View Full Version : Which PIDs to monitor to get an idea on emissions?

May 10th, 2004, 03:27 AM
I am wondering which PIDS I should monitor to get an (direct or indirect)idea about my emissions - HC, Co etc. My state inspection is coming up
again and last year I barely passed - probably because I did not bother
to prepare my tuning carefully. New York has really tightened the limits.



May 10th, 2004, 01:09 PM
No OBDII scanner will do a "tailpipe sniff" - not even a TECH II.

Without going into a dissertation on emission test methods, if your state just checks for system readiness, no MIL, and a decent visual inspection, most well maintained and minorly modified cars will pass.

However if there is actual monitoring of the exhaust, (especially beyond just an idle check) you're going to have to be real careful how far from OEM you take your registered (on road) vehicle.

The EPA has mandated OBDII systyem to warn the driver of possible emissions faults, and to aid in diagnosing these faults. There are ways to modify and enhance the cars without causing an EPA OBDII fault. However a idle, part throttle and full throttle tailpipe test is "the horse's mouth" and can't be fudged!

What are the new specs? - go to your state's Pollution Control website and find out what your model year limits are, if it's a OBD/MIL test, a dynamometer test or what. Then you will know what you are up against.

Remembe a warm, mostly stock, and well maintained car with no MIL will usually pass even the toughest "smog check"... good luck!

May 11th, 2004, 06:20 AM
The test is a tailpipe emmissions test. It seems to me that
there ought to be a way to determine from the fuel flow, the engine
temperature, engine timing and intake air and perhaps additional
parameters (cam timing etc., cats functioning properly) an estimate
of emissions or an indicator of some sort. Perhaps VE can be used
in conjunction with the oxygen storgae capacity of the cats?


May 11th, 2004, 09:21 AM
There are no PIDs or other "relayed" data that will give the following information. Now that is not to say that measurements of combustion eficiency could give the unburnt hydrocarbons amount, and a measurement of the combustion temperature will get the oxides of nitrogen amounts, but it would be a "best guess" at best...(CO2 as well as CO are produced in the combustion, and usually are not a "critical" emission check, so that's not an issue.)

Tese data values vary greatly,and a special and expensive sensor is needed to get this data.

Believe me if there was a way to "real time" monitor the emissions during drive, the "Smog Nazi's" would have it emplaced on the vehicles by now...

BTW, the stored O2 in the cats is used for the reaction/reduction process and is in no way related to the "emissions" measurement.

May 13th, 2004, 08:40 AM
I would look for misfire current and misfire history. These
are the thing that will kill you on the sniffer, the cats are
for breaking down residual error products and not gross
cylinder imbalance or misfired charge. If it's clean and
the O2s are bouncing properly, and there are no system
faults you will almost certainly pass (unless you've gone
and changed to a non-stock cam / exhaust).

If I still lived in a fascist state I think I would hardwire the
AIR pump and O2 heaters for full-time power on "special
occasions". As well as the usual; fresh oil change @ heavier
weight, fresh air filter, maybe a top end clean the week before.
Oil is hard to crack and will elevate your HCs if it's burning out
of manifold (PCV) residue or ring bypass.

May 14th, 2004, 02:21 AM
Of course the obvious - but alas, the MIL would have been illuminated on the second misfire!
AIR pump and O2 heaters are totally unnecessary once exhaust has gotten above a few hundred degrees, and turning on the AIR pump may cause undsired readings at both emission test and PCM.

If your car is un-modded, and the MIL is out, you will pass.

If your car is modded, to a point where the emissions are affected, aside from hiring a private garage to test your emissions to see what changes need to be made, your best bet is to run it hard (get it really hot) before the test, make sure ALL maintenance is 100%, don't give any reason for a more "intense" inspection, and hope for the best!