How can I tell if the cat is plugged?

[BoringLumina]

Car: 1995 Lumina with 3100V6 & 4spd auto, ~190,000 miles

The car has the highway hesitation and I plan on cleaning the EGR for that. I was wondering if the cat might be plugged too. On every car I have owned but this one I have been able to turn the key off and quickly turn it back to run and the engine "re-lights" this car will not do that. Is this a symptom of the cat being plugged?

What are the symptoms of a plugged cat? This car seems way slower than my '88 Baretta or even my '82 Citation (Both had manual trans though) In addition it is WAY WAY slower than a rental Olds Alero I drove with the 3.4. Man that car could scoot , it felt like it had 100 HP on my Lumina

[Dave]

finding a plugged cat isn't that dificult ...

put a vacuum gauage on your intake and watch it as you rev it up - hold the RPM's at about 2500 and see if the vacuum slowly starts to decline from 'round 18 - 20" down to about 5 - 10" of vacuum, you can also test drive your car in this condition too, to see if you get the same or similar results of declining vacuum.

strangely enough, you'll also notice an INCREASE in Fuel milage, but you'll also see an increase in engine tempatures, and your oil will get dirty much MUCH faster.

--Dave.

[BoringLumina]

Thanks Dave,

How the heck can there ever be an increase in fuel economy with a plugged cat? I would figure a plugged cat would give higher backpressure and increased pumping losses, thus lower efficiancy, NOT HIGHER milage. I don't have a vaccum gauge so is there any other way to check for a plugged cat? I don't hink it glows red hot at night or anything.

[Dave]

well a glowing cat is a sign of unburned fuel, caused by a spark/ignition issue, or an overly rich condition somewhere else in the system. you can get a cheap Vacuum gauage from just 'bout any parts store, sears, wal-mart or any place like that too.

a plugged cat will offer better fuel economy because of the backpressure not allowing the cylinders to fill properly - you'll lose lost of power - this is real bad on the rings tho.

--Dave.

[Juglenaut]

The computer will simply compensate for the rich signal, decrease advance, decrease air flow, and so on....This is where the increase in MPG comes from for the most part. However the Computer will only compensate to a point. A plugged cat will effect engine effeciency in operation over many areas of the system some positive some negitive.

A glowing Cat is well just that if it is plugged up bad enough it will glow. If it smells at all like rotten eggs or sulfer for that matter it's getting plugged or wearing out.

A glowing cat that is not plugged is a sign of unspent fuel or overly rich uncompensatable mixture to a point then it actually drops the temps, to much air flow, to much muffler restriction, air injection purge open all the time, overly lean mixture. Rich fuel mixture actually decreases exhaust temps.

[Dave]

Actually, too much fuel in the exhaust will cause the cat to glow, because the catalytic converter is used to burn off the extra hydrocarbons in the exhaust (along with other things) this is one of the ways to tell of you have a bad coil/icm/stuck-open injector. so while an overly rich mixture CAN decrease exhaust tempatures, it will actully increase catalyst tempatures - these guys seem to agree with me on the fact that an overly rich condition will overheat the catalytic converter ... http://www.2carpros.com/topics/catcon.htm

(they've also got a pretty good little article on how to test your catalytic converter)

--Dave.

[BoringLumina]

Thanks a lot Dave.

[Juglenaut]

If you are wondering if the EGR can cause the Cat to get plugged, well yes overtime an EGR that is openning on a cold engine or opening durning the wrong cycles or not opening at all, can cause carbon to build up on the 02 and in between the waffers of the cat either way. This will also cause the cat to get hotter as it has more restriction and the O2 will sense less oxygen by flow rate and drop the signal to rich, a o2 sense only oxygen and will signal out flags to much oxygen over fuel is a lean flag not enough oxygen over fuel is a rich flag. If I am wrong on this let me know.

A cat doen't have to be plugged to glow red with a rich uncompensatable mixture.