Borrow your stock headers?

H

HHS2K

Registered Member
Joined
13 March 2014
Messages
34
Hi everyone!

My name is Will and I'm an owner of a 91' NSX purchased off Prime two years ago locally. I've been a lurker on this site for a while and have enjoyed learning much about the car and ownership experience through the years (even before my purchase).

I now have a problem passing smog, like many Cali owners. Currently, my car is at Hilltop Auto and Don is checking things out. We believe a reversion to stock headers will get me the Pass. I have no other mods that would affect smog and car is very lightly modded overall (eg. suspension components only).

Question: may I borrow anyone's STOCK HEADER for OBD1 NSX for one day just to pass smog? All you need to do is drop car off at Hilltop in Daly City. I will cover cost of removal and reinstall of course. Happy to take you out to lunch as well and chat more about our cars!

Hope to get some help from this Forum. Just reply here or just PM me.

Very grateful in advance. Thanks guys!

Will
 
Do you know what kind of headers they are? Also, are you heating up the cats nice and good first? Like Sduff says, people damn near give oem 91-94 headers away. You could post a WTB thread here. People also sell them for very cheap in the for sale threads. Removal and reinstallation of headers that are on a car already will cost you a fortune!
 
Last edited:
Thanks everyone for your quick responses.

To answer some questions: I'm currently running DC headers. And yes, Don (and I personally before him) have been doing the usual things to try to get the Pass, incl. warming up the cats, cleaning intake temp sensor, replacing O2 sensors, pouring in a bottle of fuel cleaner. I've read so many "smog" threads on Prime already to no avail.

Will hope to hear more from folks and also post a WTB. Thanks for the advice.
 
You bet. I threw my na1 headers in the garbage. Otherwise they'd be yours.
 
OK few things:
- headers may contribute to the failure but only due to SS not holding heat as well as cast manifolds. When (date) have you failed the test? Where and for how long car was sitting and cooling down?

- How many miles on your car? Any idea on the health of your cats? When did you pass last time and in what configuration?

- Please post test results. Otherwise it's hard to understand what's going on.

- There are two sets for sale on prime right now.
 
Here are my latest readings, 3rd try in the last few weeks. Fail on slow speed. Pass on fast. But failed EVAP Test (first two times passed this one). These have all improved since Don worked on the car.

I'm at about 61.5K miles. Passed two years ago (prior owner) with same car setup (perhaps his smog station wasn't legit)? Car currently (and throughout its life) has stock cats.

Fuel EVAP Test = Fail (Passed the first two times though)

15MPH
RPM 1560
C02= Meas 14.7
02 = Meas 0.5

HC 240
Max 80

CO 0.66
Max 0.52

NO 300
Max 549

Fail

---

25mph
RPM 2560
C02= Meas 15.4
02 = Meas 0.1

HC
62
Max 62

CO
0.02
Max 0.61

NO
11
Max 507

Pass
 
What rear tire size? Just curious.

Also when you say since don worked on the car what does it mean? What has been identified as a failing part and since replaced?
 
Last edited:
I'm running 16/ 17 stock wheel combo from 94+. In terms of work done since late Dec, after my first failed smog, it's been the following:

1) cleaning intake temp sensor
2) replacing O2 sensors
3) pouring in a bottle of fuel cleaner
4) checked spark plugs (looks ok)

Potential next steps:
5) oil change (last one was April'16, only about 3k ago)
6) clean the EGR
7) another bottle of fuel line cleaner

Car is stock other than DC headers and exhaust; suspension components. Appreciate any input forum owners can give me. Thanks!
 
Looking at your HC reading for 15mph run it looks like you're running super lean or your cats are weak. Unlikely to be cats given low mileage. So check (borrow or swap with good parts) ignition and timing.
 
Thanks a lot for the observation and advice Solidol. I spent some time reading up on running lean and thinking about why this situation would happen to my car. I have a hypothesis. So I checked on all my records from when I took ownership (2 years ago). Everything I've had done has been (very typical) maintenance items, from valve gasket, to VTEC seals, to oil pan seal etc.

