1991 ZR1 Failed Emissions

[Jason 91]

Happy Thanksgiving everyone!

To my disappointment, my ZR1 failed its emissions test last week. I initially expected that it was the original injectors finally starting to go out and I was ready to replace them. I took the car to a local Corvette shop that has done work for me before and they told me that while the car may also need new injectors, one of the catalytic converters is bad.

I know that the exhaust manifold and catalytic converter are all one piece and no longer available so I’m unsure what to do at this point in time. I’d like to keep the car as close to stock as I can (if I start modifying this car, my discretionary spending won’t be able to keep up with my desires) and I need the car to pass emissions. What are my best options assuming one of the catalytic converters needs to be replaced? If I can find them, can I bolt the 1992 – 1995 exhaust manifold/catalytic converter set up to a 1991?

Here’s some other information that may or may not be of use to you.

49,300 miles on the odometer
No trouble codes
New plugs, wires, PCV valves, coolant and thermostat, fuel filter, Mobil 1 and filter and rear differential fluid
5,280 altitude

Thanks in advance for your help.

Jason

[gbrtng]

How were they able to isolate "one bad cat" since the flow from both sides gets mixed in the resonator? At 50,000 miles the original cats should still be pretty good. Better you post the test results and let us take a look before you start carving up the stock exhaust system. Also describe CO's test and what the pass/fail limits are.

[Ccmano]

Originally Posted by gbrtng

How were they able to isolate "one bad cat" since the flow from both sides gets mixed in the resonator? At 50,000 miles the original cats should still be pretty good. Better you post the test results and let us take a look before you start carving up the stock exhaust system. Also describe CO's test and what the pass/fail limits are.

I agree, I wouldn't be too fast to jump on this. Remember, shops are generally unfamiliar with the Z. I just moved from Denver in the spring, which shop are you using? Generally there are no shops in Denver that are particularly good on the Z. The are some members that may be able to help you.

Are you failing because of HC or NOx issues? Do you have an aftermarket PROM? How do you know there are no codes?

Lots of questions.... Let us know.

Hans

Give us all the details. ZR-1's can be a pain to get through the emissions program. A lot of states have been tightening standards on older cars.

[Z Factor]

Originally Posted by Jeffvette

A lot of states have been tightening standards on older cars.

All the more reason to live in a state like Florida with no emission tests.

Originally Posted by Jason 91

Happy Thanksgiving everyone!

]

Thanks

Happy Thanksgiving to you as well.

If it does turn out to be a cat, PM me as I know a guy down here with a set of stock manifolds/cats who might part with them.

[Jason 91]

Thanks for all of the quick replies – I really appreciate it.

All,

Here’s a summary of the test results on 11/14;

Emissions Test Data
Fail HC GPM = 6.2399 with a limit of 2.5
Fail CO GPM = 46.2872 with a limit of 20.0
N/A CO2 GPM = 548.0899 with no limit specified
Pass NOx GPM = 2.6330 with a limit of 6.0

Visual
Pass Catalytic Converter
Pass Fuel Filler Restrictor
Fail Air Injection System The test facility would not tell me what they believed was missing and the shop believes that this was due to them not being familiar with the ZR1.
Pass Oxygen Sensor
Pass Check Engine Light

Here’s a summary of the prior test results on 10/7/05 less 1,428 miles on the odometer;

Pass HC GPM = 2.4750 with a limit of 2.5
Pass CO GPM = 14.5508 with a limit of 20.0
N/A CO2 GPM = 495.5999 with no limit specified
Pass NOx GPM = 2.9937 with a limit of 6.0

Visual
Pass Catalytic Converter
Pass Fuel Filler Restrictor
Pass Air Injection System
Pass Oxygen Sensor
Pass Check Engine Light

Here’s a link that explains the type of test that they do;

http://www.aircarecolorado.com

Gbrtng:

I’m not sure how they were able to isolate one cat being bad. I have not had any of the normal (i.e., rotten egg smell, hesitation, rattling) symptoms so I’m not certain. The guy I deal with is not real eager to drill holes or cut up the stock exhaust either which gives me some comfort. If it is a cat, I’m particularly concerned about what caused it to go bad in the first place.

Ccmano:

I took the car to Corvettes Only on Hampden and they are the ones who confirmed no codes. The PROM is not aftermarket.

http://www.corvettesonlydenver.com

Jeffvette:

I was originally concerned about the restrictions being tighter, but they are the same as they were two years ago so something must have happened between now and then. If it was close to the limits, I would have insisted driver error on the dyno and asked for a free retest. Tough to argue with what was being put out.

