Any BMW experts out there? Just flunked smog with Hi HC and NO at 15mph. I was ok at 25. I have replaced a cracked duct between the AF sensor and throttle body. I think I am also going to change the plugs and oil (been awhile). I replaced the O2 sensor two years ago. Anything else i should be doing?

What were the exact numbers.

If your close, change oil and plugs and run 1 part Toutline (Tutoul, its a paint thinner/alcohol, It helps eliminate nocking) to 5 parts gas.

Is your cat old? You should run the engine HARD (High RPM's) to heat up the cat very hot JUST before going on the dyno, which will help it burn the HC's.

Going to do all that, and yeah, the cat is original. Someone also told me to run it down to a few gallons of gas, dump a gallon of alcohol in it, test it, then top off with gas to dilute the alky. Hmmmm.

Speaking of plugs, any thoughts on platinum vs non-plat on these things?

Don't use platinum plugs on older BMW's. Be sure to check carefully for another vacuum leak. Give it an Italian Tune Up. The suggestion to go in with a hot cat is great. Be sure your 02 sensor is functioning perfectly. It is usually the little basic things that fail you.

Being half Italian, I gotta ask, what is an Italian Tune up?

revving the enging at very high RPMs

Step One: Adjust your rear view mirror.
Then find a section of freeway with a long grade.
Run up the grade in top gear until you feel a slight miss. Back off just a little.
Repeat as needed. You'll probably find you need to attain higher and higher speeds as the process begins to work - See Step One.

Seriously - this was an OK technique 40 years ago, before platinum, iridium, palladium plugs, electronic ignition, and unleaded gas. Running the engine at high load (not high speed...) would burn off plug fouling that was the bane of high-performance cars like Ferrari and MFI Porsches driven around town.

This is a historic footnote. It does not apply to your car.

You have a cat and you're using unleaded gas. If you need new plugs and/or wires, put them in.

If you are using premium gas in a regular-gas car, that will account for high CO readings right there. Use no higher octane than is absolutely required to avoid detonation. If you want to go back to premium after you pass the test, be my guess, but other than _possibly_ having a better detergent package it's buying you nothing...

[Edit: I just saw - high HC and NO. Could be timing way too advanced...]

Being a 92', that should be a 2.5 M50 and as long as it isn't bone tired, should pass with a lot room to spare.

Start simple, just replace the plugs with good NP's or NGK's or Bosch coppers. If it's been 2+ years since your last O2, replace it.

Then drive it, get it good and warm (up to operating temps), then go back in for a test. I bet it passes just fine.