High nox, failed emissions

[Max55]

hey guys, 93 318is here. I just got her, and the prior owner said he put a new cat and exhaust on the car. The car passed ho, and co2 easy, way below, and passed nox at the 15mph speed. it failed it on the easier(so the tester said) 25mph speed test. Any ideas on what i can do get lower this? the number was double allowed.

I was thinking octane boost? possibly seafoam the engine? new plugs and an oil change? I dont know when the o2 sensor was last changed. id rather not take the car to a shop to pay 100$ just to tell me what else i need to spend money on.

[Ramblin MAn]

Those numbers mean you are running lean. You need to richen your mixture up somehow. I was able to move them a little with a higher pressure FPR on my SAAB when I had the same problem, but in the end I had to replace my AFM twice to get a unit that worked.

I'd say first thing is to replace your O2 sensor. You don't want to be running lean to begin with, regardless of your emissions, so you may well replace it first since it will also be the easiest.

[quinn11m20]

What the hell are you talking about sounds like its running lean. Are these good numbers or bad numbers? I just have to add that I live in FL. We don't have any of that gay california emissions shit. Thats to bad you can't tune your own car.

[nicknikolovski]

Here are some possible causes of high NOX emission:

Carbon buildup on pistons and in combustion chamber - might be an internal engine problem but this is last place I would look.

Over-advanced ignition timing - is the engine knocking under load? Is it using the correct fuel type and octane level?

Defective knock sensor - is the engine light on? Unlikely but you never know.

Engine overheating (check thermostat, fan, coolant level and associated components) - this is a critical one as NOX emissions are produced from very high combustion temperatures so check that the engine cooling system is working properly.

Exhaust restrictions - have a look at the cat-converter, exhaust manifold, centre & rear mufflers, O2 sensor. Check out the whole system for any blockages, is the exhaust leaking or blocked anywhere.

[Max55]

Hey, thanks for the replies guys. I broke down and took the car to a shop, saved me a lot of time.

So here what my solution and fix was FYI:

I took the car to a German specialty shop. The owner used to build and work on m42 engines for BMW so he knew exactly what to look for. He said a leaky exhaust or bad cat is almost Always the cause on these engines.


Sure enough, we looked at my exhaust, it had a few leaky spots from crappy welds. The catalytic converter was also bad, it was brand new too, but it was a cheap unit the previous owner put on.

I had a new complete exhaust system put on the car. Sounds better and the car runs smoother now. Just retested my emissions, Nox went down by more than half. At the 15mph nox test, the nox level was almost at zero, 0 . lol

Needless to say i passed