BC

While Ethanol will make you run leaner, it may not actually increase the NOX and can reduce it. At this point, its worth a try. If you make it run richer with a sharktuner or some other way, it will get better, but the other numbers will not go up as much so you should pass.

I passed on non-cat car on E85 once and another nearly did, but the car was WAY rich so it jumped up the CO numbers like 2% too high overall

BarryW

Nevermind.

tveltman

Bill, check your PMs

jcorenman

It's not just high NOx, that's what is confusing here. Low CO, high O2 and high NOx all point towards too-lean. But the HC contradicts that: If it was too lean in general then the fuel would be completely burned.

It could be an ignition misfire, lean combustion in seven cylinders creates a lot of NOx (with low HC and CO) while no combustion in another would pass a lot of unburned HC's through as well as more O2 (but no CO). But all of the ignition parts are near-new (coils, also?). The plugs might be an issue so it is worth going back to the stock plugs.

But in this case, and especially with lots of excess O2, the cats should happily oxidize all of the HC's, no?
So why is that not happening? Those are big cats on these cars, and they should happily burn any left-over hydrocarbons, given a bit of O2.

So I am going with Barry on cats, they may just be plumb worn out. And stock plugs is always a safe bet.
And maybe stick in a wbo2 sensor and make sure the mixture is OK, or a voltmeter on the NBO2 sensor should tell you if the LH is running properly in closed-loop (should cycle between 0.1 and 0.8 volts, roughly).

Good hunting!

Lizard928

Too lean will cause high HC due to a lean misfire.

Bill Ball

Yes, I see "lean misfire" as accounting for the high HC. Jim suggests the cats should have handled the HC, so the cats are in question too. I will give the ignition system a more thorough check and look for likely causes of a lean condition. I'm thinking some cleaning of the cats might make them work slightly better too.

atb

How about strapping on a supercharger?...oh never mind.

Bill Ball

Had a little time yesterday so I ran Theo's Porsche Diagnostics. No codes in the LH or EZK, everything tested worked fine. Checked all the spark plug cables and they were all correct resistance, even though a faulty one would not account for the high NO. Just covering the basics. I need more time to remove the cats, clean them, install the wideband O2 and see if I can verify a lean condition, check over the vacuum lines and other possible air leaks spots. I have spare LH and EZK with PEMs installed, and I can probably borrow a Sharktuner if it comes to that.

Hilton

Good methodical progress is the way to go.. I first tried the "oh it must be xxx" and switching parts thing, but it got frustrating. When I went for the last test, I was confident it would pass.

Its good news that all your sensors are working - that covers the really fiddly stuff to get to. Did you check the fuel pressure and also sniff the vac connection on the FPR for gas?

Keep us posted.

tveltman

Bill,

I have a spare stock cat lying around. I'm not sure of its condition, but I saved it after I installed my x-pipe. If it works for you, you are welcome to have it.

Bill Ball

Well, that seems to have been correct. See my original post for update. I PASSED!

Bill Ball

Thomas, almost any cat will probably have lower miles than mine. I'll be in touch soon.

ALKada

Congrats!

Mongo

Great! You can take care of that O2 number by tightening up any loose clamps.

safulop

I had a 944 which I sold in 2011. It failed smog one morning when I was trying to sell it, after it had sat for a while.

For the failed test, the numbers read:
15 mph RPM 2849 CO2 14.5 O2 0.24 HC 68 CO 0.21 NO 655
25 mph RPM 2862 CO2 14.5 O2 0.16 HC 23 CO 0.18 NO 750 FAIL

I drove it to a smog shop in the next town, 20 miles down the freeway. It passed that afternoon. When it passed, NO is in 300s for both speeds, HC was 135 at 15 mph, which was almost failing.

This convinced me that smog numbers are in some sense random, and only very loosely related to the condition of the car.

-Sean