Understanding the basics Of EGTs means knowing what’s a high number, and how hot is too hot. One of the most basic measures of how a diesel engine is performing can be measured in exhaust gas temperature or EGT for short. Whether you’re on the street towing a trailer or at the track making a pass, having a way to monitor EGTs is an important indicator of whether your engine is going to be in trouble or not.
Measuring EGTs is not rocket science. To accomplish this, a probe (pyrometer) is inserted into the exhaust system. In the past, pyrometers were positioned in the exhaust stream after the turbo, mainly because if things got too hot and there was a failure with the probe, it wouldn’t go through the turbo. With new probes being constructed of better materials, however, failure is nearly unheard of, and it is safe to mount most probes right in the exhaust manifold, where the exhaust temperature is the hottest. This ensures a good reading of whether the turbo is in danger of failing or not. Turbochargers are tough, but give them enough pressure and exhaust heat, and even the toughest turbo can fail.
Deciding whether to mount the pyrometer thermocouple (probe) before or after the turbine is usually a matter of utilizing a suitable mounting location. However, if you do mount your probe after the turbo, the temperature at full throttle or when under load can be 200 to 300 degrees lower than if the EGTs were measured in the exhaust manifold. This temperature difference is why the installation of the thermocouple in the exhaust manifold is considered more accurate.
If EGTs Are Important, Why No Gauges From The Factory?
One frequently asked question concerns the OEMs and exhaust gas temperature. Many want to know why trucks do not come with EGT gauges from the factory. The answer here is somewhat comical, but also somewhat obvious. Most companies figured that consumers wouldn’t understand the gauge or what it means. Some might treat it like a tachometer and try to wind it out, other consumers might view it like a temperature gauge, and get nervous at anything over half readings. Since most all trucks have pre-programmed limits for maximum exhaust gas temperatures, the Big Three consider an EGT gauge unnecessary.
How Do You Read An EGT Gauge? Part One: Start-Up And Warm-Up
Diesel engines need a certain amount of heat to operate efficiently, which is why cold starts can be such an issue if the climate is absolutely frigid. Many older engines used glow plugs to preheat the cylinders. Late-model trucks incorporate a grid heater to get things going. Regardless of which warming method is employed, as soon as the engine is running, the exhaust gas temperature will be very low — maybe only 300 or 400 degrees.
EGTs During Normal Driving
What makes EGTs interesting, is that they are an almost infinitely variable value that depends on load, engine performance, and even air temperature and pressure. If you’re tooling down a highway at normal speeds though, it should be between 500 and 800 degrees. Just like the warm-up cycle, it’s unlikely that you’re going to hurt anything at this stage, but this is where EGTs also start to get interesting, as it is an indicator of engine and vehicle performance.
Even during normal driving, exhaust gas temperature is a value that can tell you how efficiently your engine is working at a given moment, gear, and speed. In general, the lower the EGTs during cruising speed, the more optimized your vehicle is, and the better your fuel economy will be. With fuel prices what they are currently, an EGT gauge can be a welcome addition on even a stock truck, as it will tell you what speed and gear you’re the most efficient. Obviously, there are exceptions, but in the 1,000 to 2,000 rpm range, EGT during cruising can tell you a lot, or even warn you of potential problems with injectors or turbos. If your truck is DPF-equipped and your EGT suddenly goes up to 1,000 degrees or more, you don’t need to worry about that either, it’s part of the DPF cleaning cycle, and the only thing that’s going to suffer is your fuel economy.
Towing: Where Things Start To Get Dangerous
We’ve all seen spectacular engine explosions in competition vehicles, but when it comes to roasted turbos and melted pistons, towing has taken out more engines than anything else combined. From the factory, diesel trucks come with a preset EGT limit, either through basic tuning (like the AFC and fuel plate in ’94 through ’98 12-valve Dodges) or through computer tuning (virtually all newer trucks). Problems while towing can still happen with a stock vehicle, although EGTs are usually kept in check at a level of 1,000 to 1,200 degrees, which means even foot-to-the-floor upgrades aren’t going to hurt anything.
Ok, so we’ve covered stock, but what if your truck isn’t stock? There are still thousands of parts that add horsepower to your diesel truck that range from tuning, to injectors, to turbos, to modified fuel pumps. All of these pieces of the performance puzzle can increase power, but more than likely, your exhaust temperature will also go up. Those with DPF-equipped trucks are particularly vulnerable, as anything over 1,250 degrees can actually melt the substrate inside the DPF and cause a failure. Even with earlier trucks, while you might be able to push to 1,300 or 1,400 degrees for a short climb (a minute or two) we’d avoid any higher than that for a sustained period. In fact, if you’re in doubt at all, just stick to the 1,200-degree rule.
Why An EGT Gauge Instead Of An Air/Fuel Meter?
If you come from the gasoline side of things, you might wonder why most diesel engines rely on an EGT gauge instead of an air-fuel-ratio (AFR) meter. There are a few simple reasons. First, diesel engines operate within an incredibly wide range of air/fuel ratios — from more than 30:1 at idle to 10:1 in the case of some sled pullers. Running a diesel engine too rich or too lean can be a threat to engine parts. But the air/fuel ratio won’t tell the whole story. Also, most commonly used air/fuel sensors fail frequently and get clogged up with soot in a diesel application, as opposed to EGT probes which rarely have to be replaced. We all know how sooty a diesel exhaust can be…
Racing And Competition: Pushing The Envelope
If you’re looking for a pile of broken parts, diesel sled pulling, dynoing, and/or drag racing is usually a good place to start. Diesel trucks weren’t meant to drag a 40,000-pound sled or run 150-plus in the quarter mile, but that doesn’t stop a hearty few from trying it. A competition sled puller or drag truck will have a whole slew of aftermarket engine parts, including the crank, rods, pistons, and even engine block. If you’re spending tens of thousands of dollars building a competition engine, you definitely don’t want to break it.
Just like you can stick your hand in a 400-degree oven for a few seconds without getting burned, racing diesel engines can result in much higher EGT levels without melting themselves down. A temperature of around 1,500 to 1,600 degrees is good for maximum power, but peak EGT readings can be even higher, almost 2,000 degrees for a few seconds in some cases. With such extreme heat, usage is limited to about 15 seconds or less, as any longer than that can result in damage to the turbocharger, valves, and even the pistons. In some cases, the engine might have to be completely rebuilt.
EGTs And Your Engine
When it comes to exhaust gas temperatures, the most important thing to remember is these are basic guidelines. There’s no exact number that will result in damage to your engine, it depends on your specific setup, the duration of the heat endured, the load on the vehicle, and how you’re driving. If you’re nervous about numbers, stay as close to stock as you can. On the other hand, in a competition vehicle, you may be able to push it as much as the rules will allow. Just remember, you have to be able to afford to fix it too! Once you understand them, EGTs aren’t a big mystery, they’re just another tuning tool available to extract maximum performance from your diesel.