Question:How can I tell if my catalytic converter is
working properly?
Answer:The catalytic converter is our main line of defense
against air pollution, so it's important to make sure it is functioning
efficiently and passing exhaust without creating undue restrictions
that might reduce performance, fuel economy or emissions. That's
one of the reasons for periodic vehicle emissions testing. If
the converter isn't working, you won't pass the test.
If the your converter is plugged, it will create a restriction
in your exhaust system. The buildup of backpressure will cause
a drastic drop in engine performance and fuel economy, and may
even cause the engine to stall after it starts if the blockage
is severe.
The easiest test for converter plugging is done with a vacuum
gauge. Connect the gauge to a source of intake vacuum on the
intake manifold, carburetor or throttle body. Note the reading
at idle, then raise and hold engine speed at 2,500. The needle
will drop when you first open the throttle, but should then rise
and stabilize. If the vacuum reading starts to drop, pressure
may be backing up in the exhaust system.
You can also try to measure backpressure directly. If your
engine has air injection, disconnect the check valve from the
distribution manifold, and connect a low pressure gauge. Or,
remove the oxygen sensor and take your reading at its hole in
the manifold or headpipe. Refer to the backpressure specs for
the application. Generally speaking, more than 1.25 psi of backpressure
at idle, or more than 3 psi at 2,000 rpm tells you there's an
exhaust restriction.
If there appears to be an exhaust restriction, disconnect
the exhaust pipe just aft of the converter to relieve pressure
and recheck the readings. CAUTION: The pipes will be hot so wait
awhile for things to cool down. If vacuum goes up and/or backpressure
drops, the problem isn't not a plugged converter but a plugged
muffler or collapsed pipe. If there's little or no change in
readings, the converter is plugged.
Just because a converter is passing gas doesn't mean it is
okay. If the catalyst inside is contaminated or worn out, high
carbon monoxide (CO) and/or hydrocarbon (HC) readings will be
present in the exhaust. If you have access to a high temperature
digital pyrometer (or an oven thermometer will do), check the
converter's temperature fore and aft. A good converter will usually
run 100 degrees F hotter at its outlet than its inlet. Little
or no temperature change would indicate low efficiency, or a problem
with the converter's air supply. Converters need supplemental
oxygen in the exhaust to reburn pollutants, so if the air injection
system or aspirator valve isn't doing its job the converter can't
do its job either.
Check the air injection pump, belt and check valve. If you
suspect that the check valve is allowing exhaust to flow backwards,
remove it and blow through both ends. It should let air pass
in one direction, but not in the other. Examine the air injection
manifold, too, because it tends to rust out and leak air. Check
the diverter valve to make sure it is working correctly, too.
It should be routing air to the converter when the engine is
at normal temperature.
On engines with aspirator valves instead of air pumps, you
should hear and/or feel the fluttering of the internal flapper
as the engine is idling.
CAUSES OF CONVERTER FAILURES
Fouling, clogging, melt-down and breakage of the ceramic substrate
inside a converter are common conditions that can cause problems.
Plugging is usually the end result of a melt-down, which occurs
because the converter gets too hot. This happens because the
engine is dumping unburned fuel into the exhaust. The excess
fuel lights off inside the converter and sends temperatures soaring.
If it gets hot enough, the ceramic substrate that carries the
catalyst melts.
The unburned fuel may be getting into the exhaust because
of a bad spark plug or valve, but an overly rich air/fuel mixture
is another possibility. In older carbureted engines, a heavy
or misadjusted carburetor float may be the underlying cause.
But on newer engines with "feedback" carburetion or
electronic fuel injection, the engine may not be going into "closed
loop" (the normal mode where the computer regulates the air/fuel
mixture to minimize emissions).
A bad oxygen sensor or coolant sensor may be giving the computer
bogus information. A sluggish or dead O2 sensor will make the
computer think the exhaust is running lean, so the computer will
try to compensate by making the fuel mixture rich. A coolant
sensor that always indicates a cold engine will also keep the
system in open loop, which means a steady diet of excess fuel.
But it might not be the sensor's fault. A thermostat that's
stuck open or is too cold for the application can prevent the
engine from reaching its normal operating temperature. So if
your converter has failed and needs to be replaced, the engine
should be diagnosed for any underlying problems before the new
converter is installed.
Another cause of converter clogging and contamination is excessive
oil consumption. Worn valve guides or seals can allow oil to
be sucked into the engine's combustion chambers. The same goes
for worn or damaged rings or cylinders. Oil can form a great
deal of carbon, and metals present in the oil can contaminate
the catalyst. A compression check or leak-down test will tell
you if the rings are leaking, while a fluttering vacuum gauge
needle will help you identify worn valve guides.
