capewaveride Posted December 19, 2003 Share Posted December 19, 2003 2001 impreza outback sport, 5spd, 41,000miles. I've been experiencing a sort of rattle sound at certain rpms when accelerating and decelerating. Tried whacking the exhaust and shaking stuff around without any luck. Anybody had this before? Link to comment Share on other sites More sharing options...
theotherskip Posted December 19, 2003 Share Posted December 19, 2003 it probably is your heat shields. bast way to find it is to raise the car and put it on jack stands, then have somebody work the throttle until it makes the noise. while wearing heavy gloves, hold each heat shield until the noise goes away. you can then either find a way to tighten it, or remove it all together... Link to comment Share on other sites More sharing options...
vwbuge Posted December 19, 2003 Share Posted December 19, 2003 Other skip is probably right. My quick fix after I find the offending rattle is to throw a hose clamp around it and snug it down. Link to comment Share on other sites More sharing options...
alias20035 Posted December 19, 2003 Share Posted December 19, 2003 Or jam folded aluminum foil wedges in between the exhaust and offending heat shield. Since aluminum foil has low heat conduction, will not rust, and is flexible it is suitable material to use. I have used this trick on many early Legacy's with rattling catalytic convertor and resonator heat shields. Link to comment Share on other sites More sharing options...
SevenSisters Posted December 19, 2003 Share Posted December 19, 2003 Your getting good advice. My '91's ratttle was solved with the hose clamp trick. Here's the question though: Why after 10 years couldn't Subaru eliminate this minor gripe? It must drive their dealers crazy. Link to comment Share on other sites More sharing options...
theotherskip Posted December 19, 2003 Share Posted December 19, 2003 every car i've ever had has done it to some degree at some point in its life... Link to comment Share on other sites More sharing options...
alias20035 Posted December 19, 2003 Share Posted December 19, 2003 Originally posted by SevenSisters Your getting good advice. My '91's ratttle was solved with the hose clamp trick. Here's the question though: Why after 10 years couldn't Subaru eliminate this minor gripe? It must drive their dealers crazy. If you can come up with some new material that has low heat conductivity, that will not rust when exposed to road salt, that can handle instant temperature changes (like when extremely hot and then splashed with cold water), and happens to be inexpensive, be sure to give Subaru a call. Metal is one of the few substances that can handle the extreme temperature change, and even stainless steel will rust due to the high temperatures and road salt exposure. Subaru uses a very high quality exhaust system that does rattle a lot less often than most other vehicles except large trucks which usually have no heat shields, but trucks are also not required to have long life exhaust systems (10yr/150,000 mile) either. The usual rattle culprit are the interior clamps that are underneath the heat shield, and usually when they are rusted out you will not be able to get the heat shield bolts off without causing more damage to the heat shield. Which is why the hose clamp and/or aluminum foil wedges trick is recommended. Link to comment Share on other sites More sharing options...
SevenSisters Posted December 20, 2003 Share Posted December 20, 2003 Come on. You just described the attributes of aluminum. I'm not a rocket scientist, but I think this is a fair question and someone at Subaru or NASA could certianly improve the heat sheild situation. As I said, it's minor but it seems to be a rather wide spread reason for questions and complaints. Just my observation. Link to comment Share on other sites More sharing options...
myles Posted December 20, 2003 Share Posted December 20, 2003 Originally posted by alias20035 Or jam folded aluminum foil wedges in between the exhaust and offending heat shield. Since aluminum foil has low heat conduction, will not rust, and is flexible it is suitable material to use. I have used this trick on many early Legacy's with rattling catalytic convertor and resonator heat shields. I think you'll find that aluminum has _high_ thermal (and electrical) conductivity. Do you mean that it has a (relatively) high melting point? Link to comment Share on other sites More sharing options...
vwbuge Posted December 20, 2003 Share Posted December 20, 2003 It is dumb to debate over the material. It is the design that is flawed. Link to comment Share on other sites More sharing options...
myles Posted December 20, 2003 Share Posted December 20, 2003 Originally posted by vwbuge It is dumb to debate over the material. It is the design that is flawed. I was merely pointing out that heat sheilds are mainly _reflectors_, not insulators. Link to comment Share on other sites More sharing options...
alias20035 Posted December 20, 2003 Share Posted December 20, 2003 Originally posted by myles I was merely pointing out that heat sheilds are mainly _reflectors_, not insulators. Yep it is a reflector, and the heat shield clamps are deliberately small to avoid heat conduction to the heat shield. But aluminum is a poor heat conductor compared to iron and most other metals, which is why it is used so extensively for inexpensive cooking pots. Because aluminum conducts heat poorly these pots are less likely to develop hot spots like thin steel pots do, which makes them easier to cook with. Real chefs tend to avoid them for the more preferable thick copper bottomed pots, but these pots can cost ten times as much. I just recommend aluminum foil because it is easy to mold into place and will not rust. If you want to use tin go ahead, but it will last maybe a year, I have fixed Subaru's 5+ years ago with aluminun foil and the rattles have not returned. It also helps to cut out any broken clamps if you can, otherwise pack them in foil to prevent them from rattling. As for the design, I don't think much can be donw, no other manufacturer has been able to tackle this issue either, not even Lexus. The combination of high heat, water and road salt is too much for all metals, and most exotic materials such as ceramic will not tolerate sudden temperature changes which was the point I was trying to make earlier. Subaru heat shields usually start to rattle in year 5 or 6 here in Canada which probably has the worst climate and road conditions for them, so all and all not too bad. Many other cars exhaust systems are disintegrating at this point, so I would rather have a rattle that is easy to deal with. Link to comment Share on other sites More sharing options...
