CA smog help, what else is new...

Joined
11 June 2015
Messages
345
Location
Los Altos, CA
Hey guys,

I am trying to pass smog with my 1991 MT with 91K miles. Unfortunately, the car failed, and here are the numbers:
Smog_Fail_zpsxifobvm8.jpg


Here is a list of what i've done to the car:
- Oil and Filter Change
- Intake Air Temperature Sensor
- Air Filter
- Cleaned EGR Valve tested valve out of car, ok, but have not tested with valve installed.
- Replaced all 6 coil packs
- Replaced spark plugs
- Had fuel injectors professionally cleaned and tested, ok.
- Compression Tested Motor (200-230 on all cylinders)
- Replaced Rear O2 Sensor
- Replaced both catalytic converters (universal part#:332304), EO#:D-193-117
- Replaced fuel pump with Walbro 255, fuel pressure is 42 PSI at idle, and increases when RPMs are raised
- Reset ECU and re-learned the adaptives

Before the test, I drove around town for 15 minutes or so, and then the car idled at the smog shop for about 30 minutes. They could have been hotter, but I dont think it would have made the difference between passing or failing in this case.

What I find strange is that the HC reading is 168 at 15 MPH and 41 at 25 MPH with similar RPM readings. Do you guys have any ideas as to what could be causing this?

Thanks in advance for any help, you guys are an awesome community!
 
I can't comment on passing California emissions tests directly; however, a couple of observations from fiddling with after market FI systems and tuning on other cars (non NSX).

The test result is a bit hard to read; however, it appears that the failed tests are the hydrocarbons and NO. Confirming that your EGR is working and that the passages are clean is critical to meeting the NO numbers. Make sure the EGR lift potentiometer is working correctly (it could be working enough that the ECU does not generate a code). If you do a search on Prime, you will find some discussions of strategies for 'modifying' the EGR vacuum feed to EGR valve to cause it to stay open longer / more / whatever to help you meet the NO numbers.


The fuel pressure should rise as you open the throttle. The FPR is a rising rate regulator referenced to the intake manifold pressure and as you open the throttle the manifold pressure increases so fuel pressure would normally increase. Confirm that your fuel pressures meet the specs set in the Honda service manual and that is about as much as you can do (I assume you do not have any engine modifications requiring altered fuel pressure).

If you wipe the stored fuel trims from the ECU, it may take a long time for the ECU to recalculate the required trims. If the NSX ECU is like other ECUs, it will have a bunch of cells in memory defining different engine operating conditions and each cell may have a trim value. Each one of those trim values needs to be determined by having the engine operating in that range for a period of time. If you wipe the trim values from memory, you need to make sure that you drive around at 15 mph and 25 mph at the same engine RPMs used in the test to make sure that the ECU populates the cells corresponding to those engine speeds and throttle openings with the correct trim values. I don't know how long it will take the ECU to recalculate the required trims; however, don't reset the ECU and head off to the compliance test the next day.

The HC readings at 15 mph and 25 mph will be different even though the engine speed is roughly identical. Even though the engine speed is the same RPM, the throttle will have to be open more to achieve 25 mph than 15 mph. With a different throttle opening the manifold pressure is different and the ECU will be doing its fuel delivery calculation from a different cell on in its fuel map. Also, at 25 mph the engine will be working slightly harder and probably generating more heat which may be assisting the catalyst.

I note that you have an aftermarket exhaust. Is that the muffler or did you also replace the exhaust manifold? The OEM exhaust on the 91s was cast iron as I recall. If you switched from the OEM manifold to a tubular steel manifold, the tubular steel manifolds have a higher rate of heat loss than the cast iron manifold. The lower exhaust temperature may be impairing the operation of the catalytic converters. Cats generally operate better when hot; but, not too hot. Since the original compliance tests for the NSX were done with the cast manifold, the tubular manifold might be compromising your ability to meet the test standard. I doubt that this is the source of most of your problems; however, as a Hail Mary strategy if you get close to the limits, you could try wrapping your manifold to retain heat - which I think would just about be impossible with the manifolds installed on the car!

Definitely check your header for exhaust leaks or any leaks upstream of the O2 sensors and catalytic converters.

