Chuck Jones

Help me out here. Took the '87 930 in for the inevitable CALIF smog check, and it flunked the first time around. No problem...some old vacuum hoses had crumbled, new plugs, general look see and replace the obvious....no problem right? WRONG.

The last readings were CO 1.11 (Max=0.74) and NO 1340 (Max=791). It's running just a "tad bit" on the rich side. On the other hand the O2, CO2, and Hydrocarbons were all within tolerances. It measured about the same levels on the first time around before the "tune up"...not done by a Porsche indy...but done by a friend who has a shop and wanted to help me out. It passed with no problem in 04 when the PO had it.

So....Where do you suggest I start looking...other than an indy shop with all the smog gear there to drill down to the problem. Any suggestions where to look, what to look for, what relatively simple checks to perform without sophisticated equipment? Is there a particular idiosyncracy for these cars that's fairly common to this problem......Or am I fighting the invevitable and just take it to a Porsche mechanic?

Chuck

diabolos88

try leaning the CO out maybe.

JBH

Sounds like the cats are either not there or not working.

CO is reduced by increasing oxygen. Leaning out the A/F ratio will help you pass CO, but it will do nothing for NOx.

NOx increases with temperature and oxygen content so increasing O2 to pass CO could hurt you in the NOx.

That's why you need cats to pass both.

SoCal Mike

Chuck, I'm not really sure how to interpret "tad rich". I have a permanently installed lm1 and when it is smog test time run with my o2 sensor hooked up so I know the lambda circuit is keeping a/f in line and reinstall the factory cat/muffler combo. I have passed the last 3 tests(6 years) without problem. All other smog equipment on the car(save the air injection spider) is present to pass the visual, but is not capable of effecting the test outcome. I would check your fuel mixture and if that is in line, I'd be suspicious of the cat. That's basically the only thing that my car requires to pass with 100K miles. With the commonly rich mixtures that our cars run, cats don't last long. My Cat only has about 30K on it.

Chet 930

Ah yes......been down this road a few times. The 87 930 doesnt have an EGR vavle so we can eliminate that. I'm gonna assume you have the cat on otherwise you wont pass the visual test. With that, a couple of things to check......first and foremost, is the oxygen sensor connected? How old is it? Is it still functioning correctly? That sensor by itself can make or break your CIS circuit from functioning correctly to get the right CO level.
Secondly....Is the air pump working? The air pump usually works but the belt that drives it usually breaks or the cogged pulley wears down. This is important not only to drive air into the air injector lines but for the air that needs to be injected into the cat too. The 3 way catalyst your car is equipped with is designed to function with air injected into it.
If the CO needs to be adjusted down a bit, be careful not to go too lean as this will drive the NOx up as mentioned above.
So in summary.....Cat in place, Oxygen sensor connected and working, air pump working and all the lines associated with it connected and the CO within spec should let you breeze through the dyno test. I just went through this (one of the many times) not too long ago with an 87 930 I bought and flipped that had 113K miles (somewhere around there) on the original motor and cat too. I checked these same items for functionality and the car breezed right through the first time. My 77 930 was a major pain, but thats another story......

Oh...one other overlooked item......ignition timing. This is also a big contributor to poor NOx output.