JHowell37

Your car is running rich, most likely the test was done at cruise. The most likely culprit is a faulty O2 sensor.

76FJ55

I would definately replace the O2, especially if you don't know its age/history. Checking for vac leaks is a generally good idea though I doubt its your problem in this case as a vac leak will generally make you run lean. if you've gat a hand vacuum pump I would suggest pulling a vac on the FPR and fuel dampers to confirm that the diphragms aren't leaking causing your rich condition. if you don't have a vac pump at lease sniff the vac line after disconnecting it at each to verify there is no fuel smell indicating a leaking diaphragm.

123quattro

Like others have said, change the O2. As they degrade you run richer and richer. Also check to see if the fuel pressure regulator reference is connected. If that comes off it will run way rich at light loads.

atb

If you're in the car while the test is being administered, leave the car in 3, or 2 if you can swing it. 25mph in an S4 your barely off idle in 4th gear.

cwiert

just playing devil's advocate here...

if the O2 sensor was bad, wouldn't i have a warning light or something? Any chance the CATs themselves are bad? I think they're original, so that's 22 yrs old.
(BTW - I believe you all about the O2 sensor, I'm just trying to learn more)

PorKen

Air injection is diverted to the airbox at WOT so as to not overheat the exhaust system. (Extra rich mixtures are also used to keep WOT exhaust temps down.) All other times, hot or cold, air is injected into the rear 1/3 of the cats (or exhaust ports ROW).


I took apart a bad cat recently. The injection air tubes each feed one side of a central pipe which has small holes drilled down the center (facing forward, IIRC). The air pump must be able to pump at high pressure to feed these small openings. I was worried before about restricting the outlet of the pump too much, but it appears to be designed for this.

Injection air may also be diverted because the pump would be pumping more air at high rpms than can be injected, so it would just be a HP thief, and might damage the pump, too.

cwiert

that's a good point. i'll have to remember to ask the tech to do that next time. because the car should run a little more efficiently when the rpms are up a little higher, right?

76FJ55

There is no O2 warning light.

Bill51sdr

Got to respectfully disagree here Ken. The airpump is used strictly to aid in warming up the CAT on cold start-up. It has no other function. Modern cars with AI have electric air pumps that shut down upon warm-up. Three-way cats do not need air pumped into the mid-bed to function properly once warmed up.

To the OP, a failed or failing O2 sensor will not trip a CEL in any 928, unfortunately.

ptuomov

But AFAIK, these are two way cats that we have on the 928s. Am I wrong about that as well?

76FJ55

I'm going to have to side with Ken on when the Pump is actively feeding the cats. Pradumably the new cars with the electronic air pump function differently than our old 928s. you can see fron the diagram below that the only signal going to the Diverter valve (item 12) come directly off the throttle body (Item 11) at location "a" for 49 state and locatin "b" for Cali. Neither of these vac supply lovations have any kind of temperature compensation.

Bill51sdr

32V engines, at least have 3-ways.

PorKen

You may disagree, but that is how the 928 injection system works.

Air is constantly injected into the third element of 32V cats except at high load. The diverter valve is connnected directly to the TB, with no thermo valve.

Bill51sdr

Seems I need to better educate myself on that system then. I totally overlooked the lack of electronic manipulation on the diverter valve.

123quattro

On old cats they would do continuous air injection. They could run a little rich all the time to help keep temperatures down and then get the mixture back to stoich with excess air. Overheated cats used to really be a problem on cars this vintage. On some of my older Audis they actually had a thermocouple in the brick that would trigger a light on the dash to tell you when they were getting hot.

A 22 year old cat is pretty dead by that point. A $60 mail order one would be much more efficient. However, then you have to deal with fabbing it in. If the EGO doesn't fix it, then I would replace it with something cheap.