Catalytic converters get very hot -- upwards of 900- to 1,000-degrees F in some cases. These kinds of temperatures are more than enough to automatically ignite grease, oil and transmission fluid, particularly if it happens to come into direct contact with the catalytic converter, often referred to simply as the "cat." However, these kinds of temperatures also have a way of expanding metal to the extremes of its tolerance; extremes that it may or may not completely return from. Converter heat shield rattle is indeed annoying, but is remedied with a few basic materials.
Step 1 - Spray the converter's heat shield bolts with penetrating oil about an hour before you go to work on them. Converter bolts will heat-seize just like any other bolts in your exhaust system, and a bit of penetration oil can go a long way toward preventing a mishap with snapped bolt heads.
Step 2 - Remove the heat shield bolts and then the heat shield. Wash it with soap and water and take a look at its inside. Odds are that you'll find it dull and discolored. You want the inside of your heat shield bright and shiny so that it reflects heat back toward the converter instead of soaking it up and radiating it into your back into your engine. Go ahead and polish the inside of the heat shield with some 200-grit, 400-grit and then 800-grit sandpaper.
Step 3 - Cut out a few two-inch long sections of header wrap. Header wrap is a gauze-like fabric that hot rodders often wrap around header primary tubes to keep heat inside the header. This layer of insulation keeps heat inside the header where it can increase pressure and exhaust gas velocity instead of heating the engine bay.
Step 4 - Lay half-dozen washers sized to your heat shield bolts flat on the header wrap. Hit them with a quick coat of fast-drying spray paint. When you remove the washers, you'll have a perfect outline of them on the header wrap. Cut out your wrap "washers" with scissors, and trim the middle out with a penknife.
Step 5 - Clean the penetrating oil from the threads and put a pencil eraser-sized dab of anti-seize compound on them. Place one or two of your wrap washers between the converter and the heat shield and reinstall the heat shield using either the original lock washers or nuts or new ones if your car didn't come with any.
Tighten the heat shield bolts to factory spec, or to about 40 foot pounds if you don't know the factory spec. Try to jiggle the heat shield; the wrap washers should have taken up the space caused by bolt stretching and heat shield warpage, and should prevent rattling in the future.
TIPS: You may choose to use two hose clamps to clamp both the front and the back of the converter. If rattle / buzz does not stop, it may be a heat shield in the engine bay (this happened to me).
THINGS YOU'LL NEED: Basic hand tools, Penetrating oil, Sandpaper, Header wrap, Washers, Spray paint, Scissors or pen knife, Anti-seize compound, Non-nylon lock nuts or washers
WARNINGS: Make sure that your car is probably secured when up on the jack and jack stands.
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