dr bob

When I first brought the car from PO territory in Denver to its new home here in SoCal, it passed smog with flying colors. The last several tests have shown increasing NOx level as the years go by. So today it went for testing, passed with flying colors.

Since the last test, I replaced the Bosch platinum +4's with plain old Bosch copper plugs. The previous increase in NOx showed only after the original plugs were replaced with +4's after the first smog test.

perrys4

Mine has done the same thing but has gotten worse with the same plugs. I think I will start with a new o2 sensor and maybe a set of new plugs before my next test. I have heard the testing equipment can vary dramatically too.

OrionKhan

My previous test barely snuck by. The one prior to that failed. Got nailed with the dreaded gross polluter tag. Test Only shops now. About three weeks ago, I replaced the MAF and O2 sensors. I just had my test this week. The car passed with flying colors. The last two passing tests were done at the same shop. The plugs were changed between the last two tests. So I don't know what made the most difference.

Nicole

That's very interesting! It was suggested to me before that the newer style spark plugs were not ideal for the 928 S4 enging - but I had not idea why.

tv

I recently scored a .76 out of 75 (so 1%) on NOX. And this was on a euro.

Schocki

Just a question: Did you guys disconnect the vacuum line to the air pump check valve? Roger had a similar problem with his GTS. Apparently it is not very well known in the US that the air pump has to be disconnected for the smog test!?
Diconnect the vacuum line to the air pump valve and run the tesy again. This is according to the test procedure manual issued by Porsche in Germany!

perrys4

Now thats interesting....Anyone have a photo showing where this line is?

Mongo

From what I've heard, the 928 is known to have real high NOx levels at idle. Interesting how fast it goes away on acceleration

As to why Porsche's setup on the 928 is like this.....well that's a question I'd even like to ask them because I passed the NOx test by only 20 PPM. That's a little bit close, then again I've had worse cars that would pass by 7 points only.

worf928

Interesting. Schocki - is this disconnection recommended for an idle test? Or rolling dyno test?

White (in the picture - YCMV) vacuum line that runs from under the throttle body, behind the passenger side head to the air pump valve:

worf928

BTW, my GTS passed emissions with triple zeros - idle test only - with the airpump operating.

Mongo

I wonder if the air pump contributes to high NOX levels..

Amazing worf that your car has a clean bill of health like that. In the end it boils down to time, devotion and passion to get it to be happy like that

worf928

If that were only true Andy. This emission's test was last year right after I bought the car. (That Texas inspection sticker don't matter none up here in Yankee Land.) So, I'd done nothing to the car except drive it home from Texas. And at the time it was not a happy camper: it ran lean at anything higher than mid-range rpms under load. (All fixed now. And we shall see what the emissions sniffer says soon.)

I think it may have more to do with the GTS having a very healthy cat.

Mongo

I wish it were true too My coffee hasn't kicked in yet and I'm still dreaming of a perfect running shark.

I've never really had the chance to completely check out a 928 GTS before, only hear about people attesting to their experiences with them. Nevertheless, just the thought of the GTS inflicts enthusiasm and determination to pursue ownership of one someday

As far as emissions goes, did anything change significantly from the S4 to the GTS? I know the exhaust system changed that's for sure, but what about the air pump and other emission regulators?

worf928

Nothing mechanical AFAIK. The exhaust system for the GTS is not materially different from MY '89 AFAIK. I think the resonators may be slightly different. The '87s and '88 have the 'suitcase sized' resonators while the '89 and later have the 'carry-on-bag-sized' resonators.

Cams are different though.

dr bob

The actual results, for those interested:

15 MPH rolling:

%CO2 = 12.9
%o2 = 2.4
HC (PPM) Max Allowable = 110, Average = 31, measured 9
%CO Max = 0.71, Average = 0.1, measured = 0.00
NO (PPM) Max = 772. Average = 237, measured = 439

In the 25 MPH rolling:

%CO2 = 12.6
%O2 = 2.9
HC (PPM) Max Allowable = 85, Average = 20, measured 5
%CO Max = 0.59, Average = 0.09, measured = 0.01
NO (PPM) Max = 711. Average = 199, measured = 150

Readings compare with last test two years ago, except for NO which barely passed last time, and only then if the car was held in second gear in the auto trans. No significant change in HC, suggesting that the new plugs/cap/rotor did not improve combustion much, at least as measured after the catalyst.

Tune in again in another two years. This year, both the 928 and the Explorer ('92) were tagged for "Test Only" station treatment. The great and socialist republic of California has determined that these older, under-maintained cars are better tested at a place that can't try to shaft me on repairs. Extra $40 testing cost saved me from the repair demons again! I'm feeling lucky already.

As far as performance difference between the new Bosch copper plugs vs. the +4's taken out (about 25k on them...), there isn't any. The old/original copper plugs came out at about 40k, at which time the +4's were smoother at idle. I suspect that new copper plugs would have given the same results, but I suckered on the +4's anyway. Live and learn, right?

Hope this little experiment helps somebody!