Has been through twice now and looking for possible causes. 1st test results only the 15mph test failed HC too high.

15mph HC=60 CO=0.12 NO=172

25mph HC=23 CO=0.04 NO=42

Afterwards, I smoke tested the intake and replaced a leaking oil collector and also replaced the spark plugs.

2nd test results both 15mph and 25mph failed HC too high.

15mph HC=89 CO=0.07 NO=68

25mph HC=74 CO=0.04 NO=14

Frustrating to have it fail worse after working on it!

It is fairly low mileage (138k) for a 1997. I am thinking the 02 sensor or MAF cleaning might help. It may be time to check the VANOS (variable valve timing) seals but I don't think the VANOS would be involved at these lower speeds.

Any help diagnosing possible causes would be welcome.

Edit: Coolant temp once warm stays close to 200 deg. Add Chart image of OBDII readings Long & Short Term Fuel %

  • Welcome to Motor Vehicle Maintenance & Repair! Oct 19, 2023 at 21:03
  • 1
    What are your short term and long term fuel trim numbers? Coolant temperature as reported by ECU after warmup? You can edit your question to add these data.
    – MTA
    Oct 19, 2023 at 21:16
  • Thanks for the help. I added a chart of the LT & ST fuel trim %s along with the pre cat 02 sensor ST fuel trim % values.
    – 1trusted1
    Oct 23, 2023 at 1:19

1 Answer 1


Hi HC is due to incomplete combustion: there's unburnt fuel in the exhaust. There's many things which could cause this. I have an E46 M3, which has a similar engine, and from my own personal experience I would look at things in this order:

  1. Ignition system: your plugs and/or coils are the most likely culprit. As you've replaced the plugs (a smart move) I'd suggest you have worn coils. If you noticed one of your plugs was carbon fouled I'd replace that coil at least. Coil packs are hard to diagnose, but if you have one that looks a lot older than the others it's a sign that they have been replaced piecemeal, and you should consider replacing them all
  2. MAF: As you say you smoke tested your intake I'll eliminate that as a possibility, so it's worth cleaning the MAF. It's actually pretty low down on my list of likelihood, however it's very easy to get at and clean so you may as well eliminate that. Check that it's giving accurate outside air temperature readings, you can see the values using an OBD reader
  3. O2 sensors: although your upstream O2 sensors may be working electrically they may not be delivering accurate data to the ECU
  4. Old catalytic converters: it the cats are old they may be 'exhausted' (sorry) and no longer doing their job
  5. Engine operating temperature too low: The m3's inline 6es won't run efficiently if they don't reach a high enough temperature. If you're getting bad mileage as well as high HC this could be a cause. There's too many reasons and I don't know your powerplant well enough to give you the best information on this but if the above doesn't resolve it you should look into it. Coming from this, make sure the engine is completely hot before they run the test. In my car it takes 20 minutes of driving at least to really heat up. It could be as simple as that.
  • Thanks for the reply. I appreciate the help. The coils are all original and I took photos of the plugs when removed but they were all very similar with a nice beige insulators. I will check the temperatures and also the short and long term fuel trim numbers and update once I have them. Since it is not a DD, it is possible it was not warm enough when tested but I normally make sure the BMWs are good and warm prior to testing. I found a post detailing relationship between CO and HC but was not sure how to apply it to the two different test results.
    – 1trusted1
    Oct 20, 2023 at 17:42
  • A follow up to this issue is that I took the car out on a trip. The return home was a spirited 135 mile drive. Once back in town I decided to run it through smog thinking that it was nice and hot as well as cleaned out from the drive. They sniff tested it on the dyno first and it looked good then ran the test and it passed. I am not sure the reason it passed but glad it did. Passing HC was 33 @ 15MPH and 15 @ 25MPH
    – 1trusted1
    Dec 8, 2023 at 0:13
  • It's always best to smog test that engine when it's nice and hot, glad it worked out for you @1trusted1, that must be a relief!
    – GdD
    Dec 8, 2023 at 9:15

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