+1 on rusnak's "connect the brown wire" suggestion.

Elevated exhaust temp isn't where NO comes from. It is from high combustion chamber pressure coupled with high combustion chamber temps. The temperature in the combustion chamber is higher then the exhaust because the pressure is much, much higher.
The brown "CA / Japan" wire retards the timing 4° to reduce combustion pressures which is to specifically reduce NO emissions.

OK, that makes sense. But a lean mixture would also hurt here, maybe get he mixture correct and also connect the brown wires. The California cars had been delivered with those wires connected and your explanation makes sense.

Thanks.

Sal, Quicksilver replied much better than I could have. I can only add that I forgot to re-connect mine, and my 911 barely passed by the skin of it's teeth. NO under load was much higher than it should have been. Re-connecting the brown wire dropped the NO (see second test, higher RPM) dropped the measured NO dramatically. Almost by 50%.

I also have a lot of faith that a healthy O2 sensor and good DME will compensate for overly rich/lean factors. You can watch it work with an LM-1 or LM-2.

Based on the prior replies, maybe just a new O2 sensor, connect brown wires and double check base mixture may be all that needs doing.

Yes, the DME does a decent job in closed loop mode to help correct mixture but the DME ignores the O2 and switches back to open loop under these 2 conditions:
- above 50% load at any RPM
- above 5120RPMs, regardless of load
so you can't depend on the O2 signal to fix/correct mixture for all running conditions. This is why it's important to have mixture set/checked from time to time.

That is sound advice. The cat should not be bad at those miles. Maybe the test station guy didn't let the cat warm up, or maybe it's clogged with oil from the oil being too high. Hard to know without knowing the car better.

Ahhh, just checked and the brown wire is disconnected! I had forgotten that I disconnected this 10 years ago shortly after I bought the car. I passed smog in the past, so I forgot about it being a potential problem... I've now reconnected it, and will give the smog check another try.... fingers crossed.

Thanks to all for their advice, and I'll post the result.

The brown wire on my 87 was never disconnected when I failed but the CO adjustment did the trick in my case.

Keep your fingers crossed, you may be lucky.


Cheers,

Joe

I was going to say that you should talk to VW... but it sounds like you are sorted.

Something does not sound right there. Sal maybe should comment further on this. He already mentioned that the DME is programmed to maintain Lambda. If you watch the exhaust gasses on a wideband gauge, you can see that it certainly does. If you open the oil cap, the mixture will spike to lean and then quickly adjust fuel pulses to return to Lambda +/- .2. Similarly, if you unplug the vacuum line going to the fuel pressure regulator, then it'll spike to a rich mixture, then quickly return. If you open the CO mixture screw all the way until it falls out, the DME, once the O2 sensor has warmed up, will return the mixture to Lambda.

What this means is that there really is no adjustment of CO% per se. What you are doing, when you disconnect the O2 sensor and adjust CO%, is to return the mixture to baseline before plugging in the O2 sensor. IIRC, Motronic or L-Jetronic "likes" a slightly rich mixture to work from, like say 1.2 Lambda or even 1.4 according to some. If there is a huge vacuum leak letting in false air, then the system can be defeated and caused cycle up and down, constantly adjusting the mixture. Maybe baseline re-setting can make that cycle smoother, but I don't think at idle it would make much difference.

Reconnecting the brown wire seems to have done the trick.

http://forums.pelicanparts.com/uploa...1454958994.jpg

I also did the oil cap test over the weekend, and the RPM dropped when I unscrewed the cap, so I guess I don't have a vacuum leak either.

Thanks to all who offered advice!

Congrats.

Motronic is a great system.

The only change you made was to connect the brown wires, correct?
This just reduces ignition a tad and serves to detune the motor. In theory it reduces cylinder pressure slightly and thus helps keep NOx in check.

Thanks for reporting back.

Outstanding!

This thread is recent enough that I don't mind piling on. I just got my first 911, an '88 coupe. It was originally a CA car, but has since been to WA, OR, GA, and now back to CA. I took the car for a smog test today in preparation for registering it, but it failed with high NO.

Car has 121k miles. Runs great, and the PPI I had done found no compression or leakage issues. It has a stock, presumably original, cat. It also has a Steve Wong chip.

I was hoping that I would find the brown wire disconnected, but unfortunately not. Any other suggestions for things I should check? I'm new to this, but I'm handy with cars and have plenty of tools available. Here's my report.

http://forums.pelicanparts.com/uploa...1459314767.jpg

A couple of notes from things I just found in the PO's maintenance records:
New Bosch O2 sensor, plugs, cap, rotor, and wires at 111k miles.

Your HC and CO are both elevated so you are not running lean. I plugged your numbers into a Lambda calculator http://www.smogstats.com/Lambda_CAT.aspx
and your air/fuel ratio reading is .997 which is slightly rich. This should cool the burn and reduce NOx. According to the calculations, your CAT efficiency is 57.65%, not real good. Looks like the CAT could be getting weak, but the CAT is the last thing to condemn.

Couple of questions:

Does an 88 have an EGR valve? What does a Steve Wong chip do to the engine tuning?

I don't know about the EGR valve, someone else will have to chime in on that.

Steve Wong chips: 911Chips.com - Porsche Performance Chip Tuning. Under the FAQ he says they should still pass smog, and he is based in LA. There's info about the fuel map used as well. The chip I have appears to be for 93 octane gas, we only have 91 around here. Not sure if that matters. Unfortunately I don't have the original chip.

One other thing to mention: my speedometer has been intermittent since I got the car, and it was not working when I drove to the test place. But they didn't comment on it, and it worked on the drive home. Does the speed sensor feed the DME at all?

This is why I love this board - some real class acts in attendance.
Just sayin'
Bill K

Can't answer if the DME and its control is affected by vehicle speed.

If your car does have EGR, its sole purpose is to control NOx so it would need to be inspected for proper function.

Your NOx is elevated but not of the charts. Your only off by about 100 ppm on the 15 mph and right on the line at 25 mph. So if you have no EGR, you might reduce the NOx some by adding a higher octane fuel. Also, as mentioned, your CAT may be getting weak so the slightly rich mixture may be affecting its efficiency enough to put your NOx over the edge. Not sure if your car was warmed up enough before the test was done but a hot CAT works best. Another possibility, could be carbon build-up in the cylinders causing higher cylinder temps and raising the NOx. I am more experienced with non-Porsche engines so perhaps someone else can comment on whether carbon build-up is an issue with these engines. My guess is no since most of us are usually in the "Italian Tune-Up" mode of driving :) So, since you are close to passing and assuming there could some carbon build-up, a low-cost DIY attempt to reduce your NOx could be to add some premium fuel and run some Sea Foam induction cleaner through a vacuum line to clean any intake deposits and carbon. I have had good luck with these remedies on smog/Nox failures that like yours that are close.

Most of my experience is with non-Posche engines so If Sea Foam or similar intake cleaner is not recommended someone please advise.