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.

Question: I own an '08 Z06 and would like to reprogram the Electronic Control Unit (ECU) with aftermarket calibrations. Is it possible for the dealership to tell if this has been done? What portions of the warranty are voided if you have an aftermarket calibration?

We were talking about this at a Corvette club meeting, and some of the guys suggested I swap the original ECU with an aftermarket one. That way, I could swap the original one back for warranty service or reflash the ECU with the stock tune before taking it in to the dealer for service. Do you think the dealership will be able to detect a reflash or an ECU exchange? Tony Via the Internet

Answer: Yes, there are always signs of calibration changes. In fact, there are several Technical Service Bulletins (TSBs) that assist technicians in determining if a reflash or an ECU swap has taken place. The detection of any aftermarket calibration will void your powertrain warranty, including the engine, transmission, transfer case, and driveline. Many dealerships are cracking down on this! GM engineers take great care in programming new-car ECUs. Their goal is to put the best possible combination of performance, reliability, durability, fuel economy, and emissions into each calibration.

Vemp_1002_03_o Restricted_exhaust_systems 2010_corvette_modifications 3/4

While an aftermarket unit or program may give you increased performance, you'll run the risk of damaging or reducing the life span of your engine, transmission, drivetrain, and exhaust system. It may also cause your car to fall out of compliance with federal emissions standards.

I suggest that you wait until your Corvette is out of warranty before making any modifications. But if you really do want to "play," don't expect your dealership to pay.

Question: I have a C6 Z06 with 20,000 miles. It's completely stock, but I think it runs hotter than it should. I live in Miami and drive in high-traffic areas. The coolant temperature in stop-and-go traffic runs between 240-250 degrees Fahrenheit. The oil temp ranges between 220-230 degrees Fahrenheit. After talking to some friends in my local Corvette club, I was thinking of lowering my fan settings. It appears that the fan can be altered to come on at 192 degrees and stay on all the time. Do you think this is a good idea? Bob Via the Internet

Vemp_1002_04_o Restricted_exhaust_systems Corvette_temperature_guage 4/4

Answer: First, do not alter the factory setting! Running the fans all the time will cause the fan-control module to overheat and fail. The guys in your Corvette club must own C5s, which use a totally different operation system for fan control.

The C6 fans run off of a duty cycle and are capable of running at variable speeds depending on demand. The maximum duty cycle is defined as the percentage of time in a 10-minute period that a system can be operated continuously before overheating. The C6 system is not designed to run the fans continually. Try the following steps to determine why your Corvette is running so hot:

1. Check to make sure the cooling system is full and that there are no air pockets.
2. Check to make sure there isn't too much coolant in your system. A high concentration of antifreeze does not transfer heat as well as pure water or a 50/50 mix of antifreeze and water.
3. Check to make sure there is no debris in front of the radiator.
4. Check your thermostat to make sure it's working correctly and the cooling system is circulating coolant. You can use an infrared thermometer to check coolant temperature at the thermostat housing.
5. Perform a visual inspection for any kinked hoses.
6. Check to make sure the oil level is full.
7. Confirm that the fans are coming on when the car reaches operating temperature, and that the control module is responding to correct commands.
8. Check to see if there are any diagnostic trouble codes.
9. I've seen some people cut the center out of the thermostat to make the coolant flow faster. This is a bad idea because moving the water too fast through the radiator will keep the radiator from absorbing the heat from the fluid.

If everything on your Corvette is working correctly, you can try Redline Water Wetter or a similar product. It will help with heat transfer by reducing the surface tension of the water, which means that much smaller vapor bubbles will be formed. Vapor bubbles on the metal surface create an insulating layer, which impedes heat transfer. Good luck.

Got a question for our Tech Corner expert? Just jot it down on a paper towel or a lightly soiled shop rag and send it to us at VETTE Magazine, Attn: Tech Corner, 9036 Brittany Way, Tampa, FL 33619. Alternatively, you can submit your question via the Web, by emailing it to us at vette@sorc.com. Be sure to put "Tech Corner" in the subject line.

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