Why Did My Exhaust Get Quieter?

Exhaust Get Quieter

Many people have noticed that their exhaust volume is reduced after about 20-25K miles on the car, and they are wondering why this is. While many things can cause an exhaust to be loud, most of them will not cause it to become quiet. This article will give reasons for this phenomenon, and what can be done about it.

1.) Why does the exhaust get quieter?

The quieting of the exhaust is commonly attributed to oil leaks sealing up or blow-by reducing. This is correct in most cases, but not all of them. Many people think that oil may leak into the pipes via the PCV system, and thus quieting the car. While some oil will indeed find its way into the pipes through this route, there is another much more common source: blow -by past the piston rings and settling on top of and behind these shields as well as into passages that lead over time to those over-exposed areas. Oil from crankcase venting accumulates here as well. This oil slowly breaks down and becomes gummy over time, eventually blocking off these passages completely. When this happens, the amount of blow-by decreases dramatically, leading to less noise at the tailpipe(s).

Reasons Why Your Exhaust Can Get Quieter

1)Evaporation: This is much less common than blow-by, but potentially a major source for some people. If your car uses R12 refrigerant in the AC system, this gas has a very low boiling point (~ -43 F). As the car warms up on cold mornings and sits idle, the whole AC system will slowly bleed off. Eventually it gets to the point where there isn’t enough left over to act as an insulator any more, and the compressed gasses start rattling around inside trying to find someplace cool to go.

In many cases this will manifest itself as a single loud “bang” from the exhaust when you first start the car. Parts that are especially susceptible to this include the turbo inlet pipe and the intercooler. If this has occurred, a quick spray of R134a into each intake plenum chamber should restore much-needed coolant pressure until you have time to recharge your AC system.

2.) Oil soaked baffles: This is how I personally noticed that my pipes were quieting down. You can add a tenth of a gallon every thousand miles for several months before you notice any problems at all, so it’s easy to see how this goes unnoticed by many people over a 10+ year span. The large majority of cars have relatively thin baffles that are made of stamped sheet metal. When this metal flexes (and believe me, it flexes a lot when you’re driving), the steel is punctured and oil can slowly seep inside. The reason for this happening is because of turbocharging; the hot exhaust gasses passing through these openings leave them relatively stress-weakened and very easy to deform (especially in cases where there is no aftermarket downpipe). The same goes for any other path for these gasses, including around/beneath/inside of any resonator or muffler.

3.) Aftermarket catalytic converters not designed for high flow: This one might catch some people by surprise, but it’s fairly common to see an exhaust system layout cause the downstream converter to act more like a bottleneck than an actual performance upgrade. Most modern converters are designed to combat backpressure, not flow. If the exhaust pipes in front of it aren’t sized appropriately, they can provide more resistance than necessary for free-flowing exhaust gasses. This is compounded by the fact that many people do not remove or replace their O2 sensors when upgrading. Some weld in test pipes/cat delete pipes which preclude the use of these sensors completely, but most only plug them up with brand name catalytic converters sold on eBay and other sites. When you combine all of these together, you get backpressure issues at your newly upgraded rear cats (which are still plugged up), typically resulting in a loss of power and a much quieter exhaust in some cases.

4.) Leaking front and rear O2 sensors: If an upstream (before the converter) and/or downstream sensor (after the converter) is leaking, it can lower backpressure significantly because of unmetered air passing through the faulty O2 sensor. If you’re like me and your car runs like crap without them plugged in, this is something to take note of!

5.) Cracked manifolds: This one isn’t necessarily all that common; usually when I see it it’s with older N/A cars that have been through several engine rebuilds over many years. The majority of OEM turbo Subaru EJ25 manifolds were made from thin cast iron and are very susceptible to cracking, especially on turbo models running higher boost/larger injectors. This is usually accompanied by a loud ticking noise which becomes much more noticeable after the engine has been warmed up.

6.) Naturally aspirated timing belt misfire: More often than not this manifests itself as an alternating pattern of misfires that can be heard with the windows down or at idle. As the car comes under load, these misfires are drowned out by the additional airflow and they become less audible. The most common cause for this is worn timing belt guide rails, especially in engines where the guide rail bolts have been backed off to allow for belt tension adjustment (since you can’t adjust tension unless it’s enough to slide).