Z Factor:

That’s good to know. I will PM you if that’s the route I need to go.

Thanks,

Jason

[gbrtng]

Lots of HC and CO degradation in less than 1500 miles. I highly suspect a bad injector or two. The injector coil resistance can be measured at the ECM and should be in the range of 12 ohms. Anything less than 8 ohms should be replaced. Also, did you have fresh gasoline? Old fuel - more than 3 months old - might make a difference too. Were the engine and cats up to full operating temperatures when the test was run? If you have an infrared temperature gizmo you can shoot the cat temps which should be above 600 degrees or so - if one is cool, that's the weak one.

FWIW, the HC numbers two years ago were marginal.

[kenthompson]

I would probably be willing to remove mine, after I get the headers.

KT

[GOLDCYLON]

Originally Posted by Jason 91

Thanks for all of the quick replies – I really appreciate it.

All,

Here’s a summary of the test results on 11/14;

Emissions Test Data
Fail HC GPM = 6.2399 with a limit of 2.5
Fail CO GPM = 46.2872 with a limit of 20.0
N/A CO2 GPM = 548.0899 with no limit specified
Pass NOx GPM = 2.6330 with a limit of 6.0

Visual
Pass Catalytic Converter
Pass Fuel Filler Restrictor
Fail Air Injection System The test facility would not tell me what they believed was missing and the shop believes that this was due to them not being familiar with the ZR1.
Pass Oxygen Sensor
Pass Check Engine Light

Here’s a summary of the prior test results on 10/7/05 less 1,428 miles on the odometer;

Pass HC GPM = 2.4750 with a limit of 2.5
Pass CO GPM = 14.5508 with a limit of 20.0
N/A CO2 GPM = 495.5999 with no limit specified
Pass NOx GPM = 2.9937 with a limit of 6.0

Visual
Pass Catalytic Converter
Pass Fuel Filler Restrictor
Pass Air Injection System
Pass Oxygen Sensor
Pass Check Engine Light

Here’s a link that explains the type of test that they do;

http://www.aircarecolorado.com

Gbrtng:

I’m not sure how they were able to isolate one cat being bad. I have not had any of the normal (i.e., rotten egg smell, hesitation, rattling) symptoms so I’m not certain. The guy I deal with is not real eager to drill holes or cut up the stock exhaust either which gives me some comfort. If it is a cat, I’m particularly concerned about what caused it to go bad in the first place.

Ccmano:

I took the car to Corvettes Only on Hampden and they are the ones who confirmed no codes. The PROM is not aftermarket.

http://www.corvettesonlydenver.com

Jeffvette:

I was originally concerned about the restrictions being tighter, but they are the same as they were two years ago so something must have happened between now and then. If it was close to the limits, I would have insisted driver error on the dyno and asked for a free retest. Tough to argue with what was being put out.

Z Factor:

That’s good to know. I will PM you if that’s the route I need to go.

Thanks,

Jason

Jason I had reading like this on my 91 and the culprit was indeed a bad o2 sensor and a bad cat on the passenger side. Upon inspection the honeycomb was completely gone on the right cat side. The specualtion was that the cat was dropped and the bits went into the resonator. We thought it was possible somebody punched the cat however with an uncomprimised cat on the driver side we detemined that was unlikely this was the best guess we could come up with. We replaced the cat and the 02 sensor problem solved.

[tomtom72]

I'm no mechanic Jason, but I agree with Glenn. You seem to have done the tune up stuff...save the coils....they could be weak which would contribute to dirty exhaust. O2 sensors may be lazy....mine were after 16 yrs.I swapped them out and my test scores improved dramatically. I'd want to see the scan data on the O2's. Are the injectors OE? before my OEM injectors failed they did a bit of telegraphing via scan results....but I was too ignorant to see the warnings.

Maybe tap the cats a bit to see if anything is rattlin' in them? The I/R probe is proly best bet but the tap test sometimes yields results.

Our AIR pumps are electric so maybe the inspector missed that? Yours is still there, left front under the headlight? Maybe the purge canister/ evap. system is worth looking at?....to me it's a long shot but if all else fails??


Tom

Hey Jason, I'm not trying to be smart. Please look at this link and then look at your emissions' test results and think about what's going on. To me it seems unlikely that the cats are bad, old matbe but not shot. JMHO
http://en.wikipedia.org/wiki/Catalytic_converter