I'm not going to pretend to be a Nissan expert, but I can give you some generic suggestions. The only way a catalytic converter can get red hot is by burning too much unburned fuel. That will lead to a melted and plugged converter eventually. The question is why is there too much fuel?
The most common cause of too much fuel is because the Engine Computer thinks the mixture is too lean so it turns the injectors on for longer pulses. A misfiring cylinder due to loss of spark will allow unburned air and fuel to enter the exhaust stream. The oxygen sensor will detect the unburned oxygen and make the computer think there isn't enough fuel to go with it. O2 sensors don't detect unburned fuel. No matter how much extra fuel is commanded by the computer, there will still be that unburned oxygen. The unburned fuel from the misfiring cylinder, and the extra fuel from all the other cylinders burns in the converter.
A similar problem occurs if an injector isn't firing properly or is disconnected. There will be a bunch of unburned air entering the exhaust that is detected by the O2 sensor. The computer will increase the fuel from all of the other injectors. Now you have a bunch of fuel and air again that burns in the converter.
A much less obvious and hard to find problem is a leak in the exhaust system ahead of the O2 sensor. In between the pulses of exhaust flow, the momentum creates small pulses of vacuum. That vacuum draws in outside air that is detected by the O2 sensor. The computer increases fuel metering in an attempt to overcome the lean condition, but no matter how much extra fuel is delivered, the unburned air is still there.
A different but related problem can occur on cars with distributors. If the ignition timing is excessively retarded, the mixture will be ignited and start burning too late. It will still be burning when it is pushed out past the exhaust valve. This will be most visible on engines that have thin header-type exhaust manifolds. They will glow orange, but the burning is usually done before the mixture gets to the converter. Engine power will be low too.
On engines with a vacuum hose going to the fuel pressure regulator, a leaking or improperly connected hose will cause fuel pressure to go too high. That will force more fuel through the injectors than the computer is expecting. If that hose is allowing air in, you'll have extra fuel and extra air. Normally all the fuel burns in the engine, but idle speed will be too high.
As a rule, a problem with a single cylinder is usually the cause of too much fuel and air entering the converter. The clue is that idle speed is close to normal.
Wednesday, March 31st, 2010 AT 1:05 AM