Smog Test / Gunson Headaches

[ubiquity0]

Its emission testing time here so I am trying to get my SC ready for smog.

It passed easily 2 years ago, and has been running great since with the exception of some idle hunting in the first minute after a cold start. The only adjustments I can remember making to the CIS are small idle speed changes on a couple occasions, generally around when the weather turned. Howevere I recently noticed that the O2 sensor engine compartment connector had fallen apart, meaning at some point the CIS stopped recieving O2 sensor signals, and was defaulted to closed loop operation. I have spliced in a good used connector.

I bought a Gunson Gastester Professional a few months ago and figured this is a good time to put it to use. I warmed up the car then set up the Gunson well away from the exhaust (I have a 10' long tube to go the exhaust) which gave a rock steady reading, so I calibrated the display to 2.0% and then set the probe in the tailpipe.

No matter what I do w/ the mixture adjustment screw the CO seems to remain in the 2.0%-2.4% range at the tailpipe (measured with O2 sensor connected). If I lean the mixture out so much that the engine dies the Gunson still reads something like 2.2% CO! Last smog test the reading from the tailpipe was 0.37%.

The Gastester instructions claim that the 'pulse pump' in the Gastester should be audible when the probe is inserted in the exhaust. I certainly didn't notice any sound coming from the unit when I had the engine running- how noisy should the thing be? Is it possible the unit is a dud (which is going to be a pain, because I've pretty much destroyed my CIS baseline in trying to get a somewhat believable reading).

At that point I gave up w/ the Gunson, but did end up measuring the O2 output. When disconnected the voltage from the sensor was ~0.52V (steady).

The Gastester Professional does have a dwell mode which I figured I could use to check the frequency valve duty cycle but the instructions state that the lead from the Gastester needs to be connected to the negative low tension terminal on the coil,which the 911 does not have. Should I be connecting it to one of the 3 pins in the engine compartment test connector.. if so which one?

[Joe Bob]

Best to get it tested by a wrench that has a Behr unit and fire yours' up along side it. Probably a good idea to invest in a new 02 sensor as well.

[RoninLB]

mine needs about 15min to warm up.

confirming is good info.. I did it when it was new.

[Early_S_Man]

Sounds like your O2 sensor is dead! It should be cycling between 0.2 Volts and 0.9 Volts.

Just for posterity's sake ... what kind of multimeter did you use, digital or analog, make, model, etc.? O2 sensors should only be checked with high-impedance meters ... 10 MegOhms or higher input impedance!

[vash]

john at valhalla will test my car, no charge prior to smog. funny, i just got my notice today. i will let him touch it first.

[ubiquity0]

I probably gave the Gastester 15mins I think. It was in the garage at the front of the car. Back of the car well outside.

I have a digital multimeter- Equus 4320. it is 10 MegOhm input impedance.

I unhooked the O2 connector to test when I got the constant reading. Bentley Manual gives the test procedure as:
1/ hook up voltmeter to O2 wire without disconnecting the harness connector. Should fluctuate in the range of 0.1V-0.9V
2/ loosen oil filler cap to simulate lean running. Voltage should drop then recover
3/ if either 1/ or 2/ fail then disconnect harness connector & measure voltage. Should be non-fluctuating voltage somewhere between 0V-1V

I did not have any thin wire handy to get a connection without disconnecting the harness connector (which is the only way I can think of getting a voltage reading) so skipped 1/ & 2/.

Bentley suggests O2 sensor is faulty if 3/ gives no voltage reading, rather than a steady reading, and that the voltage should only fluctuate in open loop operation (sensor connected)?. Is that right?

thanks!