Today's exhaust system hasn't changed much from the ones I grew up on. It still is designed to reduce exhaust noise to an
acceptable level and to pass the combusted gases away from the cabin area for safety. It has always been tuned to maximize
the scavenging of the exhaust gases from the combustion chamber, and the only real new addition since I started in the mid-1970s
is the catalytic converter.
However, faults in this relatively simple system can have a very real impact on drivability on today's cars. The one you most
likely are familiar with is exhaust flow restrictions causing anything from skewed fuel trim to loss of power complaints.
In addition to presenting a safety issue, exhaust leaks also can lead to drivability issues related to fuel trim.
The first step is a visual inspection of the system for damage. (Photos: Peter F. Meier)
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Here are a few tips on locating these types of problems.
Exhaust Restrictions
Tapping on the pipe can locate broken substrate. Flex joints are good at hiding their leaks — inspect these closely.
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Exhaust restrictions can be caused by external damage or by overheating of the catalytic converter. So the first step is a
visual inspection of the exhaust system for damage. Check the entire system for signs of impact damage that has collapsed
the route the exhaust gas has to take to get out. Look for modifications that might alter the original ability of the system
to "pass gas." A good tap on the side of the pipes as you make your way down the system might uncover the telltale rattle
of catalytic substrate that has come loose and made its way to areas it shouldn't be.
Measuring exhaust backpressure is one method of locating exhaust restrictions. This one, taken on a 2007 Toyota at 2,500 rpm,
is fine.
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If you suspect an exhaust restriction, there are several ways to test for this condition. Some techs still use manifold vacuum
to test for restrictions. Using manifold vacuum as a clue can be done in two ways. In the first, warm the engine to normal
operating temperature and measure manifold vacuum at idle. Then raise the engine speed to 2,500 rpm and allow the vacuum gauge
to stabilize. This reading should be about the same as the idle reading. A significantly lower reading, or a gauge that slowly
drops from its reading at the higher rpm, can indicate an exhaust restriction.
By removing the oxygen sensor and inserting the probe, I can see the condition of the catalytic substrate.
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A second method using the manifold vacuum gauge is called the snap throttle test. With the engine at normal temperature, connect
your gauge and watch its reaction as you quickly snap the throttle from closed to wide open. Manifold vacuum should drop to
5 inches/Hg or so, then quickly rebound above the idle reading before just as quickly settling back down to its original position.
If the needle returns slowly, suspect a restriction in the system.
Exhaust leaks are found easier by sound, using the hose on your mechanic's stethoscope or even a length of fuel line.
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Another method is the exhaust backpressure gauge. This is a very sensitive pressure gauge that is attached to the exhaust
via the upstream oxygen sensor. Remove the sensor and replace it with the included fitting. Attach your gauge to this fitting
and check the amount of backpressure at idle and at 2,500 rpm. Most gauges are color coded for normal and unacceptable ranges.
But as a general rule, backpressure should not be more than 1 psi or so at idle and no more than 3 psi at 2,500 rpm.
Seeing is believing, so if you suspect that the catalytic substrate has come apart or has been fused together, a borescope
is a great tool to get a visual confirmation. With the model shown in the illustration, you can even take a picture of the
problem to show to your customer.
Exhaust Leaks
Gentlemen, Pick Your Engine
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Exhaust leaks usually are not that hard to find. With an assistant holding a rag over the end of the tailpipe(s), exhaust
pressure builds quickly, and any "hiss" becomes noticeably louder. But a few are a little more difficult to find, and if they
are located near an oxygen sensor, they can cause drivability problems. In these cases, this extra air causes the sensors
to think lean even if the actual mixture isn't, and the ECM adds fuel to the mixture in an attempt to respond to the sensor's
report. If you have a 5-gas in your shop, this is an easy problem to isolate by using the exhaust readings to verify the mixture
exiting the pipe.
Two methods come to mind for isolating those hard-to-find leaks. The first uses the smoke function of your shop's EVAP tester.
Using the large cone that usually comes with your tester, inject smoke into the exhaust via the tailpipe. Once the system
is full, it might help to lightly add some pressure to the system using a shop air line behind the smoke.
Another effective method uses sound to find the leak. Using your mechanic's stethoscope with the probe removed, slowly move
the hose around suspect leak areas. The hose will amplify the leak so it is easier to detect. Even a length of fuel line held
to your ear will do the trick.
Just be careful not to burn yourself when working so close to hot exhaust components.
Posted 2011-04-28 09:42:42.0
