The Truth About The AIV / PAIRC System

[Julio Bro!]

Contrary to what some of us thought, this infamous valve has an important purpose, and it's not warming up the cat.

The ALLDATA website explains that, at idle and deceleration, the ECU opens the AIV to inject fresh air into the exhaust before it enters the cat. This injection provides for afterburning of the exhaust gasses and WILL lower the CO and HC levels after going through the cat.

So, for those who have eliminated the system and don't pass the emisions test at idle, this is the main reason and need to install the PAIRC system back. Those who pass, it's because the CO and HC "good" levels aren't required to be so low in the test.

Some have noticed that later engine models don't have the PAIRC system and it's not because it's useless or caused somekind of trouble. At a tuner's magazine website (don't remember which) I learned that for those models the ECU was tuned for a leaner burn at idle and thus, the exhaust didn't need help to lower the CO and HC levels.

Is there an easy trick for eliminating the AIV and pass the emisions test? NO, unless you can reprogram the ECU for a lean mix at idle. But there is hope; the magazine website explains that JWT can reprogram their aftermarket ECUs to eliminate the need for the AIV. So, if you have a JWT ECU, check for how much that could be done or if you're getting a new one, make sure it's modified for a leaner burn at idle.

Does eliminating the PAIRC system provides more performance? The magazine website says NO. Combustion made it's work already and the AIV system is for pure emisions treatment.

Because of some people's experience, I would say it helps performance a little, but maybe not enough so as to risk passing the emisions test. For some mods, removal is a most for needed space. In any case, a reprogramed ECU should be the best bet; if it isn't a most, leave it alone and be happy.
Modified by Julio Bro! at 1:09 AM 11/6/2004

[NateDogg]

Read my post about "Emissions results w/Removed SCV"

I don't have the AIV or the butterflies and I passed pretty difficult tests here in BC (similar to CALI).

JWT ECU or not, both our 240sx have removed AIV and one has a stock ECU. They both passed easily.

About the later models- you are somewhat correct. With OBD-0 like the S13 DOHC, the O2 sensor doesn't control the idle a/f ratios. Therefore, Nissan decided to put in the AIV to help contribute to the cat and reduce emissions. So basically its a lousy system that has a rich idle (to help keep it more smooth).

Removing a malfunctioning AIV system that spews exhaust into the intake is definitely a performance gain.

Maybe I'm such a great tuner that our cars are the exception. But, I doubt it! Just keep your car in good tune and it should pass or something is wrong.

Sounds like that magazine is full of opinions and lacks real test data.

[Julio Bro!]

I haven't check what is the SCV, but in your post you do indicate that your CO and HC numbers went up at idle...which is the problem when you remove the AIV.

If you passed your emissions test with that...good for you. But there are other places in which the car doesn't pass with those numbers...and then, you need the AIV back.

[slipnfall]

I don't have the AIV or the butterflies and I passed pretty difficult tests here in BC (similar to CALI).

Nate, do you have to pass visual as well in BC? I assumed that the AIV was included in the visual portion of the test... Perhaps they don't pay as much attention to the components when doing a tailpipe sniff.

[NateDogg]

Luckily there are no visual tests here otherwise I would've failed for sure. I probably wouldn't have taken the AIV out either if we had visual though.

Julio: could you tell me what the requirements are to pass emissions in the tougher areas? Just curious.

Thanks

[Julio Bro!]

I don't know what the units are, but there you have what I got and what is needed. Remember these are for idle speed, at high rpm I pass.

HC = 405 (need 300 to pass)

CO = 2.27 (need 1.50 to pass)

Today I reinstalled the AIV, it was easy. To see which of the 2 lines was vacuum and which atmospheric...well, there was only one sucking...so, duh. Tomorrow I'll take it for the test and we'll see.

[NateDogg]

HC - my reading was 40 PPM at idle - 20PPM driving at 3000rpm on a dyno

CO - my reading was .31% at idle - .03% driving at 3000rpm on a dyno

Sounds like you have a similar test.

I doubt the AIV will affect your readings enough to pass. The AIV is certainly not the cause of your problem. I would look to fuel injectors, fuel pressure, catalytic converter, cam timing and ignition timing, air filter, etc..

[Julio Bro!]

Do you understand the units here Nate? I need 300 to pass...300 what; could it be 30.0 ppm? And 1.50...of what?

[Julio Bro!]

Alright, I got:

HC = 370 and CO = 2.00

So, I still didn't pass, but these got lower, so the AIV definitely helps.

The guy doing the test asked if I replaced several of the things I had already done. He said: "You know, I used to have a '91, but it started with the emissions situation and drove me crazy. I spent quite a few bucks trying to solve it and nothing. I know what you're going through and a lot of people with old, but nice, well groomed cars have this problem. Let's say you give me $20 and I certify your car."

Of course I did, it was just $9 more than the actual cost. So for now I'm certified, but I'm still curious about the problem...I'll dig deeper and Nate...you we're right, theres something else going on...or...our cars are too old for this.

I have a new quirk though, which I'll post separately...see if any of you can help me there.

[InsanityInc]

I'm betting it's a leaky injector or a bad MAF. High HC means you're running abnormally rich. At least one of my injectors is leaking currently, and I need to order a replacement.

[Julio Bro!]

Hmmm, how may I be sure, could this appear in a code?

[NateDogg]

The easiest way to tell if an injector is leaking is to pressurize the system and then pull the plugs and look for raw gas on them.

Have you messed with cams at all? Does the coolant temp sensor show the right values? Have you cleaned the MAF?

[Julio Bro!]

You mean the spark plugs Nate or a part of the injector?

MAF is clean, cams are OEM, and temp sensor was replaced about 2 weeks ago.

[eddiec]

leaky injector will throw a code.

my 91 failed emissions twice brfore it passed. not sure what the eventual cure was, as i replaced quite a bit of stuff. a short list would include new injectors and o-rings, plugs, wires, temp sensor and a new cat. also took the car to someone with a consult and they found that my throttle was mis-adjusted causing it to stick open very slightly. but was enough to run rich. also dailed back the timing and lowered the idle rpm.

[InsanityInc]

leaky injectors throw a code sometimes. Mine threw a code once, even though it's been leaking for a long time (been a cause of problems), and it's still leaking, but I don't get an engine light.

Smell around your injectors. If any of them smell like gas, you probably have a leaky one.

[NateDogg]

If any of the plugs are wet then that cylinder's injector is leaking.

If all the plugs look the same you've got another issue.

[Julio Bro!]

Turns out my Random Tech cat was damaged, almost 4 years in use.

Got a Magnaflow cat at the muffler shop; I suppose I'll take the car for a test, sometime this next year, to see if the cat was the problem.

Also I have to find a good conditioned MAF, mine suddenly does the idle hunt...not always...sometimes. That's a bitchy thing!