air fuel ratios at idle?

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air fuel ratios at idle?

Postby cederrowe » Fri Jun 11, 2010 2:50 pm

I keep getting in the range of 16 to 17 to1 at idle and goes right down to 15 to 1 cruising. Does anybody else have the lean idle?

And under boost all the way down to 10.9 to 1
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Re: air fuel ratios at idle?

Postby klue » Fri Jun 11, 2010 3:44 pm

idle is slightly lean, is that the original AFM?
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Re: air fuel ratios at idle?

Postby MWP » Sat Jun 12, 2010 12:42 am

What are you measuring the AFRs with?

But, yes, that is quite lean.
Ive found best, smoothest idle is around 13:1... which is the same for most engines.

Still, if it is idling ok at that AFR, then its not a problem.
Basically, AFR is not important at idle, whatever it idles the best at is what you use.
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Re: air fuel ratios at idle?

Postby cederrowe » Sat Jun 12, 2010 1:03 am

Aem wideband

Yes it's ideling fine, today it was near 15.1

Yes original AFM, 113,000 miles
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Re: air fuel ratios at idle?

Postby Simba » Sat Jun 12, 2010 1:29 am

You've got something causing an idle lean condition. As it's only obvious at idle, I would suspect a vacuum leak, o2 sensor issue or AFM issue. Check them all out per the TSRM/BGB.

Under normal circumstances, a stock computer/fuel setup will bounce around 14.5-15.0 or so.

15 at idle or cruise is not terribly out of the ordinary. 16-17 is very much so. If you're that lean under cruise, don't drive it until you fix the cause.

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Re: air fuel ratios at idle?

Postby MWP » Sat Jun 12, 2010 2:13 pm

I drive my car everyday at a lean cruise of 16:1.
Cruising an idle at those lean AFRs is perfectly ok.

Is when you get lean under power that you start doing damage.
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Re: air fuel ratios at idle?

Postby cederrowe » Sat Jun 12, 2010 3:19 pm

"right down to 15 to 1 cruising"

Like I said it goes down were it is supposed to be under cruise and drops all the way to 10.9 under 11 psi boost.

I was just wondering what other people are getting on idle, i have had some cars idle at 15 to 1 just not 16 to 17 to 1. It idles smooth though.
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Re: air fuel ratios at idle?

Postby klue » Sat Jun 12, 2010 4:55 pm

12.5-13.5 for cold engine idle and load
14.5-15.5 for idle, which ever is smoother on your set up(and more rich is a waste of gas)
14.5-15.5 cruise, any leaner and your heating up the engine and will be lacking torque
>10.5 in boost.

edit: 16-22 should only be seen under deceleration,
my guide is for stock pistons, forged will be much more tolerant of leaner AFR's. If you have stock pistons, and like them in a solid stat (as apposed to liquid) then keep it rich baby
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Re: air fuel ratios at idle?

Postby Mafix » Mon Jun 14, 2010 3:42 pm

that looks good for a stock motor klue.
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Re: air fuel ratios at idle?

Postby Simba » Tue Jun 15, 2010 1:02 am

MWP wrote:I drive my car everyday at a lean cruise of 16:1.


Running it that lean generates a lot of heat, for a fairly marginal mileage increase.

Point being, with a relatively stock setup, you should never see anything that lean with any kind of throttle.

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Re: air fuel ratios at idle?

Postby MWP » Wed Jun 16, 2010 4:53 am

Yes, but at cruise, timing is so far advanced and the load is so light that the extra heat is negligible.
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Re: air fuel ratios at idle?

Postby cederrowe » Thu Jun 17, 2010 2:57 am

I noticed i'm getting 14.5 at up to 6to7 psi boost at 1/4 throttle then if i hold it there it will eventually drop suddenly to 12 and dropping to 10.9 and the car starts to make more power?
It almost hesitates untill fuell ratio drops the it makes power,
possible air leak?
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Re: air fuel ratios at idle?

Postby coyoteboy » Thu Jan 06, 2011 6:10 pm

I'm just dealing with this at the moment, misfiring if I get leaner than 14.5, but then I'm not running the stock ECU so I've no idea what timing it uses at idle - any ideas?

Incidentally, the accepted "safe" AFR for a load situation on these engines (according to a few experienced tuners I've spoken to) is 11.5:1, I'd not be up near 12.5 under boost, certainly not into the teens. It's still MILES better than the 8.something I was seeing with the stock ECU at 12psi :)
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Re: air fuel ratios at idle?

Postby MWP » Sat Jan 08, 2011 2:26 am

Above 0psi (ie, making boost) you should NEVER be leaner than 12.5:1. Even 12:1 is risky.
Doing so is just asking for a melted piston... and you will eventually.
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Re: air fuel ratios at idle?

Postby klue » Sat Jan 08, 2011 2:57 am

Lean burn with advanced timing is asking for major trouble.

I like to run a stock piston 3sgte with no leaner than 11:1, on my own car the leanest i gets is 10.7 or so.

under no circumstance should the afr get leaner than 14.7:1 on this engine, from my own experiments and experience the torque significantly drops past 14.4 or so AFR. under cruise. What i would do is drive the car on a steady cruise, and slowly lean out the mixture while maintaing a steady speed and load(best on the dyno). I could clearly feel, hear, and see(coolant temp) that the engine was not happy with anything past 14.4 during loaded cruise.

I had trouble getting a steady idle with a lean mix, so i stick to about 13.5-14 at idle. little richer when cold. around 17 or so degrees of timing with my set up. Remember that fuel is for thermal management, not just getting the car to move. so a richer mixture is going to keep things cooler, leaner is going to make it hotter.

it is very important when asking questions about what would be a safe timing/fuel mix that you state your mods in detail. Where you could run a ct26 vs ct20b vs 20g safely is very different

hope this helps
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