Signs & Symptoms of a Bad Turbocharger Intercoolerby Richard Rowe
Technically speaking, an intercooler as most of us know it is actually and "aftercooler." An intercooler is a heat exchanger that goes in between the two turbos in a series turbocharge system, while an aftercooler goes between the final turbo and the engine. But whatever you want to call that radiator thing in your turbo plumbing, it plays an important role in helping your engine withstand the abuses of boost. Intercooler failure is fairly rare, at least compared to failures in the plumbing between it and other components.
All substances in this universe -- at least those warmer than absolute zero, or negative 459 degrees F -- contain a certain amount of energy in the form of heat. Compressing a gas takes the heat energy it contains and concentrates it into a smaller area, thus increasing the net temperature of the gas. Dropping the temperature of a given volume of gas will drop its pressure. Cooler air contains more oxygen per cubic inch, which boosts horsepower, and it reduces the odds of fuel pre-ignition and uncontrolled detonation. The intercooler is a radiator that sheds some of this heat energy, cooling the intake charge.
Intercoolers and turbo plumbing are fairly simple things by nature, and there aren't too many ways they can fail. If an intercooler or the plumbing between your turbo and engine leaks, they'll vent boost pressure to the outside and cause a net loss of oxygen going into the engine. This will throw off your air/fuel ratio, causing the engine to run extremely fuel-rich. Since there's not enough oxygen available to burn the fuel going into your engine, you'll end up spewing raw fuel out of the exhaust pipe in a cloud of black smoke. Since every particle of black smoke is a drop of fuel, you can expect a severe drop in fuel economy as well as performance.
Those symptoms will always manifest to some degree or another, but turbo control systems and computers can use any number of approaches to help compensate for a boost leak. If the leak is fairly small, your turbo wastegate -- the exhaust bypass valve -- can help to keep boost pressure in the green zone by increasing turbo rpm. In cases like this, you may notice nothing more than a bit of lag upon acceleration and some extra turbo whine. In more severe cases, the computer will respond to the rich condition by going into its failsafe "limp home" mode, which is designed to make the car nearly undriveable by dropping power and restricting rpm.
Some cars -- notably the Subaru WRX STi -- utilize an air-to-water intercooler surrounded by engine coolant instead of an air-to-air intercooler mounted in front of the radiator. The only way an air-to-air intercooler can drop in efficiency is if something physically restricts airflow through the radiator or covers the cooling fins; dried mud will do it. Air-to-water intercoolers rely on engine coolant and can get clogged with mineral deposits such as calcium, lime and rust just like your radiator or heater core. An efficiency drop will raise intake air temperatures, causing a reduction in horsepower and an increased tendency toward engine knock and ping.