But last April or ~1000 miles ago, I put in a DF Side Scoop and Uni Filter combo from SOS. This combo replaced my stock scoop and stock paper filter. I'm running stock airbox by the way. This swap was performed so quickly and easily by my shop at the end of last year's OLF that I almost forgot about it. But when picking up the car this week, I was reminded of this mod by the awesome sound that it makes at throttle.

Then, I started researching about the effects of a pre-oiled air filter like the Uni and some folks believe these types of filters can clog up the intake, MAP sensor and other parts downstream. With so little time and miles since this mod, I am not sure whether this is would have caused my smog issue. But I did read about dirty/ failing MAP sensors which would then lead to high HC levels. It's clear thought at change of ownership 2 years ago, and obviously before the scoop went it, all Smog readings were fine. I have no reason to believe that the smog report from PO was not legitimate.

Setup when I bought the car was: Stock intake, stock scoop, and paper filter, DC headers, Thermal exhaust = Passed smog
Current setup (from April'16 on): Stock intake, DF scoop, and UNI filter, DC, Thermal = Failing smog
Car has always been NA. Not chipped (I believe).

Could my current setup be the cause?

Thanks for the forum's continued advice here!
 
Well that certainly lets more air in but brain should be able to adjust and drop required fuel amount to be at 14.7 and such. I hate those filters though - the best for stock or so NSX is paper filter. Sure if you go with boost you can see gains as paper is more restrictive. It's a trade-off. And it requires maintenance too. Never a fan. Prefer better filtration over CFM. If Map is acting up you would know. And IAT sensor you have already cleaned up. I would check coils, spark plugs, igniter. And put paper filter back :)
 
Thanks gents for the advice and offer to help. I'm going to try to keep my header and exhaust setup for now. Looking back at smog records from PO, I'm thinking those components are good, and cats are good. The only setup thing that I have changed (that affects Smog) in the limited time and miles of my ownership is the scoop and air filter so I'm reverting back to paper tonight. I really hope it's just the Uni and impact of the oil from it.

Oh, new spark plugs were put in before the last smog test (numbers above from two weeks back). I thought Don was just going to check on my existing plugs but he put in a new set anyway. As you can see, I still fail.

So when I have a bit more time, I'm going to check on what solidol suggests and perhaps check if there's a leak somewhere as well. Parts that haven't been checked/cleaned/ changed are fuel filter, MAP, EGR.

Thoughts?
 
HHS2K said:
Thanks gents for the advice and offer to help. I'm going to try to keep my header and exhaust setup for now. Looking back at smog records from PO, I'm thinking those components are good, and cats are good. The only setup thing that I have changed (that affects Smog) in the limited time and miles of my ownership is the scoop and air filter so I'm reverting back to paper tonight. I really hope it's just the Uni and impact of the oil from it.

Oh, new spark plugs were put in before the last smog test (numbers above from two weeks back). I thought Don was just going to check on my existing plugs but he put in a new set anyway. As you can see, I still fail.

So when I have a bit more time, I'm going to check on what solidol suggests and perhaps check if there's a leak somewhere as well. Parts that haven't been checked/cleaned/ changed are fuel filter, MAP, EGR.

Thoughts?
Click to expand...

From my experience, I can tell you that I did EVERYTHING (and I mean everything...) shy of factory manifolds/cats and my car (1991) would not pass smog. The two factors that were changed, and that I can attribute to the car finally passing were:
- Factory manifolds and cats
- Scheduled an appointment with an NSX familiar smog shop (Eddie at West Sac Smog). This way, I was able to get my car nice and hot, and get right on to the rollers for the test. This will prevent the car from cooling off.

Be aware that the 1991 model year has tighter maximum allowable emissions than it originally had. I'm not sure what year this was done, but the limits were changed, and this may have affected the difference between a pass during prior years, and a fail today.

