Recently I put my car on the 4 gas analyser, and it gave me these results at idle:

CO2: 10.64 %
CO: 4.98 %
O2: 1.94 %
HC: 148 ppm
Lambda value: 0.93/13.7AFR

I have to mention that it concerns an older carbureted vehicle, so the tuning is rather usual. There is no EGR or secondary air injection, and the exhaust has no leaks. I am wondering how it is possible for the engine to leave almost 10% of the inducted oxygen unused while tuned rich. I would assume all the oxygen should be used in the combustion with this tune. Poor atomisation may cause poor combustion, but I wouldn't expect that to have such dramatic results.

The only other thing i can think of, is that intake air gets mixed in the exhaust at the time of gas exchange. But with these results, that would mean that the gas exchange here is extremely good. Is that assumption probable?

The valve timing is:

IO 16deg
IC 56deg
EO 56deg
EC 16deg

  • How about that the combustion process has finished and all you are left with are the products...
    – Solar Mike
    Commented Oct 5, 2017 at 14:23
  • @SolarMike But then there shouldn't be any oxygen left, or else this combustion process is highly incomplete, right? The high CO% shows that there's plenty CO left to be burned. How can CO and O2 co-exist at these temperatures and not react?
    – Bart
    Commented Oct 5, 2017 at 14:48
  • but is the concentration sufficient to support combustion - what are the other gases to make 100% ?
    – Solar Mike
    Commented Oct 5, 2017 at 14:52
  • @SolarMike I suppose so? The temperature to initiate or continue combustion is high enough i'd say. Would you say these results aren't abnormal? I don't have a cat so it differs from what i'm used to see on the analyser anyway, so i can't say. And what gases are you referring to? After the gases on the analyser result, only nitrogen is left.
    – Bart
    Commented Oct 5, 2017 at 15:07

2 Answers 2


TL DR: I would suggest you have an exhaust leak up stream of the sensor location. I know you said it doesn't have a leak, but I'd suggest you just haven't recognized it.

Valve overlap only occurs between the exhaust cycle and the intake cycle. Valve overlap will only allow exhaust gasses to revert (called reversion) back into the combustion chamber during the intake cycle. You would never push part of the new intake charge through the exhaust valve opening because the cylinder would be under vacuum at the time. Overlap can be used in place of an EGR, if done correctly.

I would throw one caveat out there. All bets are off if the engine is turbo/supercharged. If there was enough pressure, it is conceivable there could be some intake charge forced out of the exhaust port if there was enough overlap. You don't mention it in your question, so I doubt this even comes into play here.

  • I can never be 100% sure about exhaust leaks, but it's an all stainless system, and only 1.5 years old. Nowhere it showes any sign of a leak, like sooth spots or something. But it could still be leaking somewhere, that'd explain the 2% oxygen. Assuming it doesn't, is this a normal result? It's normally aspirated as you presumed.
    – Bart
    Commented Oct 5, 2017 at 16:34
  • Exhaust leak seems like the likely culprit to me. It would not be from valve overlap and that's really what I was getting at here. I guess there could be one other culprit, that being a leaking intake valve ... but that would give you other running issues as a result. One other thing, the 13.7AFR is actually a tad leaner than what normal operation should be. Most tuners (even for carburetors) likes the AFR to be in the 12-12.5:1 range, which is actually a bit richer than what you're running. I realize 13.7 is still richer than stoich, but cars don't run at 14.7:1 by design. Commented Oct 5, 2017 at 16:48
  • Then my question, which was about valve overlap, is answered. It's still interesting to find the cause of that oxygen though. I'll double check my exhaust for leaks for a start. And if there's a leak, then the real AFR will be significantly lower than what is calculated with the leak. If it drops to 0,5% then the AFR will near 12.5. Tuning it richer doesn't give me extra power at the moment, and it smells like unburned fuel too bad for my liking already.
    – Bart
    Commented Oct 5, 2017 at 17:26

There many potential reasons why all oxygen isn't consumed during the combustion process. In older engines the combustion chambers may be inefficiently shaped which doesn't allow the entire intake charge to burn.

In recent times there has been massive investment in optimizing valves, pistons, heads, airflow etc to make combustion engines more efficient, specifically combustion efficiency. Something that took a back seat to other concerns in the past.

Running rich doesn't guarantee efficient combustion, in particular fuel cools the intake charge, too much fuel may cool the combustion, foul sparkplugs, etc.

Engine development has been a long continuum that continues today and without a specific technical analysis of any given engine its difficult to pin lack of efficiency to any specific cause(s).

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