Went and got my smog test for a '87 911 and it failed for it's biannual smog test.

It's got SSI's on it and Tom Amon ( ) fabricated a testing muffler for me by taking the muffler section out of a Bursch and replacing it with a Miller Catalytic converter - both of which are brand new.

After swapping the 73 Dansk muffler for the "customized" one, it failed the first time because of the new NOx (PPM) numbers.

I thought the car had been plenty warmed up for the test - especially after running the adjustments back at Tom's garage. Perhaps I needed to drive it a bit longer from the garage to the testing station (1 mile perhaps?). In hindsight, after reading threads here on the rennlist and other posts on the net, maybe I should have driving it longer for that first test before mucking w/ the allen screw.

When I looked back 2 years ago when the previous owner had it smogged, the results didn't have NOx. New column = new problems for people? Sounds like it from the investigation I've done thus far. The engine is good - no problems that might lead to questionable results. I put very few miles on it and only use good 91 octane gas.

Tom was thrown by this number, and he's very well respected out here on the west coast. The smog guy said it might be from carbon build up. When I left Tom's garage he put from Jectron and Valve Clean in the gas tank. I'm heading down on Saturday to a different shop that has the dyno and wants to do some compression tests and perhaps cylinder inspections.

The reason for the post is

(1) Anyone know of any simple gotchas that would cause that NOx number to be high?
(2) Where do they pull these MAX numbers from anyway?
(3) Think the additives will help based on the numbers I've posted (below)?

Test 1 : Just swap the mufflers
Test 2 : Adusted richness - increased
Test 3 : Disconnected O2 Sensor (why not )

Here's the readings for the tests (sorry for the formatting):

--------------------------------------------------------------------------------------------
%CO2| %O2 | HC (PPM) | CO(%) | NO (PPM)
--------------------------------------------------------------------------------------------
|MEAS| MEAS | MAX | AVE | MEAS | MAX | AVE | MEAS | MAX | AVE | MEAS
--------------------------------------------------------------------------------------------
15mph (test 1) - FAIL
14.8| 0.3 | 121 | 31 | 115 | 0.76 | 0.10 | 0.45 | 807 | 237 | 958
25mph (test 1) - FAIL
15.0| 0.34 | 96 | 20 | 20 | 0.65 | 0.09 | 0.06 | 746 | 199 | 841

15mph (test 2) - FAIL
13.4| 3.7 | 121 | 31 | 112 | 0.76 | 0.10 | 0.62 | 807 | 237 | 576
25mph (test 2) - PASS
13.5| 2.5 | 96 | 20 | 20 | 0.65 | 0.09 | 0.08 | 746 | 199 | 930

15mph (test 3) - PASS
13.3| 2.0 | 121 | 31 | 152 | 0.76 | 0.10 | 1.04 | 807 | 237 | 1232
25mph (test 3) - FAIL
13.7| 2.3 | 96 | 20 | 96 | 0.65 | 0.09 | 0.22 | 746 | 199 | 286

 

No "NOx" on a CA test two years ago? I wonder what that's all about, CA has been testing NOx for years.

If you are a regular Chevron user the additives probably won't help, if you use some other brand they might, but not the amount that you're looking for.

This Miller cat, I'm not familiar with it. Is it a two-way or three-way? No insult intended, but if it's a two-way it's not capable of dealing with NOx. A 3-way cat takes NOx (nitrogen oxides made up of nitrogen dioxide and nitric oxide) and reduces those two poison gases to nitrogen and oxygen. A 3-way cat works best to control NOx when the engine is running slightly richer than the ideal mixture for perfect combustion, known as the stoichiometric point. That said, it controls CO & HC best when the engine is slightly lean. The answer is to be positive that the fuel mixture is adjusted, using a properly calibrated machine, to the exact Porsche spec, at the exact temperature and idle speed.

 

Pete,

When I looked at the test from 2 years ago, there was no 3rd column. So I was only guessing that this advanced test wasn't done 2 years ago?

Miller is located in Hayward and when I talked to them earlier in the day he said he gave me a OBDI not ODBII. Stated that by giving me OBDII I'm covering up a problem. Not trying to cover anything up - just want to be able to get those stickers.

The OEM cat wasn't ODBII was it? Should I go back to Miller and tell them to give me an OBDII instead?

Here's a pic of it: http://flickr.com/photos/11889048@N0...7603150895383/

 

Hey Pete...

Just talked w/ some guys and apparently Santa Cruz doesn't check the NOx (or didn't 2 years ago) - which is where the car was registered previously. Car now lives in San Francisco.

The fun continues...

 

I don't know anything about the terminology "OBD1" or "OBD2" cat - there was no OBD when your car was built, and I have no idea how either would work on your car. If your OXS sensor is placed where the one shown in the pictures is, it might not be getting hot enough. The cat that you used may not be large enough (the one that the factory used on the '86> Turbo is four times as large as one shown in the pictures) to cope with the exhaust that your car produces. If you go back to square one and are positive that your mixture is set exactly to factory spec, you might have to install the original system in order to pass the test.

I found the file for my '82 SC, and its 2002 smog test shows NOx numbers of 231 ppm (15 mph) and 117 ppm (25 mph) - good enough to pass requirements for an '87 car! Porsche started using 3-way cats on 1980 models for the sole purpose to meet 1981 NOx requirements.

 

Those terms are what the guy at the company that made the cat was telling me. I'll give him another call tomorrow - as he was pointing his fingers elsewhere when I asked about the cat itself.

Must say that not having a muffler and that test pipe in place really sounded evil when I was driving back and forth to the test station.

 

I had a high nox in an 85 ROW Carrera. I put in a new cat, new 02 sensor, and changed the oil. At re-test the NOX was way under the limit, no problem, sailed thru.

Steve

 

Cat is brand spankin' new. Maybe it's the O2 sensor.

Going to head to a shop in Burlingame and see if they can do some checks and also run an oscilloscope on the sensor. Some porsche shops round the penninsula say that I should be fine even w/ out the cat - which is the way it was tested prior.

Setting the mix to spec... where does this info reside? I've got the Bentley service manual - but don't see it in there.

 

Any Porsche technical manual that covers your year car should have the specs and the work procedure to set the mixture...

 

As a final update, it finally passed last night.

The problem? A combination of adjusting the mass airflow sensor and the replacement of the head temp sensor. Apparently the CAT was big enough - but honestly I was starting to second guess the size of it as well.

As always, thanks for the info Peter.

 

Good news, thanks for the post!