Date: Wed, 18 Dec 1996 19:49:15 -0800
From: Mike West <mwest@cdsnet.net>
Subject: Catalytic Converters

 I figure this is a safe subject, only three people have one on this list.  :-)

 How Cats Work:

 Without being altered in any way itself, the catalytic agent has a
big effect on a chemical reaction.

It just has to be there, it doesn't get used up.

The basic two-way catalytic converter causes the oxidation of HC
and CO in the exhaust, making them combine more efficiently with O2,
which reaction results in producing CO2 and water.

Read "After-burner".

 It works well enough to let the engine be a little off and still
pass a smog test.

 The metals platinum and palladium are the catalytic elements of the
two-way converter.

 Only a very thin layer of these metals is required.

The metals are either applied to ceramic pellets, like a GM unit,
or to a ceramic honeycomb.

They even put these in wood stoves, can you believe that? ! !

Since any lead in the gasoline would quickly coat the catalytic
surface and slow or even stop the reaction, non leaded must always
be used.

I don't know how fast a converter can be killed but have heard an O2
sensor is dead in just a few hours.

As the gases flow through the pellets or honeycomb, they start to
burn rapidly, and the temperature can reach 1,600 deg. F.

 Extra oxygen in the exhaust stream is needed to support the blaze,
and that may be provided by a very lean air/fuel ratio, pump-type
air injection, or an aspirator valve setup.

Along with the main converter, which is mounted between the head-pipe
and muffler, a smaller "light off" catalyst is sometimes used.

 Since it's located right at the outlet of the exhaust manifold,
it heats up to working temperature quickly to burn the emissions an
engine produces while cold.

Triple-acting:

The three-way or reduction type catalytic converter first appeared
in 1976 , with the introduction of the O2 sensor, and has since
taken over completely.

It not only oxidizes HC and CO, it also breaks NOx down into
ordinary nitrogen and oxygen through the action of rhodium.

It requires the "stoichiometric" air/fuel ratio, 14.7 parts air to
one part fuel by weight, the ideal blend for complete combustion
or it just won't occur to any useable degree.

 Regular carburetors or fuel injection systems are too inaccurate
to keep the mixture close enough to that ratio, so electronic control
"closed loop" had to be added.

 This requires a computer, and several sensors, one that reports on
the oxygen content of the exhaust stream, or O2 sensor, and a
carb or injection setup that can adjust the mixture according to
the computer's commands, which is the reason such systems are
called "feedback" or"closed-loop."

In any ordinary three-way cat, the NOx-reducing section is located
upstream of the oxidation section, and air injection pump output
is routed to a chamber between them.

 This is necessary to keep the added air from upsetting the mixture
balance created at the intake.

 A switching system directs air injection to the exhaust manifold
during warm-up to help burn the large amounts of HC and CO that are
produced in that mode, then shifts it to the middle of the converter
once normal operating temperature is reached.

That's when NOx production begins and the system enters closed
loop.

What kills cats:

 I know  you've heard of lead fouling and mechanical damage.

Beyond that, the most common cause is an engine that pumps out too much
unburned fuel, which can overheat or clog the catalyst with soot and other
contaminants such as metals.

Another cause of cat clogging and poisoning is oil burning.

 Bad valve seals, for instance, can cause a lot of carbon formation,
and metals present in the lube will coat the catalytic agents.

You can test for plugging with an ordinary vacuum gauge.

Take a reading at idle, then hold rpm at 2,500.

The needle will drop when you first open the throttle, then
stabilize.

 If it then starts to fall, suspect excessive back-pressure.

 To check back-pressure directly:

 With air injection, you can disconnect the check valve from the
distribution manifold, and plug in a pressure gauge (you want one
with a low scale).

Or, remove the O2 sensor and take your reading at its hole in the
manifold.

 Since specs for the normal pressure range vary considerably, look
it up if possible.

 As a rule of thumb, anything over 1-1/4 psi at idle, or more than
3psi at 2,000 rpm, means you've got blockage somewhere.

 You can also buy a kit that lets you check pressure both fore and aft of
the unit. Fairly cheap I'm told.

 These have a hole punch for an air chisel,a self-tapping hollow
nipple, and some screw in plugs for the holes you'll be making.

 If there's any noticeable difference between the readings, you've
found a restriction.

Next, get the beast up in the air and do a "stress" test.

Put on a heavy glove or use a rubber mallet and give the converter
a couple of solid whacks.

 With the pellet type,you should hear some rattling. If not, the
pellets are probably stuck together with heavy contamination.

 On the monolithic type,(one piece) a rattle means the ceramic honeycomb
substrate is at least partially broken, altho it may not be bad enough to
need replacing.

With the engine fully warmed up and running, you can use a surface
thermometer or a pyrometer to compare the temperature of the pipe
just ahead of the converter with that of the outlet.

 If the outlet temperature is at least 100 deg. F. higher than the
inlet temperature, the catalyst is working.

 Its job is to cause HC and CO to burn up, and that process
makes heat.

If both readings are nearly the same, or the outlet is cooler than
the inlet, the oxidation reaction isn't happening.

The problem still could be lack of air instead of dead cat.

Fresh air:

In cases where emissions are high, or there's not much difference
in temperature between the in and out pipes, you still can't be
sure if the cat itself is bad, or if it's just not getting enough
air to support the blaze.

 Check the air injection system, which has the same effect as a
blacksmiths bellows.

First, do the belts and hoses routine on the air pump and the etc.'s.

Then, see if the check valve has rusted out.

Sometimes you'll hear an exhaust leak or unusual sounds coming
from the pump (the rattle of death).

It should let air pass in one direction, but not in the other.

If you suspect that the valve is allowing exhaust to flow backwards,
pull it and blow through both ends.

Look at the air injection manifold, which also tends to
corrode through.

The diverter valve should dump the pump output when the throttle is
opened, then it will snap closed when you let the throttle close.

Listen and feel for air escaping from the little muffler on the
valve at idle.

 Before condemning the diverter valve, make sure its vacuum
line is in good shape and properly connected.

 There are three types of aftermarket cats:

Two-way (oxidation of HC and CO only), with  air inject.

Three-way (adds reduction of NOx), and three-way with air
injection tap and diverter setup.

 Use the emission code from the EPA tag under the hood to
identify the proper part as listed in the catalogs.

 west