Hopefully this points you in the right direction. It was a tough pill for me to swallow to sell my DC headers and replace them with factory manifolds, but it's nice knowing that I can get through smog with little trouble every two years...
 
I have no clue why some of you guys do not pass. I have D.C. Headers, have had several cat back exhausts, a ctsc, a previously NA motor, a uni filter, a stock filter, a k&n intake, a downforce scoop, no scoop, two scoops, etc... Never ever had 1 time where I did not pass and I don't grease palms either. I follow the laws. :confused: That must be such a pain in the ass.
I think the only thing different I may have done years ago was change every single coil pack on the car.
 
Have you changed your gas cap and vacuum tubing on the car yet?
 
kertong said:
jinks, you're up by sacramento right? unenhanced smog? down here we have to pass "enhanced" smog tests where idle as well as load conditions are measured (goes on a dyno-like roller). apparently much tougher to pass, and just keeps getting tougher! :(
Click to expand...

I feel everyones pain here guys truely. But I live in Pinole. On the back of the Berkeley mountains near wild cat canyon and the san pablo dam. I also smog my car at a different smog check station almost every two years. I just wish I could find the reason for this? As I mentioned I had no issues whatsoever truthfully. Just trying to help out. There has got to be some logical explanation for this? If not, I will gladly share my smog check spots within my last 6 years of ownership...
 
Last edited:
kertong said:
nice! i know the area - used to go fishing by san pablo dam/reservoir in my college days! and the in n out / best buy plaza off the 880 was a frequent outing after some, eh, "recreational" college activities haha.

ah, sorry to go off topic but i have some very good memories of that time and place of my old life!
Click to expand...
No worries man. No offense taken. This weekend I will pull up my last 3 smog checks during the past 6 years. I want to help people who have issues here. I still scratch my head wondering why people have issues. It is quite a conundrum.
 
kertong said:
Hmm, given you changed the manifolds and cats at the same time, I'd attribute the changes in readings to the cats - headers shouldn't change emissions, should they? If you have factory cats, I'd say it's worth giving headers a try again - fwiw, i have stock cats but comptech headers and I passed last year (and that's with a badly threaded plug #2 , haha!)
Click to expand...

Ya Ray, the cats were BRAND NEW, and still wouldn't pass with the DC headers on there. These were the EARLY comptech/DC headers, some of the best ever made for this car, but guaranteed to fail smog. It has to do with how much heat is dissipated before the exhaust gases can reach the cats. I am sure I could find some modern aftermarket headers that bolt on to the factory cats that I now have, get them SMOKING HOT, and probably pass. I'm just done with the headaches of worrying every two years. I've been down that road with my turbocharged 88 Prelude... Maybe some day in the future someone will have me as a believer in a certain brand of headers that wont be an issue, but that's a pricy gamble. Right now all I worry about is 12 bolts and about 15 minutes to swap out my test pipes with cats every 2 years, I can live with that. Yeah i'm down about 10 HP, but hey, if I wanted HP, I would not have bought an NSX, lol...
 
The older D.C. Headers had the worst design ever as it had a flex joint which would always crack. The D.C. Headers I bought a few years ago had the ball joint mounting. No issues what so ever.
 
Hey gents, good to see the continued advice here. I've read all the smog threads on this forum, and other forums too it seems, but to no avail. I haven't touched the stock cap, changed the gas cap, or changed vacuum tubing. Indeed, only thing I did since my last update was to replace the Uni with a paper filter. I'm not going to smog it a 4th time until I have a better idea of what's going on. I've paid the reg for the year.

After a few days of looking over the smog records, I noticed a few things.