7.) Catalytic converter damage/failure: Unfortunately I have no pictures for this one, but it’s something that should always be a potential concern when you’re not seeing a check engine light or any other discrepancy on your car’s dash at all. A good way to determine if there is an issue with your downpipe or cat(s) would be to bring the car up to operating temp (or let it idle until it does so automatically), then open up your hood and look for signs of distress. If everything looks okay, rev the engine briefly to high RPM and see if you can somewhat hear whether or not your cat(s) are still working. If they’re within spec, no issues should be occurring; but even a slightly burnt/off smell could indicate pending failure.

8.) High-boost knock: This one is especially applicable to those running an aftermarket EJ25 turbo that’s pushing more than the standard 20psi of boost (22+ psi). When enabled, subaru’s knock control system works by setting off the CEL when it detects pre-ignition events taking place in the cylinder. The issue with this particular code is that so many things can cause high-pressure fuel injections systems to light off early (bad tune, fuel pressure issues, etc.) that the O2 sensors may falsely detect knock even when it’s not taking place. Take a look at this article for more information on how knock is detected in modern engines .

9.) Catalytic converter resin migration: This one isn’t too common, but could happen to some cars if they use cheap aftermarket oil additives. A good way to determine whether or not you’re using the correct oil for your engine would be to check with your dealer and use what they recommend. If you still want something different, ensure that all oil used meets Subaru’s LL-01 specification for your car’s year/model. Using an oil appropriate for turbo models will NOT prevent this problem either; Subaru specifically recommends against using synthetic oils of any kind in their turbo models. If you’re having this issue, the only way to fix it would be to have your downpipe/cat(s) replaced by a dealership since it’s unlikely that they’ll ever pass emissions testing once clogged up with resin.

10.) Substantially low boost levels: Just as an example, if someone is running the stock EJ25 turbo on a GT30R upgrade, but doesn’t have sufficient fuel pressure for the setup because they haven’t upgraded their injectors/lines etc., it will cause problems because that engine needs at least 42psi of boost at WOT to sustain safe operation under load. Because the computer doesn’t know how much boost is being generated (since there is no MAF in that setup), it will run the engine far leaner than necessary. In fact, in many cases the computer will be running the engine around 15:1 air/fuel ratios at ~60% throttle when it should actually be leaning out to between 10.5-11.5:1 when under load with a properly tuned GT30R upgrade in place.

Short Term Fixes

While you are looking into long term fixes for these problems (as outlined below), there are some temporary options you can consider if your exhaust volume is really bothering you.

1.) Change the oil. The easiest way to quiet your car at this point (if you haven’t done so already) is to change the oil and filter. Doing this will remove much of the build-up in your exhaust before it becomes a problem for good, and help clean up existing gummy areas in the system. Also make sure you do not use higher viscosity than needed when changing your oil (example: 10W-40), as almost every engine is designed with 5W-30 in mind.

2.) Clean out any PCV passages that may still contain oil from crankcase venting. Many cars have one or two vacuum lines running to their PCV valve assemblies that usually plug into tiny passages on top of the valve covers. Over time, oil sprayed in through the PCV system can find its way in there and slowly build up to the point where it clogs them off. This will only make the problem worse, but a quick cleaning with some brake cleaner on a long thin tool will make this a thing of the past.

3.) Add “Oil Breather Valve” to your crankcase ventilation system. Racing Beat has come out with a new product called an Oil Breather Valve that is designed to allow gas from your crankcase to escape into the atmosphere without allowing oil from your engine’s sump into that same space. These valves are very easy to install (less than 5 minutes) and can provide you years worth of extra horsepower by reducing the amount of oil that you allow into your intake manifold by sheer virtue of reducing the pressure in your crankcase. As an added benefit, they also help quiet your exhaust (to a small degree) and increase MPG by reducing pumping losses caused by excessive crankcase pressures.

Long Term Fixes

The best way to fix these problems is to remove what is causing them: Blow-By and Oil Leaks.

1.) Remove Existing Gaskets and Replace with FelPro MLS (Multi-Layer Steel) Gaskets: When gaskets fail, they tend to do so either from too much torque applied at installation or from not being flanged out enough where needed. This leads to areas between two mating surfaces that are incompletely sealed. The longer you wait, the more of a chance that some oil will find its way down to these incomplete areas and accumulate over time. Eventually this oil will start to break down and can even lead to a blown gasket if proper preventative maintenance is not performed on the engine.

