Smog trouble - high HC and NO

N

NorCal91NSX

Registered Member
Joined
6 September 2005
Messages
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My 91 NSX is having trouble passing smog. I am hoping maybe someone here might be able to shed some light on what is going on with my car.

Here are the numbers:

15mph @ 2243RPMS
CO2% 13.2
O2% 1.3
HC 208 (max 114)
CO% 0.53 (max 0.73)
NO 929 (max 785)

25mph @ 2500RPMs
CO2% 13.8
O2%: 0.2
HC 138 (max 88)
CO% .13 (max .61)
NO 1049 (max 724)

As you can see, just the HC and NO readings are high. Do you guys think it could be the cats? I'm really hoping it isn't, as they seem to be very expensive to replace on the NSX.

I'm located in the California Bay Area.

Thanks a lot for the help guys.
 
$ $ $ $

On a fourteen year old car, it is not at all uncommon that the catalytic converters have consumed (expended?) the catalyst, thus resulting in poor gas emissions.

As much as you do not want to hear this.......get ready to buy new cats. I hear they are not cheap for the NSX.
 
I'm interested to find out why. I have a 91 with 95,000 miles as well as Comptech Headers and Exhaust when I had a smog test last year. Mine passed with no problems.
 
All right, here's what I think I figured out so far:

EGR system problems -> high NO
Running rich (O2 sensor, MAP sensor) -> high HC, high CO
Bad cats -> potentially high HC, CO, or NO, not necessarily at the same time

If I'm correct, it indicates that the car could possibly be fixed without replacing the cats if the EGR system is clogged AND the car is running rich due to bad O2 sensors or the MAP sensor...

However, given that the car is high mileage and isn't throwing the check engine light, it sounds to me like the problem is probably the cats.

If anyone has other ideas or can correct me, please reply. Thanks guys.
 
NorCal91NSX said:
My 91 NSX is having trouble passing smog. I am hoping maybe someone here might be able to shed some light on what is going on with my car.

Here are the numbers:

15mph @ 2243RPMS
CO2% 13.2
O2% 1.3
HC 208 (max 114)
CO% 0.53 (max 0.73)
NO 929 (max 785)

25mph @ 2500RPMs
CO2% 13.8
O2%: 0.2
HC 138 (max 88)
CO% .13 (max .61)
NO 1049 (max 724)

As you can see, just the HC and NO readings are high. Do you guys think it could be the cats? I'm really hoping it isn't, as they seem to be very expensive to replace on the NSX.

I'm located in the California Bay Area.

Thanks a lot for the help guys.
Click to expand...


a typical gas analysis for hc
your 2500 rpms your hc is ok, it should be between 25-150 ppm
but should not be that high at 2243.
when your hcs go up you have a rich mixture
could be a restricted air cleaner, restricted pcv valve, stuck open egr valve, if i were you i would find someone who can give you a 5 gas test and check the readings, they could find it in a second
your co is ok
and no, i think you mean nox which is oxides of nitrogen.
and that is the main pollutant of emissions.
whats weird is when nox goes up it goes lean.
i would say it was your egr but your nox wouldnt go up.
the rodium in the cat converter controls nox so it could be that.
get a 5 gas test, hope that helps :smile:
 
If you have a small hand vac pump, egr is easy to test as well as egr flow. A cat is a possibility but unlikely to leave such low co numbers. Also unlikely O2 would run so lean if in bad condition.
You need to test egr operation and flow before you buy any other parts.

Is all your maintenance current? Plug, air filter, etc.

MB
 
My 94 with the hi-boost CTSC and Comptech I/H/E failed smog the previous week for having HC levels too high at 1500 rpm. Everything else was okay though. I took my car to Don at Hilltop this past Monday and he suggested that older NSXes need to get their cats really hot right before the smog check is performed. He referred me to J&K Auto, also in Daly City. Tung over there knows how that the cats have to be hot before the test is performed, and he was able to get my car to pass.

Normally, he charges $37 + $8.25 for the document fee for the smog test, but he charged me $85 total due to the extra time it took to heat up the cats. Try and get him to commit to the $37 price when you go in. He upped the price after my car was done, saying he needed to charge me extra for the extra time he spent. I didn't bother to argue with him, since I was just glad that my car passed.

-CiaoBoy


NorCal91NSX said:
My 91 NSX is having trouble passing smog. I am hoping maybe someone here might be able to shed some light on what is going on with my car.

Here are the numbers:

15mph @ 2243RPMS
CO2% 13.2
O2% 1.3
HC 208 (max 114)
CO% 0.53 (max 0.73)
NO 929 (max 785)

25mph @ 2500RPMs
CO2% 13.8
O2%: 0.2
HC 138 (max 88)
CO% .13 (max .61)
NO 1049 (max 724)

As you can see, just the HC and NO readings are high. Do you guys think it could be the cats? I'm really hoping it isn't, as they seem to be very expensive to replace on the NSX.