theotherskip Posted December 20, 2003 Share Posted December 20, 2003 i believe you have your metals and physics mixed up. aluminum is a _good_ conductor, of both heat and electricity. much more so that iron. you use a cast iron frying pan when you want a very even heat, beacause it slowly absorbes heat and gives it off very evenly. aluminum, on the other hand, absorbs and gives off its heat very quickly, which can cause hot spots. heat transfers in a number of ways, mainly conduction, convection, and radiation. conduction is basically a direct exchange of heat, such as holding one end of a metal bar in a flame and the other end will get hot. convection is the transfer of heat energy through a heated substance - like a forced hot air heating system or the cabin heater in a car. finally, there is radiation, which is how the sun heats the earth. this is what heat shields are protecting you against. while conduction requires a direct contact, and convection needs close proximity when in a large space, radiation can be over much greater distances. the only to protect against radiation is to put something in its path, since it is line of sight. by putting on the heat shields, they have to absorb the radiated heat, which is a very lossy process, then reemit the radiated heat away, but since you have gone through a conversion, a lot of heat energy has been lost. this protects you when you park over combustable materials (grass, leaves, etc), and also protects things in the vicinity of the exhaust pipes. basically, the heat shields were designed and installed for a reason, so it is in your best interest to try and reinstall them rather that rip them off. Link to comment Share on other sites More sharing options...
Setright Posted December 20, 2003 Share Posted December 20, 2003 AL will not "rust", but it certainly will corrode! First a thin layer of anodised AL will form and later, as the salt makes it's way, it will create fault lines in the AL. Link to comment Share on other sites More sharing options...
Commuter Posted December 20, 2003 Share Posted December 20, 2003 I work in the heat treat industry. Thermal conductivity (Btu/lb-ft-F). This is the "speed" at which heat energy will travel thru the material. Specific heat (Btu/lb/F). This is the heat energy that the material can "store" within itself. Thermal conductivity values: Aluminum - 136 Mild steel - 26-38 Stainless steel (304, 430) - 8 (I would expect this to be representative of most 300 and 400 series stainless steels.) As you can see, Aluminum has a much higher heat conductivity than steel or stainless steel. What is missing here is the measure of how easy or difficult it is to get the heat "into" or "out of" the material. (There is another property that gets into this, but I'd have to spend more time looking it up, refreshing myself, etc.) There are the 3 modes of heat transfer that were discussed ealier - Convection, conduction, and radiation. Solid Aluminum is processed below radiation temperatures, so that is essentially out. Conduction requires "touching" a hot surface (eg, pot on stove element). Convection is the heat transfer from a fluid, usually hot air. Aluminum is typically heat treated by convection, and lots of it. It is more difficult to heat Aluminum than steel in this mode. It just doesn't take on the heat the same as steel. Conversely, it doesn't give up its heat the same either. I always have to warn new employees about the dangers of an Aluminum heat treat shop. One can walk by a bin of (say) 500F steel parts, and you will feel the heat being thrown off of it. You can walk by a bin of 500F Aluminum parts and never realize that they are hot. Until you mistakenly touch one that is! I mentioned the specific heat property for interest sake. It does not have any real bearing on this whole heat shield issue as far as I can see. For what it's worth, Aluminum stores about twice the heat energy as steel or stainless steel (0.24 vs 0.11 to 0.12). What grade of stainless does Subaru use in the exhaust system? I know that mid-grade 300 series has been used. More and more 409 is being used. The shields might be different again for all I know. Just curious. So far, my 97 OB has not suffered from rattling shields. However, there are a few strategically placed screws turned into the ends of shields around the Y pipe. I presume this was done before I got the car. I agree with keeping them in place if at all possible. I have used the metal strap worm gear clamps successfully in the past. Commuter 1 Link to comment Share on other sites More sharing options...
shortlid Posted October 23, 2014 Share Posted October 23, 2014 Anyone just end up taking a sawzall or air cutting wheel the shields and be done with them! 1 Link to comment Share on other sites More sharing options...
spazomatic Posted October 24, 2014 Share Posted October 24, 2014 (edited) My baja's been doing this lately too. I was kinda afraid that maybe the catalyst was breaking down and coming loose, though to my ear it sounds like a heat shield (have an s10 blazer that sounded nearly the same, right before the catalyst self destructed). Im gonna try the hose clamp trick first. If that fails, ill drill some holes, blunt the ends of some sheetmetal screws, then run them through the shielding, against the cat to create a tension. (Less surface area for heat to migrate through) P.s. Some of y'all are pretty smart fellers! hyuck hyuck Edited October 24, 2014 by spazomatic Link to comment Share on other sites More sharing options...
forester2002s Posted October 24, 2014 Share Posted October 24, 2014 It's actually finding where the rattle comes from, that is the hardest. I've had some luck with hose-clamps. But I usually resort to stuffing St.St. 'pot-scourers' in between the heat-shield and the exhaust-pipe. I tear the pot-scourer into clumps, and use a flat-bladed screwdriver to stuff the clumps in behind the heat-shield. It works quite well. 1 Link to comment Share on other sites More sharing options...
upnorthguy Posted October 24, 2014 Share Posted October 24, 2014 Anyone just end up taking a sawzall or air cutting wheel the shields and be done with them! Totally. I love the missing rattle now that my a couple of the offending shields are gone. 1 Link to comment Share on other sites More sharing options...
Recommended Posts
Please sign in to comment
You will be able to leave a comment after signing in
Sign In Now