John Speake

At idle with full manifold vacuum, the airpump valve will be blowing lots of air into the exhaust. Less so as the higher roller speeds / throttle used in your tests.

But Jim appears to be saying that if the airpump is blowing into the exhaust partially (the other part is vented into the engine airfilter box) then this can raise O2 and NOX ?

Is this why running in a lower gear, needing a little more throttle can help with NOX levels ?

killav

My Nox was cut in half by the operater keeping the car in 2 and performing the test with the throttle opened up a bit more, which adds some fuel to cool the combustion chamber which lower's the Nox.

.02

Jim Morton

State of CA started to do SMOG tests with load to better simulate "around town" driving where the emission systems should be doing there job.. so yes, there should be more realisitic manifold pressures with the emission systems working per off idle condition(s).

For Bill's situation, I'm suggesting that the high O2 value from the gas analyzer suggests an air source where not intended... many possible sources, one being a faulty check valve, or bad gasket(s).

Personally, when sluething SMOG issues, I like the KISS principle and tend to track down the explanation for why the gas analysis can be explained. For Bill's case, the NOx and O2 levels are high relative to the proportion of the other gases, HC,CO and CO2. Per Dr. Bob's post, the cat wants a fairly narrow range of emission gases and with this in mind, the amount of O2 level seems the most suspicious to me.

Again, my $0.02, there could be other explanations... hard to tell without being at the car with some good diagnostic tools... like ST and a good gas analyzer.

IcemanG17

Its running quite lean.....

Lots of great ideas.....I like vac leak & bad 02 sensor..... Jim has a 5 gas analyzer and a dyno.....I'd say its worthwhile to "test" it at his place prior to smogging it again...

When I had a street 928...I found the "italian" tune up was HUGE.....ideally run the car HARD HARD HARD just before you pull up ot the smog shop and they take you straight on....but the lean readings are odd.....since Sharky only struggled with high HC, never lean....but only when cold...once warm it was fine...probably still is....

Jim Morton

Actually, it's only a four gas... but it still has value for the problem at hand.

;^)

GregBBRD

Actually, it is so close to passing NOX that if you took it for testing on a rainy day...the combustion chamber temperatures might be down enough for it to pass....

Just need to wait for a rainy day.....oh wait....not too hard to find, right now.

Also, even though all of the testing machines are "supposed" to be exact and calibrated....I've found huge differences from testing place to testing place.....many stories about that!

Randy V

No help on the NOX issue, but this may be helpful for posting your replies:

https://rennlist.com/forums/rennlist...-function.html

blown 87

And from day to day they will change.

I have also heard that some folks will hook a small water bottle with a needle valve with a hose hooked up to a vacuum source for cars that have a converter that is not really doing all it should on Nox.

Speedtoys

"which adds some fuel"

How?

You'll have a higher RPM, but will be making the same HP to run the roller speed..and consuming the same fuel..right?

SeanR

Because the car is not lugging in high gear at low speed.

Speedtoys

Well, is 2500rpm cruise, lugging?

Bill Ball

That does help. I never quite understood how that worked.

SeanR

For us they use speed, not RPM on the tests. So they do a 15mph test and then a 30mph test. It's something I've had to argue with techs over and over. Once I finally get a tech station to follow me, I get a good 6 months of use out of them until the tech either decides to let his cert go or he quits, then it's all over again.

GregBBRD

Imagine a job where all you do is breathe exhaust gasses all day long. Has to suck.

dr bob

Just don't don't inhale, Mr. President, and you'll be fine.