Last time passing smog successfully was Oct 2014 when PO sold the car to me at 57.9 miles. DC, Thermal, Stock Cat (as said in previous post). Readings were:
[TABLE="width: 896"]
<colgroup><col width="64" span="14" style="width: 48pt;"></colgroup><tbody>[TR]
[TD="width: 64"][/TD]
[TD="class: xl66, width: 64"][/TD]
[TD="class: xl66, width: 64"]CO2[/TD]
[TD="class: xl66, width: 64"]O2[/TD]
[TD="class: xl66, width: 64"]HC[/TD]
[TD="class: xl66, width: 64"][/TD]
[TD="class: xl66, width: 64"][/TD]
[TD="class: xl66, width: 64"]CO[/TD]
[TD="class: xl66, width: 64"][/TD]
[TD="class: xl66, width: 64"][/TD]
[TD="class: xl66, width: 64"]NO[/TD]
[TD="class: xl66, width: 64"][/TD]
[TD="class: xl66, width: 64"][/TD]
[TD="class: xl66, width: 64"][/TD]
[/TR]
[TR]
[TD][/TD]
[TD="class: xl67"]rpm[/TD]
[TD="class: xl67"]meas[/TD]
[TD="class: xl67"]meas[/TD]
[TD="class: xl67"]max[/TD]
[TD="class: xl67"]ave[/TD]
[TD="class: xl67"]meas[/TD]
[TD="class: xl67"]max[/TD]
[TD="class: xl67"]ave[/TD]
[TD="class: xl67"]meas[/TD]
[TD="class: xl67"]max[/TD]
[TD="class: xl67"]ave[/TD]
[TD="class: xl67"]meas[/TD]
[TD="class: xl66"][/TD]
[/TR]
[TR]
[TD="align: right"]15[/TD]
[TD="class: xl65, align: right"]2550[/TD]
[TD="class: xl65, align: right"]15.2[/TD]
[TD="class: xl65, align: right"]0.1[/TD]
[TD="class: xl65, align: right"]80[/TD]
[TD="class: xl65, align: right"]25[/TD]
[TD="class: xl65, align: right"]79[/TD]
[TD="class: xl65, align: right"]0.52[/TD]
[TD="class: xl65, align: right"]0.03[/TD]
[TD="class: xl65, align: right"]0.11[/TD]
[TD="class: xl65, align: right"]549[/TD]
[TD="class: xl65, align: right"]233[/TD]
[TD="class: xl65, align: right"]188[/TD]
[TD]pass[/TD]
[/TR]
[TR]
[TD="align: right"]25[/TD]
[TD="class: xl65, align: right"]2362[/TD]
[TD="class: xl65, align: right"]15.2[/TD]
[TD="class: xl65, align: right"]0.1[/TD]
[TD="class: xl65, align: right"]62[/TD]
[TD="class: xl65, align: right"]20[/TD]
[TD="class: xl65, align: right"]58[/TD]
[TD="class: xl65, align: right"]0.61[/TD]
[TD="class: xl65, align: right"]0.03[/TD]
[TD="class: xl65, align: right"]0.09[/TD]
[TD="class: xl65, align: right"]507[/TD]
[TD="class: xl65, align: right"]194[/TD]
[TD="class: xl65, align: right"]185[/TD]
[TD]pass[/TD]
[/TR]
</tbody>[/TABLE]

Now, with same DC/ Thermal/ Stock Cat + Scoop/ Uni + new spark plugs, new O2 sensors, air intake sensor cleaned + bottle of guaranteed to pass + hot cats, my 3rd try in Jan 2017 at 61.4 miles was this:

