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Failed first time with a rich mixture resulting in high HCs and COs but very low NOx with my 1987 S4.
Did a few things - New O2 sensor, new spark plug (wr7dc), wires, rotors, changed oil, changed coolant, fuel additives (guaranteed to pass smog 2x),
new fuel filter, cleaned MAF and against my better judgement put lacquer thinner in fuel to clean catalyctic converter.
Result was a lot better but still failed a second time - pic of before and after smog reports below.
I finally decided to replace catalytic converter to Magnaflow units.
Result was HCs and COs are impressively very low but NOx was double the acceptable limit!
Question is how can results go from one extreme of rich mixture to the other extreme of lean mixture with just a changed of catalytic converter?
Why would my NOx shoot up from 5ppm to 1600ppm? Does not make any sense...
Things that I am going to look at are MAF recalibration, change fuel injectors, check vacuum leak. Should I do a De-Carbonization?
But would be open to other suggestions or maybe point me in the right direction. Or maybe I still have lacquer thinner in my fuel mix that contributed to a high air to fuel mixture? Thanks in advance.
Prior smog results with original Catalyctic Converter Current smog result with new Catalyctic Converter.
Really lean, or really hot, will do that. Dont have EGR, so thats not broken, not a "bad cat"...so thats off the list too presumably.
Not sure if MAF cleaner was a wise choice.
What cats did you end up with?
Well, I made sure the engine was hot before the smog test so I had it running hot and never turned it off until the smog person took the car for the smog test. And it was a tad hotter than normal. Do you think this contributed to a rich NOx? I used CRC MAF cleaner and it had good reviews here on Rennlist so I tried it and followed directions to a T.
I went with Magnaflow cats. Got it installed just two hours before I went in for the smog test today.
While cats may mask the symptoms for testing, the problem will be solved when you replace injectors. You need even flow through all 8 nozzles. You can have yours cleaned and flow balanced by a service, or replace them with new. There are better 4-hole injectors that will drop in. Contact your favorite 928 parts vendor.
I agree that German cats are durable so it’s unlikely the problem. The MAF deteriorate so it may cause lean condition. That was the case when I bought my 88 as parts car because the owner could not pass smog to sell it. The car test drove fine when I bought it.
While cats may mask the symptoms for testing, the problem will be solved when you replace injectors. You need even flow through all 8 nozzles. You can have yours cleaned and flow balanced by a service, or replace them with new. There are better 4-hole injectors that will drop in. Contact your favorite 928 parts vendor.
His results pre new-cats/maf cleaning was the same as me with all new everything, injectors maf spark fuel stuff..everything.
New cats put his HC numbers where they should be, but something huge happened with his NOX..unlikely to be injectors. (?)
I agree that German cats are durable so it’s unlikely the problem. The MAF deteriorate so it may cause lean condition. That was the case when I bought my 88 as parts car because the owner could not pass smog to sell it. The car test drove fine when I bought it.
Then why did new cats erase his HC numbers if the OEM cats were good?
Thats what dead cats look like (if all else is healthy) fuel wise.
Sometimes it is difficult to figure out what is wrong with a particular car, when it is sitting in front of me and I have all the diagnostic tools in my shop at my disposal.
In this case, there is no diagnostic effort...just someone standing back and throwing parts at it and hoping.
Don't get me wrong, sometimes you get lucky and that works, but sometimes it makes no sense.
There are steps to take....and the very first, before anything is done is: What is the voltage output from the O2 sensor, when it is connected to the computer?
It turns out that Sharktuner not only gives us the ability to tune a car, but also has great real time diagnostic capability. I frequently install a set of brains with Sharktuner Pems, just to see what is happening with knock sensors, hall sensor, temperature sender, throttle switch, O2 sensor, O2 adaptation (tells you MAF condition), etc.
The third step is a "smoke test"....looking for intake leaks. Don't forget to check the charcoal canister purge system and HVAC system for leaks, as well as checking the fuel pressure regulator and dampers for fuel leakage into the intake.
Fuel injector issues are hard to detect. However, if (and that's a giant if) I were to believe the testing sheets I get back everytime I have a set of injectors cleaned, it's amazing that the engine those injectors came out of ran, at all!
It's really rare to have a bad cat, but it is possible to have a weak cat. "Fresh (perfect) 928 engines need very little "help" from a cat to pass a California emissions test. Engines with a 100,000 miles need more help...and the amount of help required goes up more as the engines age. (Sorry to all you people that think a 928 engine lasts forever....it's just not true.)
11-03-2018, 10:58 PM
Failed CA smog again - Can anyone explain?
