Mark SF

I just took the 89 S4 in for a smog, and it failed on high NO. The CO and HC are fantastic, essentially zero.

So it seems to be running too lean.

The car seems to drive very well, apart from random, occasional hunting idle.

Any tips on causes of this failure on the 928? EGR, vacuum leak?

Also is there a source of CARB-legal cats for California, if it comes to that? (other than the dealer, of course)

DG84S

My 84 failed for high NOX. I replaced the O2 sensor and it passed.

neilh

Check out :
http://www.car-sound.com/04basics/04carb.asp

anonymousagain

Mark - you didn't state it, but prior to hitting the rollers you made sure to put the car through some spirited runs to heat the cats up sufficiently, yes?

Some operators will ask, but mostly I tell them to let me know when I can take it out for 10min prior to testing, to warm up the cats --> initial fail changed to pass with flying colors the first encounter.

IcemanG17

its lean for sure..... what were the exact #'s?

If its close....making sure the cats are good and hot will help.....a new O2 sensor is another good idea, but I thought the failure mode for dead O2 is to run rich?

I think a dying MAF will run a bit leaner than usual......its easy to swap out with a known good one from a local 928er.....

Years ago I thought my MAF was going out (forgot why).....I swapped it out into Bills supercharged S4....his S4 ran BETTER with my MAF...it was just a touch smoother than his was.....

Scotts87928

I have two documents. One has a recent CA and a 2008 Colorado test and the other is my second CA test. As you can see on the CA tests they show NO. I failed on the low speed test on 1/28/11 and then on 4/23/11 I failed on the high speed test but failed on the low speed test. Notice how clean it is running on in each of the other tests.

The car has original style and heat range plugs, fairly new wires caps & rotors (under 4K miles). New fuel pump and filter. Clean air filter and air pump filter. No vacuum leaks or the Vacuum was the same as my '89 that ran great at about 21" here at sea level. New O2 sensor, and I swapped the cats with the '89. No change. (this was the done prior to the second emission test in CA). The TPS checks out per specs. I replaced the Ezk and the LH from the '89 and did not notice any change the way the car runs. The car does idle low once it is hot (around 600 RPM). It starts good when cold but needs say three cranks to start when warm and it helps with a little gas. I have replaced the MAF from Kevin at Injection Labs (now in Boulder, CO).

I did check the timing with a timing light and compared the marks on #1 and #5 cylinders and it appeared the timing marks were similar to the timing marks on the '89 at idle and advanced seem consistent between the two.



I'm going to recheck the timing marks (on the cams) this weekend to see if I'm 1/2 tooth out. Other than that I'm stumped and open to suggestions. I replaced the injectors with Ford and the car ran better then but it hasn't been used much in the past 9 months. And just around the neighborhood in the past 6 months since it isn't "emission" legal yet. I did pay the CA taxes and registration but still need to get it to pass emissions before I get a CA plate.

Please forward this on to anyone that will listen and cares to weigh in.

Scott Smith
'87 Gemballa 5sp Red/Black
'89 S4 Black/Linen off to Phoenix complements of Paul's uncle

Hilton

Yes, as MAF's get old, they tend towards lean (they output lower voltage, so the LH adds less fuel).

Degradation is not very noticeable on a car with an O2 loop, until the MAF is so far out of whack that the closed loop can't compensate for it.

The factory cats are very long-lived - I'd be checking other things first.

The other cause of lean-running is unmetered air. Are you sure there are no vacuum leaks? Has the intake refresh been done?