The voltage should always be switching above and below .5 volts. In a perfect .5 could be acheived. It is constantly switching as adjustments are made to achieve .5 volts, due to varying conditions. Backprobing sensor connected lets you know what is happening. You are running rich, thats why co is high and hc is slightly high. NOX is low because cylinder temps are low due to a rich condition. Nox would be high if cylinder temps were high, like in a lean condition. Sensor is easy to eliminate.

 

Update - I had some spare time this afternoon so I set out to test the temp II and oxygen sensor.

The temp II tested normal (342 ohms at 79/80C).

The voltage on the oxygen sensor (disconnected) varied between 0.21 and 0.22 volts.

Is the indicative of running rich?

I plan to check the resistance at the MAF sensor next.

Is there anything else I should be checking that would cause the engine to run slightly rich?

Thanks,
Adam

 

Adam--

Test the voltage with the sensor connected. The controller needs to see the sensor voltage too so it can adjust the mixture while you are testing. It should cycle as others point out, but its the controller that adjusts the mixture based on the sensor signal, the response of the sensor to the adjustment, then the response of the controller to that sensor change... that's what makes it seem to cycle. The cycling only happens ad idle and near idle, when the time from adjustment to reading is longer. As engine speed and load increase, that lag time diminishes to the point where it is more like real-time response. Modern (more modern...) systems have adaptive tuning so there's no cycling at all.

BTW, this is a less-than-ideal way to test the sensor. It's more a controller test, since the sensor is both the process variable and the verification value. Considering how inexpensive the senor is, it's really not a bad idea to just replace it, especially if it's been more than 25k or so since it was last replaced. In my limited experience with these things, a mean 50k life is probably a little optimistic, so anything after 25k will pay you back. You can buy a somewhat generic Bosch 3-wire sensor and splice the wires, rather than buying a 928-specific sensor. I think my last one was less than $40 at 928 Int'l. Call Jim in the next half-hour before he dances off to Texas on vacation, and you can have it in your hands tomorrow.

 

Thanks Dr. Bob!

I'll call Jim/Tom and order up a new O2 sensor. As far as I know, the current O2 sensor is the original. I'll test the current one tomorrow evening with it hooked up and then compare it to the replacement.

I checked several other threads and one item to check is the fuel regulator for high pressure or a leak.

This is frustrating - but I am determined to find the cause of the enrichment...........
Thanks again Dr. Bob!

 

It appears that your 02 is bad Low voltage, below .5 =lean. High voltage, above .5=rich. With fixed lean voltage, LH is adding fuel to richen mixture.

 

Zeus+,

Thanks for the input. I've spent the last hour trying to understand the impact the O2 sensor output has on the fuel injection system.

According to NJ's emissions, I'm running rich and the O2 sensor says I'm lean and enriching the A/F mixture. Makes sense as to why the CO and HC is up.

Hopefully, the new O2 sensor will arrive Friday and I can do the R&R Saturday morning.

I'll keep everyone posted.

Thanks Zeus+!
Adam

 

Adam,


Good luck pulling that old one out. I tried an O2 sensor wrench, and various other ones, then I actually removed the entire exhaust system from the car and had it sitting in the driveway, two days worth of Liquid Wrench, and I still couldn't get it to budge.

My only solution: Get a new set of crossovers from 928 Motorsports with the high flow cats, add a new O2 sensor . Good as new, passed emissions.

Just tell the Mrs. that somebody on Rennlist recommended it as a solution to your problem.

 

Hi all,

I took the 928 for its emission re-inspection this morning and it passed!

Here's the results:

NOx standard=1184; reading=491; result= PASS
HC standard=157; reading=7; result=PASS
CO% standard=0.88; reading=0.00; result=PASS

Work done this past month:

Replaced O2 sensor (what a job - had to remove the exhaust system and used alot of heat on the old sensor to get it out. It looked like it was the original O2 sensor).
Replaced air filter and air pump filter
Replaced distributor caps and rotors
Replaced sparkplugs
Replaced rotted out CO test tube on passenger side

I think the biggest improvement came from the O2 sensor - the engine idles nice and smooth now. I believe Zeus+ and drbob made the correct diagnosis that the O2 sensor was bad.

Thanks again everyone for your advice and support!

Adam

 

Great to hear,

Based on my unplugged sensor I was guessing that yours was working right. Sounds like it was the likely the problem all along.

Does this meen that we will see you at Frenzy 11??

Michael

 

Congrats Adam! Good job. I know you sweated that O2 sensor and CO tube for a long time. You have the patience of a saint.

That means you'll be ready and on the road for next week's 'Sharks at the Beach'. Talk to you later.

 

If you just unlplug the O2 sensor, then the LH will revert to a mid range setting of the O2 loop adjustment. Then measure the CO at the test ports before the cats (I assume you still have cats?)

You should then get a truer idea of what is going on.

Try adjusting the idle CO with the pot on the MAF (anticlockwise to weaken).

Make sure you have good vacuum on the fuel pressure regulators as a vac leak with raise the idle fuel pressure.

 

Hi John,

I'll check the CO at the test tubes (my brother has an exhaust gas analyzer).

Should I be concerned that the CO reading is now 0.00% and the HC is 7 after the oxygen sensor change and new plugs, caps and rotors?

Although it passed the emissions test, is the engine running too lean (based on the test results)?

I appreciate your thoughts,
Adam

 

Hello Adam,
Glad you are now legal !

0.00% at the tail pipe is believable.

If the O2 loop is working correctly, as it now appears to be, then the A/F ratio at idle has to be 14.7:1

I just found this table that shows CO versus AFR. http://www.perfectpower.com/technical_info/afr.asp

The 928 WSM says to adjust idle CO at the test pipes to 0.6 +/-0.2%

So that would give a AFR of about 14.2:1 or less if measured before the cats. This is well within the range of the O2 loop which will be set to keep idle CO around 14.7%.

From the chart you can see that at 14.7:1 the CO is 0.1% before the cats...