Restricted Exhaust Systems - Tech Corner

James Berry Nov 26, 2009 0 Comment(s)
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Question: I have an all-stock '99 Corvette with 69,000 miles. It won't accelerate past a crawl, whether the engine is completely cold or at operating temperature. It also has a hard time climbing hills. When I try to accelerate, I notice a hissing noise coming from under the hood. There are no current error codes. I suspect a faulty sensor, but I don't know which one. What do you suggest? Derrick Via the Internet

Answer: The first thing to check for is a restricted exhaust. This can be caused by a clogged catalytic converter or muffler, or a kinked tailpipe. A restricted exhaust prevents the engine from breathing properly.

There are a few tests that can be performed to determine if the exhaust is restricted or the converter has failed:

1. Visual inspection: Check for a pinched or kinked exhaust pipe, physical damage to the insulator or converter shell, cracked or broken seams, excessive rust, or a plugged tailpipe.

2. Rattle test: Gently strike the center of the converter with a rubber mallet. If the converter rattles, the substrate has disintegrated and the converter should be replaced. Remember, the converter gets very hot; this test should only be performed on a cool converter.

3. Lead-contamination test: There is a special lead-detecting paste available that will check for the presence of lead at the tailpipe.

4. Restriction test:
a) Attach a vacuum gauge to an intake manifold source.
b) Allow the engine to reach operating temperature.
c) Note the vacuum reading at idle.
d) Raise the revs to approximately 2,000 rpm.
e) The vacuum reading should be close to the idle vacuum reading.
f) Quickly release the throttle. The vacuum reading should momentarily rise, then smoothly return to the normal idle reading. If the reading does not quickly return to the normal level, the exhaust system is likely restricted.
g) To confirm the exhaust system is restricted, you can remove the exhaust pipe from the exhaust manifold. Take care not to have the exhaust dragging the ground or to have the hot exhaust burning paint or any components.
h) Once the pipe is removed, testdrive your vehicle. If power is restored, you've confirmed that you have a restricted exhaust system.

If the exhaust system is restricted, it may be the result of another underlying problem. The following are the most common causes of catalytic-converter failure:

Vemp_1002_02_o Restricted_exhaust_systems Corvette_exhaust_system 2/4

1. Poor engine performance or an overly rich air/fuel mixture. This can cause the converter to overheat, causing the substrate to melt down. One sign of this is a converter that is red hot. Symptoms include loss of power at high engine speeds, a hissing noise when accelerating, prolonged cranking, and poor acceleration and fuel economy.

2. Leaded fuels can also cause the converter to overheat and the substrate to fail.

3. Coolant leaking into the exhaust system or silicone from sealants can coat the catalyst and render it useless. If the exhaust system has had coolant or silicone contamination, the oxygen sensor and catalyst will have a white coating or glaze that should be visible when the parts are removed.

4. A kinked exhaust pipe, physical damage to the insulator or converter shell, cracked or broken seams, excessive rust, or a plugged tailpipe may also cause converter failure.

5. A less common cause is thermal shock. This occurs when the converter gets hot then rapidly cools (as when exposed to cold temperatures), causing it to physically distort and disintegrate.

Good luck, and don't get burned.




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