I don't know anything about those universal cats. Unless others have used them and confirmed that the car passes tests with them installed or the vendor certifies (and has test results) that they will meet the compliance tests on a 1991 NSX, give some consideration that you may never pass the tests with those cats. All cats are not created equal!

Finally, I suggest this if the more obvious solutions don't pan out. I assume that you are still running the OEM narrowband O2 sensors? If so, do you have access to a high impedance voltmeter or a portable scope or some other data logging device that would allow you to record the voltage on the O2 sensor? You would have to rig up a temporary connector to the O2 sensor to measure the voltage (you need to leave the connection to the ECU intact while doing this). The ECU uses the stock narrowband O2 sensor to attempt to control the fuel mix so that the AFR is right around 14.7. This is where catalytic converters work best at trying to reduce all three exhaust emission components.

http://www.endtuning.com/afr.html

Your CO values are good and your NO values are high. If you were running somewhat leaner than stoich, this tends to push up the NO values and give lower CO values which appears to mimic your results. Something to check.

If the AFR is at 14.7, the O2 sensor will typically give you a voltage of 0.5 volts. If the AFR is slightly rich the voltage will swing up to 0.9 - 1 volt and if it goes slightly lean it should drop to 0.1 volts. If you drive around and monitor the voltage, the voltage will likely bounce around; but should average out around 0.5 volts. If it is stuck at .9 volt or at 0.1 volt or is consistently above or below 0.5 volts, you may have a fuel mixture problem which is compromising the operation of your catalytic converters. This test should only be done after the ECU has had a chance to repopulate the fuel trims. Trying this test with a voltmeter will be a pain because the voltage will likely fluctuate quite a bit. The best arrangement is if you have something that can log the voltages on a laptop.

All of the above are suggestions and comes with no guarantees!
 
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I passed CA SMOG test in April this year with my all stock 1991 NSX, MT, 125,000 ml. Just prior to SMOG test both O2 sensors were replaced because of bogging problems, which took care of the problem. The day of the test I drove for about 30 minutes on the freeway in 3rd gear to keep RPMs up and to get cats good and hot. At the SMOG station I kept the engine running and waited less than 10 minutes before the technician (which was familiar with testing an NSX) started the test. I think getting the cats really hot is key, having to wait a half hour may allow them to cool to much.
 
Second the EGR lift potentiometer mine was bad. But only found that after replacing every other device in the EGR system.
Another item to check is the coolant temp sensor.
Also it would be good to monitor the vacuum pressure on the EGR to confirm it is active. The gear, gear ratio and wheel size will change the rpm and the amount/magnitude of command the ECU sends.
There is also a troubleshooting flow chart in the NSX shop manual if I recall correctly.
 
Thank you for the helpful and thorough responses guys!

My next step will be to test the EGR valve while applying vacuum with the engine running. I have not verified the passages are clear yet. I will post my results in the next few days after performing this test.

I agree that the cats not being hot enough could contribute to a high 15 MPH, but a low 25 MPH. This being said, I can always try a test with them hotter, but i'd like to find something a bit more tangible explaining how cool they can become. This leads me to the exhaust... There is a comptech exhaust after the cats, but I don't know if the headers are factory before the cats. I have looked at the headers, and they dont appear to be cast, they appear tubular. However, I have not found a single logo or CARB EO emblem on the headers. Also, there is a DC Sports sticker on the firewall, not sure if thats a clue...

Does anyone have a description or picture of the factory 1991 headers?

I have read that aftermarket headers can cool cats down and fail smog, so I would like to look further into this.

Thanks again for all of the help guys!
 
The stock 91 does not have headers, it has a cast iron manifold.
 
Found this pic online. Are these factory on the right?

nsx%20manifold_zpsrhmrstrp.jpg


If so, i've definitely got aftermarket headers.

It is worth installing factory manifolds? Should I expect significant results?
 
The ones on the right are the stock ones and I do not know if you will see an improvement (emission wise) with them. You could always have your headers ceramic coated or wrapped to hold the heat in them so the exhaust does not cool off.
 
Yeah I may explore that route. Or borrow a set of OEM manifolds...I'll test the EGR tonight, but maybe it's time a swing by Driving Ambition and talk to Shad. I only live about 10 minutes away.
 
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