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Old 02-26-2010, 01:25 AM   #1
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Default Why do cold engines run rich?

I can understand an engine running rich if it's in open loop mode as it's not getting readings from the O2 but what if it's already in closed loop mode? I mean the engine is still cold but why would the ECU be programmed in such a way to run more rich than when it's warmed up if it's in closed loop mode? Wouldn't an engine that is running at 15 or 16:1 (slightly lean) cause the engine to heat up quicker and therefore not be cold? I mean one reason why engines don't run lean during normal operation is because of the risk of detonation but that shouldn't be an issue when it's still cold, right? I realize the O2 sensors in most cars are narrow band but it's still possible to estimate somewhat inaccurately how lean the mixture is, right?
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Old 02-26-2010, 01:43 AM   #2
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Default Re: Why do cold engines run rich?

If an engine is cold and ran lean, it will stumble and run rough. Adjust it lean enough and it simply wont run. The fuel mixtures for a warm engine differs greatly from a cold engine. I cant get all scientific on you but this is normal.

Not sure what you meant when you stated closed loop operation and cold engine. Do you mean after a few minutes of run time where the O2 sensor heats up but the coolant temp is still low? If so, same priniciple applies, cold engine = richer mix regardless of O2. A side note..various factors can cause detonation, not just a warmed up/hot engine.
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Old 02-26-2010, 02:26 AM   #3
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Default Re: Why do cold engines run rich?

So you're saying an engine that is designed to run lean can't run lean if it's cold? I mean some engines will run rough regardless of whether or not they're warmed up because they're not designed to run even a little bit lean but why would temperature of the engine matter when it comes to whether or not it can run lean? As for closed and open loop part, yes you understood what I was getting at when I mentioned that.
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Old 02-26-2010, 04:55 AM   #4
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Default Re: Why do cold engines run rich?

colder denser air is harder to ignite so it requires more fuel (rich) for it to run properly. also it helps in warming up the engine, o2 sensors and cat-converter quicker.
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Old 02-26-2010, 06:03 AM   #5
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Default Re: Why do cold engines run rich?

I agree with the info these guys have given.

Its just a "Choke" that you don't have to mess with as the ECU adjusts it automatically.

Verses a choke on a Carb. engine such as a dirt bike, ATV, Lawnmower where you have to actually flip a lever, etc.

A cold engine as the other guys said needs a richer mixture to get fired up, take away that rich mixture to soon and it will die.

You could see that perfectly on a lawnmower or something if you wanted to try it out, start it with full choke, once it fires up and is running, flip the choke off, it will die, unless the air temp outside is already really hot(IE: Summer), then it might stumble and keep running.
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Old 02-26-2010, 10:08 AM   #6
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Default Re: Why do cold engines run rich?

Fuel doesn't vaporize very well when cold and tends to fall out of suspension in the intake...thus the need for a richer mixture.
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Old 02-26-2010, 10:33 AM   #7
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Default Re: Why do cold engines run rich?

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Fuel doesn't vaporize very well when cold and tends to fall out of suspension in the intake...thus the need for a richer mixture.
Not necessarily a richer mixture - but you need to injector more to compensate for that fuel falling out of suspension and coating the intake runners. Honda K20/24Z series engines have air passages in the intake manifold to "inject" air into the pintle caps of the injectors during cold operation, to help break up the fuel and atomize it better in cold conditions. As the engine heats up, a thermo. controlled valve closes off those passages as the fuel begins to vaporize on the closed intake valves (like normal).

Cold air is more dense, so it has more oxygen, and therefore you need more fuel by volume for a stoich. mixture. But warm up enrichment isn't there to make the car run rich, it's just there to compensate enough to get back to stoich. (or whatever the target afr is).

Also, a lean engine is hotter than a rich engine, so if building heat is the objective, running richer is the wrong direction.
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Old 02-26-2010, 11:02 AM   #8
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Default Re: Why do cold engines run rich?

I was only eluding to it being a richer mixture as it seems the OP is looking at that way. But yes, you are correct about it not really being about a richer mixture.

Fun Fact: Honda also used the Air Injection system on the 99-00 Civic Ex's.
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Old 02-26-2010, 05:28 PM   #9
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Default Re: Why do cold engines run rich?

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Originally Posted by EE_Chris View Post
Fun Fact: Honda also used the Air Injection system on the 99-00 Civic Ex's.
I was just about to post that....


Ok so you guys are saying that once the car is in Closed loop mode, it's already operating like a warmed up engine but with cold fuel and cold air intake?? I am aware that the temperature of the fuel is more important when trying to improve fuel economy compared to temperature of the air but....it sounds like an engine that is 70F, closed loop. but with 50F fuel and air is going to use the same amount of fuel as an engine at 180F but with 50F fuel and air??? Is the reason why an engine idles fast when warming up is because it can't properly atomize the fuel mixture like a warmed up engine can? Does having a cold block not allow for enough of the explosion of the mixture to go to work as most of the heat is mostly getting absorbed?
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