1993 BMW 325is emissions pass but CO high

CO is an indicator of a rich/lean mixture condition. The lowest reading of CO is when the fuel/air mixture is at 14.7 to 1 and the CO will go up on both sides (meaning either rich or lean) of the curve. Too rich of a mixture will cause a shortened life span of the cat and the O2 sensors can only perform so much fuel trim. Other than offering information and support, what can I help you with?

The O2 sensor was replaced on this car about 1.5 yrs ago as well, and it didn't have any significant effect on the CO readings before or after. With my tuneups I've been able to pretty much cut the HC and NOx in half, I'm just wondering what part of the picture could leave the CO in this condition where it doesn't seem to be shifted in a positive direction by anything (except the apparently defective MAF which made the HC go sideways). The catalytic converter is original.

The mixture and the MAF are "joined at the hip" and how much fuel is metered is dependent on the MAF readings. 122k miles should not be a factor in the health of the cat but too rich of a mixture could shorten its life.

Based on what you said before about the CO elevating equally once the mixture drifts towards either lean or rich mixtures, would you say that the MAF which reduced the CO is in better shape?

In that case the HC doubled, should I interpret this as other MAF being better and exposing a different problem regarding the HC?

An interesting question, I dont know the answer as to which one is better, I would choose one that could keep the pre-cat CO level at the correct percentage to maximize catalytic converter life span. Elevated or near tolerance readings can indicate a tired cat, all those miles add up, especially if driven in a spirited fashion, and those miles are cumulative in their progression towards a compromised converter.

This is my dilemma, choosing which is the lesser of two evils for my cat, the elevated CO or the elevated HC. I've spoken with a few aircare (emissions) specialists & BMW mechanics, as well as checking results on older cars like this. Most BMW's with this engine or generation of engine seem to eventually progress to this level where the CO is elevated and the HC / NOx are low. The well maintained ones anyway ;-)

I'm just trying to determine if this is a worn cat or a common problem that most people never notice or resolve once the miles start piling up. In my case it's been doing it since the car was approx. 5 years old and less than 100,000 km. That would lead me to believe it's not a cat problem, as it should have degraded and failed over the last 100,000.

Charles, when you get it figured out, let us know.

I decided to change my coolant temp sensor, and CO scores are now 3 times lower than before, and now beat an average emissions pass by about 2.5x on that emission type. This score had been elevated to the previous level since at least 2002 / 115k km. The NOx has now increased slightly to 0.29 (max pass is 1.24) and HC increased to 0.30 (dead on for an average pass and about half the limit). I'm happy now that all my scores are consistently good (CO and NOx are about 20% of the allowable). I wanted to get this issue under wraps as I am chipping the car with the Dinan performance chip, which is hopefully arriving tomorrow.

The inconsistent behaviour of the car under different load conditions has now disappeared. I think the car was compensating for the bad coolant temp sensor readings so much than any significant change in conditions would be beyond its limits of adaptation and cause it to have driveability symptoms, mostly below 3000 rpm. With this fix, the car is nearly unfazed by the load of the air conditioning, which previously would cause noticeable degradation and power loss, especially at low rpm.

I have seen and heard about quite a few BMW engines with this type of emission score pattern and similar driveability problems, so I will be spreading the word in the enthusiast forums...it's a cheap part and a quick fix. In my case the parts / labour for both sensors (this car has one for the cluster reading as well) was $120 (Canadian). Thanks for your feedback in understanding the CO emissions.

Replacement of questionable sensors is always favorable if the values are outside the operating parameters, if and when those values are known, that's what makes answering questions in the forum so much of a challenge, I will polish my crystal ball to possibly avoid this issue in the future lol

As a follow up to all of this, I installed the Dinan chip after I figured out the CO problem, and the car is running beautifully. I had a Motorvac cleaning done now that all of my emissions tuning is complete, gave it a couple days for the DME to re-learn and I ran it through emissions testing today.

Now all my emissions scores are 5-10x lower than an average pass with the new chip producing an extra 18 hp and 20 ft-lbs of torque. Needless to say I am pretty pleased...the new emission scores are now 0.03 (HC), 1.19 (CO) and 0.07 (NOx). Not bad for a 16 year old car with 200,000 km and its original cat ;-)