[TABLE="width: 896"]
<colgroup><col width="64" span="14" style="width: 48pt;"></colgroup><tbody>[TR]
[TD="width: 64"][/TD]
[TD="class: xl67, width: 64"][/TD]
[TD="class: xl67, width: 64"]CO2[/TD]
[TD="class: xl67, width: 64"]O2[/TD]
[TD="class: xl67, width: 64"]HC[/TD]
[TD="class: xl67, width: 64"][/TD]
[TD="class: xl67, width: 64"][/TD]
[TD="class: xl67, width: 64"]CO[/TD]
[TD="class: xl67, width: 64"][/TD]
[TD="class: xl67, width: 64"][/TD]
[TD="class: xl67, width: 64"]NO[/TD]
[TD="class: xl67, width: 64"][/TD]
[TD="class: xl67, width: 64"][/TD]
[TD="width: 64"][/TD]
[/TR]
[TR]
[TD][/TD]
[TD="class: xl68"]rpm[/TD]
[TD="class: xl68"]meas[/TD]
[TD="class: xl68"]meas[/TD]
[TD="class: xl68"]max[/TD]
[TD="class: xl68"]ave[/TD]
[TD="class: xl68"]meas[/TD]
[TD="class: xl68"]max[/TD]
[TD="class: xl68"]ave[/TD]
[TD="class: xl68"]meas[/TD]
[TD="class: xl68"]max[/TD]
[TD="class: xl68"]ave[/TD]
[TD="class: xl68"]meas[/TD]
[TD][/TD]
[/TR]
[TR]
[TD="align: right"]15[/TD]
[TD="class: xl66, align: right"]1560[/TD]
[TD="class: xl65, align: right"]14.7[/TD]
[TD="class: xl65, align: right"]0.5[/TD]
[TD="class: xl65, align: right"]80[/TD]
[TD="class: xl65, align: right"]25[/TD]
[TD="class: xl66, align: right"]240[/TD]
[TD="class: xl65, align: right"]0.52[/TD]
[TD="class: xl65, align: right"]0.03[/TD]
[TD="class: xl66, align: right"]0.66[/TD]
[TD="class: xl65, align: right"]549[/TD]
[TD="class: xl65, align: right"]233[/TD]
[TD="class: xl65, align: right"]300[/TD]
[TD]fail[/TD]
[/TR]
[TR]
[TD="align: right"]25[/TD]
[TD="class: xl65, align: right"]2560[/TD]
[TD="class: xl65, align: right"]15.4[/TD]
[TD="class: xl65, align: right"]0.1[/TD]
[TD="class: xl65, align: right"]62[/TD]
[TD="class: xl65, align: right"]20[/TD]
[TD="class: xl65, align: right"]62[/TD]
[TD="class: xl65, align: right"]0.61[/TD]
[TD="class: xl65, align: right"]0.03[/TD]
[TD="class: xl65, align: right"]0.02[/TD]
[TD="class: xl65, align: right"]507[/TD]
[TD="class: xl65, align: right"]194[/TD]
[TD="class: xl65, align: right"]11[/TD]
[TD]pass[/TD]
[/TR]
</tbody>[/TABLE]

You can see that a few things jump out immediately on this new test. 15 mph RPMS are very low (smog folks tell me it has to be this low), and thus my HC and CO are resultingly quite high.

As I shared with you all, the only mod I've done that would (possibly) affect my smog would be the scoop and/or Uni (but some of you have told me this wouldn't). And if maintenance items have failed since 2 years back, I've taken care of them prior to my January test. And with the relatively low mileage, I can't imagine my original cat would fail.

At this point, I'm almost positive that with so little mileage since the last pass, and assuming low chance that the car could have developed issues since, my NSX simply cannot meet the stringent standards for 15mph running at such low RPMs.

What a frustrating experience.
 
Seems as though with such low rpms at 15mph they are trying to smog it in too high of a gear.

In looking through the So Cal smog check thread, another owner was having a similar issue when the tech was trying to run the car in 2nd gear for the 15mph test. He was able to get the tech to run the 15mph test in 1st gear, resulting in a pass.
 
Last edited:
Don't give up HHS2K. Your so close!
 
Jinks said:
The older D.C. Headers had the worst design ever as it had a flex joint which would always crack. The D.C. Headers I bought a few years ago had the ball joint mounting. No issues what so ever.
Click to expand...

That could have been it for me... May be worth looking into CA smog legal headers soon :biggrin:
 

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