What Can Happen When a Heat Shield Falls Off of a Manifold?

Heat shields aren't just another pretty face.

While this might not be a problem for some cars, bear in mind that your car's manufacturer didn't spend millions of dollars in engineering time to install the heat shield for decoration. Odds are good that, if you have a heat shield, then you've got a cramped engine compartment and components in it that don't respond well to sustained high temperatures. In short, the entire engine bay was designed with a heat shield in mind.

The Basic Problem

Heat and infrared energy aren't quite the same thing, but they are two sides of the same coin. Infrared energy is light, and like any kind of light it transmits in a straight line. Placing anything in front of that light creates a shadow. That's why you suddenly feel cooler when you're standing near a fire and put a hand up in front of your face: because most of the fire's heat energy is reaching you through infrared radiation. The inside of the heat shield is -- or should be -- polished like a mirror, reflecting that light back toward the manifold.

Melted Parts

Modern cars use an enormous amount of plastic on and around the engine. Plastic is cheap to manufacture and can hold up well for years, but is easily damaged by prolonged exposure to heat. The infrared energy pouring off of a 500-degree manifold will bake anything made of plastic or rubber with a direct line-of-sight to the manifold, including the fan shroud, radiator hoses, plastic electrical connectors and anything else sensitive to heat. Prolonged exposure to heat will eventually weaken and crack plastic, and will degrade rubber and nylon. And that's saying nothing of the computer, which may not be far from the engine.

Reduced Engine Efficiency

The outside of your heat shield is only half of the equation. What happens to all of the energy that bounces back toward the manifold? The manifold itself is dark in color, which absorbs light energy. That heat energy goes back into the exhaust manifold. The hot manifold raises the pressure of the gas inside, forcing them to speed up in order to get out of the manifold. The engineers that built your car designed the manifold with certain gas velocities in mind, so reducing pressure in the manifold could ultimately wind up costing you horsepower and torque.

Loss of Fuel Economy

A catalytic converter acts something like a blast-furnace, using unburned hydrocarbons from the engine to maintain a thermal reaction that converts harmful pollutants in your exhaust stream to more inert forms. Removing the heat shield deprives the converter of needed thermal energy, and the computer is likely to compensate by injecting a bit more fuel to bring the converter back up to temperature. The net result is a loss of fuel economy that will vary depending upon how much your cat relies on the heat shield. Again, not all cars are designed this way, but those utilizing converters designed with a heat shield in mind generally are.

References

• Engine Management: Advanced Tuning; Greg Banish
• A Technician's Guide to Advanced Automotive Emissions Systems; Richard Escalambre