How do I determine if the catalytic converter needs replacing?
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Joined: May 2014
Posts: 184
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From: Seattle, WA
Car: '84 Camaro Z28
Engine: 305 HO
How do I determine if the catalytic converter needs replacing?
So, I've been in the process of restoring the wife's '84 Z28 that had been sitting for 12 years. I've completed much of the usual stuff that goes along with this, but now, I want to get it where I'm getting decent gas mileage.
I've been reading a lot about catalytic converters and O2 sensors. I intend to replace the O2 sensor because of the age, and, quite frankly, I don't trust it to still work properly.
My question is: What about the catalytic converter? I've read that, if I disconnect the O2 sensor (and I'm assuming a known good sensor) and the engine begins to run better, then it's probably time to change the catalytic converter. Has anyone else read this or has experience with it?
I'd like to hear how you go about making a determination if the catalytic converter needs replacement.
I've been reading a lot about catalytic converters and O2 sensors. I intend to replace the O2 sensor because of the age, and, quite frankly, I don't trust it to still work properly.
My question is: What about the catalytic converter? I've read that, if I disconnect the O2 sensor (and I'm assuming a known good sensor) and the engine begins to run better, then it's probably time to change the catalytic converter. Has anyone else read this or has experience with it?
I'd like to hear how you go about making a determination if the catalytic converter needs replacement.
Joined: Sep 2005
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Re: How do I determine if the catalytic converter needs replacing?
Easiest way is just to take it off and look at it. Which is pretty eeeeeezy actually, since the HO uses the 4-bolt flanges on both ends.
If the stuff inside isn't all busted up, and you can still see right through it, then it's probably still OK.
If the stuff inside isn't all busted up, and you can still see right through it, then it's probably still OK.
Joined: May 2004
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From: Lexington, SC
Car: 1987 SC/1985 TA
Engine: 350/vortec/fitech
Transmission: T56
Axle/Gears: 3.27 9-bolt
Re: How do I determine if the catalytic converter needs replacing?
Disconnecting the O2 sensor will only keep it from going into closed loop. The engine may 'appear' to run better to some. The idle can be smoother because the ECM is no longer adjusting the mixture based on O2 readings and the carb is no longer 'hunting' for the perfect mixture. No relation to cat function.
A bad cat usually has that 'bad cat' smell. A cat for the HO can be had from Rock Auto pretty inexpensively.
A bad cat usually has that 'bad cat' smell. A cat for the HO can be had from Rock Auto pretty inexpensively.
Thread Starter
Member
Joined: May 2014
Posts: 184
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From: Seattle, WA
Car: '84 Camaro Z28
Engine: 305 HO
Re: How do I determine if the catalytic converter needs replacing?
Well ... that would be great except mine isn't bolted. Apparently, it got replaced or something at some point, and clamps are used. Of course, the pipes have been heatsoaked, cooled, etc a million times, and removal ... I might as well get a new CC.
Thread Starter
Member
Joined: May 2014
Posts: 184
Likes: 0
From: Seattle, WA
Car: '84 Camaro Z28
Engine: 305 HO
Re: How do I determine if the catalytic converter needs replacing?
Disconnecting the O2 sensor will only keep it from going into closed loop. The engine may 'appear' to run better to some. The idle can be smoother because the ECM is no longer adjusting the mixture based on O2 readings and the carb is no longer 'hunting' for the perfect mixture. No relation to cat function.
A bad cat usually has that 'bad cat' smell. A cat for the HO can be had from Rock Auto pretty inexpensively.
A bad cat usually has that 'bad cat' smell. A cat for the HO can be had from Rock Auto pretty inexpensively.
Edited: I found out what you mean by "closed loop". I found a pretty good explanation of open, closed, and "limp mode", here: http://www.hotrodhandbooks.com.au/eB...Line-03-2.html
Last edited by mlbinseattle; Sep 27, 2014 at 05:17 PM. Reason: Updated to include additional info.
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From: Oakdale, Ca
Car: 89 IrocZ
Engine: L98-ish
Transmission: 700R4
Re: How do I determine if the catalytic converter needs replacing?
Two ways I know how to check for restricted exhaust...
