What Causes Exhaust Manifolds to Glow Orange on a Diesel?

All objects warmer than absolute zero glow with a certain amount of infrared light, a result of the vibrations of the metal's atoms. As the metal gets hotter, the light emitted leaves the infrared spectrum and moves into visible light; first red, then orange, yellow and finally white. The iron used in exhaust manifolds typically starts to glow in the visible red-light spectrum between 700 and 900 degrees Fahrenheit; it's fully red by about 1,200 degrees, turns orange about 1,800 and yellow somewhere near 2,500 degrees.

Diesels differ from gasoline engines partially in that they are fuel-throttled instead of air-throttled, which means that rpm is controlled by metering fuel instead of air. The more fuel goes in, the more power the diesel makes. But don't take that to mean that the diesel can make infinite power as long as you keep injecting fuel -- oh, no. Like all engines, diesels ultimately hit their power limitations according to how much air goes in; inject too much fuel too late in the cycle and the slow-burning diesel will run out of air and time to burn, and enter the exhaust manifold still burning several hundred degrees hotter than it should.

High EGTs -- in a diesel, at least -- are almost always the result of an excessively rich air/fuel ratio. This can happen one of two ways: either by injecting too much fuel for a given amount of air or by not having enough air to burn a given amount of fuel. The former typically happens after installing aftermarket diesel power tubers, which inject more fuel to make extra horsepower. A few things will cause an air -- or, more accurately, oxygen -- restriction in an engine. Boost leaks, clogged air filters and high altitude will result high EGTs. Any malfunction in the intercooler will decrease air density in the intake, which starves the engine of oxygen and increases EGTs.

Glowing red parts aren't particularly good for any engine, no matter how stout it may be. Sure, it would be hypothetically possible to make an engine capable of withstanding 2,000-plus degree operating temperatures, but why bother when it's never supposed to see more than 1,300 degrees? Exhaust manifolds themselves may crack, but that's the least of your concerns. Turbocharger failure is almost certain, and engine damage is even more likely once that heat backs up into the cylinder and block. A higher-flowing exhaust system is the quickest way to drop EGTs, but eventually you'll need to either decrease the amount of fuel injected or install a larger turbo to supply more air.