From: Wally Plumley [wplumley@bellsouth.net]
Sent: Tuesday, January 09, 2001 4:48 PM
To: 928
Subject: [928] Re: Catbypass - Failed Emmissions- Help!

At 06:09 PM 1/8/01, jhormaechea wrote:
>By the way, do factory "cat" wear out with time and affect pollution and 
>performance levels?
>How do we know when the cat is clogged (of course before the emissions man 
>tell us)?


There are several possible failure modes for catalytic converters.
1. Exterior physical damage from impact, etc. This is pretty rare. The 
housings are tough, and the cats are usually well protected. Nonetheless, a 
visual inspection is a good idea any time that you are under the car.
2. Interior physical damage from impact or vibration. Pretty rare on 
factory cats, not quite as rare on cheaper replacement cats. The "pellet" 
type is not common anymore, and the monolithic substrates are pretty tough, 
as anyone who has ever tried to gut a cat knows. It is unlikely that an 
impact could internally damage the cat without leaving visible damage on 
the shell.
3. Interior physical damage from overheating. One of the easier ways to 
kill a cat. If a plug wire or plug fails, you get a cylinder full of 
unburned fuel every other revolution. This can quickly slag a cat at 
highway speeds. The later 928s have an ignition monitoring system to 
prevent such overheating.
4. Poisoning. Leaded fuel and silicone sealers are the most common killers 
here. A poisoned cat can sometimes be revived by extensive 
high-speed/high-load usage, but not often.
5. Plugging or clogging. Excessive oil consumption or an internal coolant 
leak are the most common causes for this. Crud builds up on the leading 
edges of the monolithic substrate passages, partially blocking exhaust flow.
6. Wear-out. The cats slowly lose the precious metals that are the 
catalysts and that are plated onto the substrate blocks. The cats just 
slowly become less and less effective at burning the pollutants.

Cat failure usually takes one of two forms: blockage and ineffectiveness.

Blockage, from damage or plugging, partially blocks the exhaust flow. The 
symptoms usually are that the engine cranks easily and runs well, but loses 
top-end power. The exhaust note may change, becoming smoother and less 
resonant, eventually developing a hissing quality. The best way that I know 
of to find a partially blocked exhaust is to hook a vacuum gauge to an 
intake manifold vacuum source. Crank the engine, and rev it to about 2500 
RPM. If the exhaust is NOT blocked, the vacuum gauge will initially drop as 
you open the throttle, then quickly climb to a reasonably high value, such 
as 20 - 22 inches of mercury, and stay there as long as you hold the 
throttle steady. If the exhaust is partially blocked, the vacuum gauge will 
initially drop as you open the throttle, then quickly climb to a reasonably 
high value, such as 20 - 22 inches of mercury, but will then start slowly 
dropping. If the exhaust is badly plugged, you can end up with the throttle 
wide open, but only 2500 RPM with zero vacuum.

An ineffectual cat shows up on the rejection sticker from the emissions 
test. I don't know of any home test for effectivity.

Wally Plumley
928 Specialists