Put a vacuum line on the intake, at idle should be 18-20in/hg...now take the engine up to about 2500 RPMs, should remain at or near the idle setting...if falls off lower...indicates plugging, the lower it falls, the more restriction...IIRC, service spec is 2inches below initial reading...keep in mind you're also relying on the muffler as well.
Next is to pull the O2 sensor, screw a low pressure gauge into it, or use a rubber adapter that comes with fuel pressure/vacuum kits (you'll want a low pressure one...0-15psi), fire the car up and see how much back pressure is in the system...FWIW, I think when I checked my '89 years back, was about 1PSI...I'd assume if you see 2-3, etc, you have too much restriction.
Put a vacuum line on the intake, at idle should be 18-20in/hg...now take the engine up to about 2500 RPMs, should remain at or near the idle setting...if falls off lower...indicates plugging, the lower it falls, the more restriction...IIRC, service spec is 2inches below initial reading...keep in mind you're also relying on the muffler as well.
Next is to pull the O2 sensor, screw a low pressure gauge into it, or use a rubber adapter that comes with fuel pressure/vacuum kits (you'll want a low pressure one...0-15psi), fire the car up and see how much back pressure is in the system...FWIW, I think when I checked my '89 years back, was about 1PSI...I'd assume if you see 2-3, etc, you have too much restriction.
Thread Starter
Member
Joined: May 2014
Posts: 184
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From: Seattle, WA
Car: '84 Camaro Z28
Engine: 305 HO
Re: How do I determine if the catalytic converter needs replacing?
Two ways I know how to check for restricted exhaust...
Put a vacuum line on the intake, at idle should be 18-20in/hg...now take the engine up to about 2500 RPMs, should remain at or near the idle setting...if falls off lower...indicates plugging, the lower it falls, the more restriction...IIRC, service spec is 2inches below initial reading...keep in mind you're also relying on the muffler as well.
Next is to pull the O2 sensor, screw a low pressure gauge into it, or use a rubber adapter that comes with fuel pressure/vacuum kits (you'll want a low pressure one...0-15psi), fire the car up and see how much back pressure is in the system...FWIW, I think when I checked my '89 years back, was about 1PSI...I'd assume if you see 2-3, etc, you have too much restriction.
Put a vacuum line on the intake, at idle should be 18-20in/hg...now take the engine up to about 2500 RPMs, should remain at or near the idle setting...if falls off lower...indicates plugging, the lower it falls, the more restriction...IIRC, service spec is 2inches below initial reading...keep in mind you're also relying on the muffler as well.
Next is to pull the O2 sensor, screw a low pressure gauge into it, or use a rubber adapter that comes with fuel pressure/vacuum kits (you'll want a low pressure one...0-15psi), fire the car up and see how much back pressure is in the system...FWIW, I think when I checked my '89 years back, was about 1PSI...I'd assume if you see 2-3, etc, you have too much restriction.
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Thread Starter
Member
Joined: May 2014
Posts: 184
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From: Seattle, WA
Car: '84 Camaro Z28
Engine: 305 HO
Re: How do I determine if the catalytic converter needs replacing?
So, let me see if I understand correctly. The O2 sensor function has really nothing to do with the catalytic converter. I already understood that it detected how much oxygen was in the exhaust where it exited the engine and is used to tell the ECM to adjust the mixture at the carb. And disconnecting it will not give an indication of whether the CC is bad or not. Correct?
So ... (1) If I need to cut out the existing CC just to tell if I can still see through it or not (and again, if I do this, I might as well replace it with a new CC), (2) if doesn't have a "bad cat" smell, and (3) pressure indication is good (no exhaust restrictions), then I suppose the only other thing to determine if the CC is still good is to take it through emissions? (And in Washington state, I don't need to take anything older than 25 years through emissions, so I won't worry about it.) Does all this make sense?
If the pressure reading is good indicating there isn't any backpressure, I'm probably not going to worry about the CC at this point. However, at some point after I get everything else completed with the restoration, I probably will get the CC replaced.
So ... (1) If I need to cut out the existing CC just to tell if I can still see through it or not (and again, if I do this, I might as well replace it with a new CC), (2) if doesn't have a "bad cat" smell, and (3) pressure indication is good (no exhaust restrictions), then I suppose the only other thing to determine if the CC is still good is to take it through emissions? (And in Washington state, I don't need to take anything older than 25 years through emissions, so I won't worry about it.) Does all this make sense?