2.) Service Manual Recommendations: In almost all cases, your car’s manufacturer recommends a more frequent service schedule than what most people actually follow. Most cars only need their valve clearances adjusted every 30K or more miles for example, but it is more common for people to go several 100K before this basic service is done. If you are having these types of problems with your car, I highly recommend either taking it for one of those recommended services or doing the job yourself. Most of these types of problems are relatively easy to fix, but make sure you follow your car’s service manual instructions to avoid additional problems during repairs.

3.) Inspect Your Car For Oil Leaks While Washing It: Whether you are performing this task manually or having it done at a car wash that has large brush assemblies on the ends of their arms, you should take this opportunity to check for oil getting into places that it shouldn’t be going. Look closely around the valve covers and even inside the spark plug holes while washing any area under your engine compartment looking for tell-tale signs of oil leakage from anywhere including damaged gaskets and old rusted through studs. These small spots can add up over time so keep this in mind if you are storing your car for long periods of time.

4.) Check Your PCV System: With the engine off, check all hoses and tubes for proper connections and obvious signs of damage. Once again, brake cleaner comes in handy here as it will clean away any oil that may have accumulated on these surfaces to give you a better view of what is going on with your engine’s breathing assembly. I am amazed at how many people repair their cars’ “oil problems” even though they never bothered to actually look into their ventilation system just to see if there was something simple like a collapsed hose or kinked tube. There is no sense spending hundreds (or thousands) on an engine rebuild when all it takes is a quick check of your car’s ventilation system to determine the root-cause of the problem in most cases.

5.) Replace Your PCV Valve: If you have been having oiling problems for a while, it is highly possible that your existing PCV valve has become clogged with deposits from combustion gases and/or oil that was contaminating its surface from past leaks or from simply being run too long. Its location varies depending on the make and model of your car but in most cases, it is located somewhere around where a crankcase breather tube would be attached near an engine mount. Just look around under the hood for a one-way valve connected to a hose going into an opening big enough to fit your cupped hand through. Replace this inexpensive part with a brand new one and you shouldn’t have to worry about oiling problems for quite some time.

6.) Add A Fuel Recirculation Valve: Some car owners have had great success in prolonging the life of their engines by installing special fuel recirculation valves that they claim help them achieve 200K miles or more on a single engine rebuild. These devices are relatively easy to install and once again, I would recommend either taking your car to a shop for this service or doing it yourself if you feel up to the task. The exact location of where these types of valves should be installed varies from car-to-car but as a general rule, it should be located as close as possible to the throttle body on the intake side of your engine. These are normally closed systems that are only opened when you are at full throttle to reduce the amount of fuel entering the combustion chamber, but some engineers claim that these devices do more than just that while others say they aren’t worth a crap. All I can tell you is that some people swear by them while others think they’re one big joke so you be the judge on this one.

7.) Remove Your Exhaust System: On some car models with V6 or V8 engines, it may not be possible for you to remove your entire exhaust system completely due to space constraints in your garage or driveway. If this is the case, all I would recommend is loosening each pipe connection one-by-one without completely removing them from the manifold or the vehicle to see if you can move all 6 or 8 connections enough to check for wear and tear. Just make sure that you secure each connection back onto its respective flange or bracket after doing this (anything loose I would recommend re-tightening) before moving on to the next step. If you feel up to it, go ahead remove one of your mufflers at a time but be careful when lifting these out as they are extremely heavy especially when filled with exhaust gases.

8.) Remove Your EGR Valve And Clean It: This is not one of those “burnt toast” type cleaning jobs where simply spraying it down with carburetor cleaner will do so I would recommend buying an actual cleaning kit from your local auto parts store. These kits normally run around $20 and include a special brush to clean your EGR valve as well as a can of spray-on cleaner. Just follow the directions on the package (which is usually very simple) and you should be okay. After cleaning it, it is highly recommended that you remove any excess liquids with brake parts cleaner and compressed air before putting it back into place. For those of you without access to these types of chemicals, simply leave the EGR valve facing down in an open container overnight and all the liquids should drain out on their own by morning assuming there’s nothing trapped inside preventing this from happening.


With a little luck, following these 8 easy steps should have your engine performing as good as new again and give you that same feeling of accomplishment you had when you first bought it. Again, if you feel uncomfortable doing any of this work yourself or simply don’t want to bother with it, your local repair shop should be more than happy to help out.

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