What is a boost leak, and why should I test for one?
If you're new to the forced-induction world, there's something you should be aware of: boost leaks. They cause reduced power, increased black smoke, lower engine efficiency (fuel economy), and high exhaust gas temperatures. They can also cause turbo damage over time due to over-speeding.
Modern-day diesel trucks, including the 89+ Dodge Cummins, 92+ GM Diesel, including the 6.5L Turbo Diesel and the 6.6L Duramax, and 94+ Ford Powerstroke diesels all used forced induction via a turbocharger. The turbo ingests air and pressurizes it much higher than atmospheric pressure, pushing it into the engine's intake. This allows a machine to make a LOT more power than it could naturally be aspirated (an air intake tract that is not pressurized). Being the intake tract between the turbo and the intake manifold/plenum, the air connection is pressurized as little as 7PSI on a stock engine to over 100PSI at full load on heavily modified engines.
The entire tract must be sealed entirely between the turbo and intake of the engine. If there are leaks in the system, be it from a torn intercooler boot, improperly clamped intercooler boot, damaged or leaking intercooler, or a blown-out intake manifold gasket, you suffer from a boost leak. Your turbo is attempting to pressurize the intake tract, but some air leaks or "escaping" to the outside atmosphere. This means some of the air that was meant to be forced into the engine to make power and properly burn the diesel fuel, is in effect, wasted. If the system isn't sealed 100%, you aren't making the most use of your turbo.
Intercooled diesel engines, found in 91.5+ Dodge Cummins, 1999+ Ford Powerstrokes, and 2001+ Duramax Diesel Engines are more prone to boost leaks due to a higher number of connection points. They generally have a boot connection at the turbo to the intercooler pipe (leak potential #1), a boot connection at the intercooler (leak potential #2), the intercooler itself (leak potential #3), the intercooler to pipe boot at the intercooler outlet (leak potential #4), the intercooler pipe to intake plenum boot (leak potential #5), and finally, the intake manifold/horn to engine gaskets (leak potential #6).
Stock intercoolers and intercooler boots are designed to hold factory boost pressure levels. When you add a chip, power module, programmer, or larger-than-stock fuel injectors, you will increase the boost pressure because all of the above items add fuel to achieve the desired horsepower gain. More power equals more boost. More boost can equal burst intercooler boots, blown out intercoolers (especially those with plastic end-tanks), and in some cases, blown intake manifold/intake horn gaskets.
Sometimes under heavy acceleration, an intercooler boot may "blow-off." The boot is still intact, but the clamp holding it may not have been torqued enough to control the boot onto the intercooler pipe, intercooler, or turbo outlet. They will often make a vast BANG when they blow off due to sudden expulsion of boost pressure, and the tube may have hit something under the hood at the same time. When this happens, you may lose up to several HUNDRED horsepowers, as the engine is now, in effect, naturally aspirated. In addition, you may notice smoke out the tailpipe and high exhaust gas temperatures (if you have a pyrometer installed), along with a general lack of power.
Repair is relatively simple, depending on the truck and which boot "blew." You'll have to loosen the intercooler clamp, slide the boot (and clamp) back onto the pipe/intercooler/turbo, then re-tighten the clamp. Generally, this is enough to solve the issue, but sometimes there is a fine line between getting a clamp just tight enough and stripping it out. Be careful. If you don't have a spare, you may end up limping your truck to a parts store to try to source a clamp.
Some trucks vent the crank-case vent into the intake of the turbo. This means that any blow-by/oil vapors will end up in the intake tract and intercooler. The OEM boots, in most cases, are not oil-resistant. Over time the exposure to oil will break down and soften the boots. This makes them more prone to bursting or blowing off. Heavy-duty aftermarket boots have a better oil resistance and are usually thicker and much more robust than factory intercooler boots. Because most aftermarket intercooler boots are denser, it is a good idea to purchase new clamps at the same time to make installation easier.