Chalkboss

I recently had a smog test done and the tech commented that the car was running pretty clean. I stared at the numbers for a while and got curious if these could tell me whether I am running lean or rich. I had been told the car was running very lean, and that was prior to me doing a full intake refresh. I'm sure this exhaust gas thing is not exactly new news to most of you guys (especially mechanics) but I wanted to dive into it on my own. I'm on vacation haha.

I found the Brettschneider equation and plugged my numbers into it and got a lambda of 1.015. This is on the lean side it seems, but only very slightly.

I have a few questions:

1) Is this a valid computation of lambda?
2) Is the lambda value of concern? If not, at what value should you start to worry if it's either too rich or lean?

Here are the numbers at 25mph:
C02= 14.3%
02= 0.4%
HC= 28ppm
CO= 0.08%
NOX= 128ppm

Thanks

GregBBRD

It actually looks pretty good, for a street car.

The NOX go crazy if it is too lean.

V2Rocket

Agree on those numbers looking great.
"Perfect" is a 1.00 lambda so you are just a hair over that.
1.00 is ~14.67:1 AFR, 1.015 would be ~14.92.

CO2% is a good indicator of AFR it seems...around 14.7 AFR the CO2% tends to be about 15%, in my experience.
Basically it is a sign of "perfect combustion".

Chalkboss

Thanks. Now I'm curious about the numbers prior to all this freshening work. I'm going to dig them up and post them when I can.

Crumpler

What's an emission test?

V2Rocket

Where on the temp gauge does the car run? I have noticed with tuning the DME in my 944, leaning it out a little for cruising makes it run hotter (just over the mid range) versus adding a little more gas (just under mid range), on the same road/air temperature/driving conditions.

Wondering if maybe there's still a tiny little vac leak somewhere, lots of hoses and clamps to miss on an intake refresh, maybe?

The O2 sensor, if functioning correctly, should be able to correct for that amount of "lean". How old is the O2 sensor?

I wouldn't worry about it though...and I bet the car gets great gas mileage

dr bob

Don--

The tailpipe tests are done with exhaust that's passed through the 3-way catalyst. So the validity for tuning/diagnostic purposes is pretty much none. To find out how well the engine is actually running, grab that gas sample upstream of the cats.

Speedtoys

Someth'ng many should know more about, sooner or later...

Chalkboss

It has always run between the middle and upper temp lines. Confounding the lean/rich is that I also put a new radiator in and new water pump, hoses, expansion tank. It seems to not wander towards that upper temp line as eagerly now but again I threw a lot of parts at it.

Oxygen sensor has about 30K on it. I had the MAF rebuilt last year, it was just out of spec.

John Speake

Was the air pump working when the tests were carried out ? If so, that will make lambda look weak.

Chalkboss

Yes the air pump was installed and functioning. Has to be to pass the visual inspection. Sounds like to really get a better understanding I need to use a wideband 02 sensor.

V2Rocket

Or, it's time for some PorKen chips

John Speake

The air pump is pumping air into the cats, which is fooling the true lambda measurement.

If you locate the diverter valve on top of the passenger side cam cover, then unplug the vac hose to it, then all the air pump output will be fed into the bottom of the engine air filter box. and the lambda will measure OK.

You need to plug the open vacuum line of course.