I'm located in the California Bay Area.

Thanks a lot for the help guys.
Click to expand...
 
Took off the EGR valve today and though there was some carbon build-up it didn't appear to be bad enough to affect operation. Does anyone know of a shop near the East Bay (Walnut Creek) that is knowledgable about NSX's and would have a reasonably priced and accurate check to see what's wrong with the smog system. Thinking about trying to reset the ECU, run the car very hard and try smogging it one more time. Last time the smog shop left it running at 3,000 RPMS (not under load) for about 8 minutes and 2,000 RPMS for about 5 minutes though the car sat at idle for about 5 minutes before they ran the test. Any suggestions are much appreciated.
 
NorCal91NSX said:
Took off the EGR valve today and though there was some carbon build-up it didn't appear to be bad enough to affect operation. Does anyone know of a shop near the East Bay (Walnut Creek) that is knowledgable about NSX's and would have a reasonably priced and accurate check to see what's wrong with the smog system. Thinking about trying to reset the ECU, run the car very hard and try smogging it one more time. Last time the smog shop left it running at 3,000 RPMS (not under load) for about 8 minutes and 2,000 RPMS for about 5 minutes though the car sat at idle for about 5 minutes before they ran the test. Any suggestions are much appreciated.
Click to expand...

go to a good shop or a dealership and ask for a 5 gas test they will see what you read and find out whats wrong, especially at the dealer they have charts to read to tell them what it is
 
I agree with the suggestions to just get the cats really hot first - mine, as well several other NSX's that have been identified over the course of several of these threads have had the same issue. Just get it REALLY hot before you take it in & try to keep it hot with minimum wait time for the test.
High CO & NOx do point to the cats but you will improve their efficiency dramatically (probably enough to pass) by getting them hot enough to aid the reduction of the NOx and complete the oxidation of the CO to CO2.
 
Any other suggestions before I attempt one more time to get the cats as hot as possible before testing?

Also, if I fail again, where's the cheapest place to get a set of new cats? I saw Magnaflow replacement cats for sale on ebay for $56.34. This seems extremely inexpensive to me since they're normally upwards of $250 apiece. Does anyone have experience with these cats.
Thanks in advance.
 
Last edited:
You seem to be stuck on the cat converter idea. Factory cats last time I checked were around $900 each and one of them was unavailable at that time.
I have had the same problem and it took quite a few smog checks to pass last time (Aug 04). My HC and NO were all over the place over several different checks. I think you need to try getting the smog check place to do all their preliminary work and let you drive it hard. Then run it right in for the test.

A quick and dirty check on your Cats. Drive the car for a while. Take an oven thermometer(infrared tester would be nice if you have one) and hold it with pliers against the exhaust pipe(may have to dismantle the thermometer housing) on the inlet to the cat and then the outlet side. Should be approx 50 degree rise on the outlet side if they work efficiently.

You will also see how fast the exhaust temps drop at idle. Holding the engine at a high RPM, with no load, while waiting for the test seems to be pretty worthless, altho I suppose better than letting it idle.

You didn't mention if your car is stock. Intake, headers, etc.?

Try the cheap things first. EGR, PCV, fresh oil change, getting the cats hot.
Sulley
 
Actually, I don't know if it is stock other than that. Does anyone have any idea if any other aftermarket parts could be causing this smog trouble? Visually all I've seen so far are the comptech headers, comptech exhaust, dali coolant tank, drilled/slotted front rotors, pearl paint, red painted engine cover, red vacuum hoses, clear engine cover diamond etched with: NSX by Gutten and it had aftermarket rims that i've since replaced with stock 2000 rims. All the small things lead me to believe it's possible there's other performance modifications to the vehicle I don't know about. Anything I could look out for that might be causing this smog failure? Thanks for your help.
 
Have not tried this on the NSX (have not had to) but with many other cars, mostly Hondas, we put in a couple bottles of gas treatment in the tank the day before, from what I am told, the gas treatment burns completly clean. Has worked many times, when the numbers are close.
 
red vacuum hoses
Click to expand...

We might have a winner.

If you haven't fixed this by now, you are going to have to trace down all the hoses and verify. What fun you are going to have verifying all the specs and operations.

I have found on other cars that some hoses have internal check valves and restrictors. These are not obvious: they can be deeply lodged into the OEM hose.

Pay particular attention to the hoses for the charcoal canister. I would even remove the canister and see if any fuel drips out.

Drew
 

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