Higher CO2 with 02 sensor connected
 

Hi,
I have a 911 SC '83 (USA). When the 02 sensor is disconnected, the C02 emissions are normal. The strange this is that when I connect the 02 sensor, the CO2 emissions go up to 3 times as high. This seems very strange to me. I've already replaced the 02 sensor, the 02 sensor relay and the ECU under the passenger seat has been checked and works as it should. Any ideas where to look? Is it a good idea to just leave the 02 sensor disconnected?

Thanks!

How hot is the car when this is checked?

The car has been checked with a warm engine. Hope this is correct?

Sure you don't mean CO? Nobody ever fails for too much CO2 as far as I know.

If not at operating temps, O2S will modulate more to compensate.

You say normal but What are your readings?

Normal CO2 readings are over 14%., however. If the vehicle has an air injection system it can be lower around 13%.

Btw, the higher the CO2 the better as it's an indicator of engine combustion efficiency.

Would also help to know the HC and CO levels.

CO & HC are the emissions measured. When CO is high HC is usually lower than limits. An adjustment can be made on CIS engines to balance the emissions within limits, assuming the O2 sensor and cat are good.

As far as smog testing goes HC, CO is measured, however, HC and CO go up and down respectively until the mix is too rich or too lean causing a misfire. Yes, CIS can be tweaked to balance things and get a car to pass, but A good CAT can make a dirty engine seem like it is cleaner than it is. Its good practice to utilize all the info from the 5 gas analyzer HC, CO, NOx, O2 and CO2.

If the OP is not confused and talking about CO2 (not CO), what he is experiencing seems normal. With the O2 sensor disconnected, lower CO2 is possible due to little or no fuel control and the computer in open loop. This causes less efficient combustion and the CAT to not work as well. Correct fuel control (and good compression, timing, etc.) gives complete combustion, which burns more fuel producing less HC and CO. O2 is low too because with all the fuel being burned, it eats up the O2. With everything right and a good hot CAT, the tailpipe emissions are mostly CO2 and water.