Choked up over exhausts

Jan. 1, 2020

The exhaust system often is overlooked as a cause of drivability problems.

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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.

Here are a few tips on locating these types of problems.

Exhaust Restrictions

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.

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.

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.

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

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.

About the Author

Pete Meier | Creative Director, Technical | Vehicle Repair Group

Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels.

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