General Question

2davidc8's avatar

Can someone help me understand these vehicle smog test results (details inside)?

Asked by 2davidc8 (7795points) April 21st, 2014

In California, we have to put our cars through a “smog” test every other year. In this test, HC (hydrocarbons), CO (carbon monoxide), and NOx (nitrogen oxides) emissions are measured (among other tests).

So, my vehicle passed, but just barely. But here’s what I find a bit strange:

At 15 mph or low rpm, HC, CO and NOx emissions were close to the maximum allowed. But at 25 mph or high rpm, these same emissions were WAY below average. So, I’m wondering how could this be? Could it be that the fuel mixture is too “rich” at idle, or is this irrelevant?

I’ve also heard that you should drive your car around for a while just before taking it in for the smog test. I forgot to do it this time. Maybe that explains the results?

Observing members: 0 Composing members: 0

16 Answers

FireMadeFlesh's avatar

You are correct on both counts. When an engine is not yet at operating temperature, it requires a richer fuel mixture to run. This is due to the greater friction of the more viscous oil – more power is needed at a given rpm to avoid stalling. If you’ve ever driven a car with carburettor induction, that is the function of the choke. Late carbie cars had an automatic choke, and modern cars are fuel injected, so the car takes care of this for you. As you increase the revs, the engine is receiving more than enough fuel/air to run despite the increased friction (there is no risk of stalling, like at lower revs), so it reverts to a more standard fuel/air mixture. You’ll also notice that the engine will idle slightly faster when it is cold.

Also, when the engine is cold, the components are not at their operating size. When metals heat up, they expand. Therefore a cold engine is running with slightly larger gaps between moving parts, which decreases efficiency, and allows more unburnt fuel through into the exhaust manifold. This will increase your hydrocarbon reading.

It also isn’t the best idea to be revving an engine too much when it is cold, as the greater oil viscosity means more wear on the moving parts. I personally change gear at 2,500 – 3,000 rpm in my car when cold, but can rev out to 7,000 rpm when warm.

kritiper's avatar

The car should be fully warmed up before the test, and this can throw off the readings a bit.
At different operating RPMs, the engine fuel requirements differ, hence the different readings. At idle, for example, an engine needs a richer fuel mixture compared to high RPM where the engine can use a much leaner mixture. Carburetors old the old cars that had them, required seven (7) different circuits to provide the gas to the engine when required. New engines with computerized fuel delivery must meet the same engine needs.
Engines aren’t designed to operate at cold temps but the fuel delivery must cover those needs until the engine is at full operating temperature. That is what the choke did on the old cars.

2davidc8's avatar

@FireMadeFlesh @kritiper So, the ensure that my vehicle passes the smog test next time, would you suggest that it might be a good idea to have my mechanic adjust the fuel mixture at idle to be leaner AND for me to drive the car around a bit until it is fully warm before the test?

SecondHandStoke's avatar

My cousin’s new POS Chevy Sonic doesn’t even have a temperature guage.


kritiper's avatar

@2davidc8 It depends on how old your car is. Newer cars have no adjustments because the fuel injection is computer controlled. Make sure your air filter is fairly clean and do warm the car fully before the next test.

FireMadeFlesh's avatar

@2davidc8 As @kritiper said above. Make sure you service your car at the scheduled intervals, and it should do just fine. The fuel mixture should be what the engine was designed for – too rich or too lean can lead to problems. Keep it in the sweet spot.

FireMadeFlesh's avatar

@SecondHandStoke Some cheaper end cars like that don’t have a temperature gauge, but they should have a temperature warning light. That would at least give you enough information to pull over and turn off if your cooling systems failed. Your cousin’s car runs a small, modern engine, so it shouldn’t take more than 3–5 minutes of normal driving to get up to temperature.

kritiper's avatar

One more item: be sure to use gas with the octane rating that is recommended for your car. Some people think higher octane fuel is better: it isn’t. It won’t make your car run better or stronger or last longer. The proper octane will make the engine run at it’s best, with the cleanest emissions and highest MPG.
I have my car tested after driving about 15 miles.

2davidc8's avatar

@kritiper @FireMadeFlesh OK, thank you for your advice and for the guideline figure of 15 miles. My car is pretty old, though it doesn’t have as many miles as you might expect. It doesn’t have a carburetor, but I think that some adjustments are possible. I’ll ask.

SecondHandStoke's avatar


The lack of a proper temperature gauge is merely the latest in a very long series of non-innovations designed to isolate the motorist from automobile operation experience.

It’s nothing short of contemptible.

Am I the last driver on Earth that allows his car to communicate with me through the subtlest of sensations and sounds?

Far too many cars these days are clearly designed to assume that the operator has no idea what the hell they are doing.

Are these the sort of motorists we want piloting their roadgoing tanks while checking Email and reading newspapers?

SecondHandStoke's avatar


By far the greatest problem you’re facing is the fascist “Republic” of California.

No other state has such a crushing chokehold on a motorist’s options.

It’s not terribly difficult to make cars work better, especially older ones.

It doesn’t take a genius to make a slew of improvements to a car to increase throttle response, acceleration, top speed etc.

But heaven fucking forbid. the California Emission Nazis take the excesses of investigation to an entirely new level.

If that throttle body gasket is too thick they call it a spacer and waste your time and money in the form of failing you car on a freaking VISUAL inspection.

“Oh we’re terribly sorry but your catalytic converter is mounted an inch too far back.”

“Too bad so sad.”

Sadly, In California you make all manner of costly and time consuming improvements just to have to remove them before subjecting your sacred ride to the emissions Gestapo.

FireMadeFlesh's avatar

@SecondHandStoke You’re not the last, believe me. I still drive a performance car with a dirty N/A engine and a proper manual gearbox (none of this paddle rubbish). The only feature that questions my driving skill is the reversing sensors – but rear visibility is so bad I kind of need them.

FireMadeFlesh's avatar

Alfa Romeo GT, with the 3.2. You?

SecondHandStoke's avatar

DC1 Integra with performance modifications.

FireMadeFlesh's avatar

Nice. I come from a Honda family – got to love old school VTEC!

Answer this question




to answer.

This question is in the General Section. Responses must be helpful and on-topic.

Your answer will be saved while you login or join.

Have a question? Ask Fluther!

What do you know more about?
Knowledge Networking @ Fluther