders. In a gasser, EGR isn’t too obnoxious because gas engines don’t create much soot. But in a diesel, well, we’re sure we don’t need to tell you about all the gritty, dirty carbon that little EGR bully throws back through your intake, cylinder head(s), and combustion chamber.

f your diesel is saddled with an evil EGR system and you’re suddenly noticing your mileage plummeting and your performance suffering, do yourself a favor and pop off your MAP sensor to make sure it’s not coated in a nasty, gooey, sheathing of soot. Our ’07 6.7L Cummins was. It took about 5 minutes to remove, clean, and reinstall our MAP sensor in our Dodge Ram. The reward was a small bump in fuel economy (about mpg) and much snappier throttle response and top end pulling power.

What Is EGR?

Exhaust gas recirculation (EGR) is a system that takes exhaust gas from a diesel’s exhaust manifold and reintroduces it into the intake of the engine to reduce the combustion chamber temperature.

Why do modern diesels have EGR? you ask. Basically, EGR is used to reduce the formation of NOx emissions, which are created when the combustion chamber temperature exceeds 2,370 degrees. Pumping exhaust (EGR) back into the engine helps to cool the combustion chamber because EGR has almost no oxygen in it. Without oxygen, the temperature stays below the NOx-producing threshold, and the emissions coming out the tailpipe are cleaner.

Unfortunately, the EGR gasses from a diesel tend to bring a sooty mess with themit’s a lot like cholesterol in your body. That sooty mess tends to gum up the EGR valve, EGR cooler, intake ports, and any sensors that are exposed to it. Take a look at these two 6.0L Power Stroke EGR valves. The one on the right is almost new, and the one on the left is so gummed up from EGR that it stopped working. And keep in mind, that as bad as EGR is, if your diesel is tuned to blow black smoke out the exhaustyour EGR system is being exposed to that same stuff!

Courtesy of David Kennedy of Diesel Power