If the pressure reading is good indicating there isn't any backpressure, I'm probably not going to worry about the CC at this point. However, at some point after I get everything else completed with the restoration, I probably will get the CC replaced.
Joined: Jan 2009
Posts: 887
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From: Golden, CO
Car: 87 IROC
Engine: L31 350
Transmission: T56
Axle/Gears: 4.10 D44
Re: How do I determine if the catalytic converter needs replacing?
Catalyst = promotes chemical reaction without being consumed. The CC cleans up your exhaust. It works best when the exhaust is as clean as possible in the first place... not too rich and not too lean. The O2 sensor provides feedback to the ECM, which adjusts the mixture for better emissions. (And more power, of course! 
)
Read http://en.wikipedia.org/wiki/Catalytic_converter.
)Read http://en.wikipedia.org/wiki/Catalytic_converter.
Supreme Member
Joined: Jul 1999
Posts: 5,183
Likes: 42
From: Oakdale, Ca
Car: 89 IrocZ
Engine: L98-ish
Transmission: 700R4
Re: How do I determine if the catalytic converter needs replacing?
So, let me see if I understand correctly. The O2 sensor function has really nothing to do with the catalytic converter. I already understood that it detected how much oxygen was in the exhaust where it exited the engine and is used to tell the ECM to adjust the mixture at the carb. And disconnecting it will not give an indication of whether the CC is bad or not. Correct?
Correct
So ... (1) If I need to cut out the existing CC just to tell if I can still see through it or not (and again, if I do this, I might as well replace it with a new CC), (2) if doesn't have a "bad cat" smell, and (3) pressure indication is good (no exhaust restrictions), then I suppose the only other thing to determine if the CC is still good is to take it through emissions? (And in Washington state, I don't need to take anything older than 25 years through emissions, so I won't worry about it.) Does all this make sense?
Yes, makes sense..but suggest you at least get a scan tool to monitor rich/lean...even if you remove the Cat...if it runs clean, why even spend the dollars for a Cat, put in an "off road" pipe.
If the pressure reading is good indicating there isn't any backpressure, I'm probably not going to worry about the CC at this point. However, at some point after I get everything else completed with the restoration, I probably will get the CC replaced.
Correct
So ... (1) If I need to cut out the existing CC just to tell if I can still see through it or not (and again, if I do this, I might as well replace it with a new CC), (2) if doesn't have a "bad cat" smell, and (3) pressure indication is good (no exhaust restrictions), then I suppose the only other thing to determine if the CC is still good is to take it through emissions? (And in Washington state, I don't need to take anything older than 25 years through emissions, so I won't worry about it.) Does all this make sense?
Yes, makes sense..but suggest you at least get a scan tool to monitor rich/lean...even if you remove the Cat...if it runs clean, why even spend the dollars for a Cat, put in an "off road" pipe.
If the pressure reading is good indicating there isn't any backpressure, I'm probably not going to worry about the CC at this point. However, at some point after I get everything else completed with the restoration, I probably will get the CC replaced.
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Member
Joined: May 2014
Posts: 184
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From: Seattle, WA
Car: '84 Camaro Z28
Engine: 305 HO
Re: How do I determine if the catalytic converter needs replacing?
Good point. If it's already running at or near the desired ratio, why worry about it? I suppose that I just wanted everything to be in working order and to be "right". However, if it's not hurting anything, then why bother to spend the money and put out the effort? If the day ever comes that WA state (specifically King County) changes its mind about old cars, then I'll worry about it. And yes, I do need to get a scan tool, too. Thanks for the input. 
Thread Starter
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Joined: May 2014
Posts: 184
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From: Seattle, WA
Car: '84 Camaro Z28
Engine: 305 HO
Re: How do I determine if the catalytic converter needs replacing?
Catalyst = promotes chemical reaction without being consumed. The CC cleans up your exhaust. It works best when the exhaust is as clean as possible in the first place... not too rich and not too lean. The O2 sensor provides feedback to the ECM, which adjusts the mixture for better emissions. (And more power, of course! )
Read this.
)Read this.
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