air fuel ratios at idle?

cederrowe

New member
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
1988 st 165
 

MWP

New member
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.
 

Simba

New member
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.
 

MWP

New member
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.
 

cederrowe

New member
"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.
 

klue

New member
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
 

Simba

New member
MWP":1rvfqa0u said:
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.
 

MWP

New member
Yes, but at cruise, timing is so far advanced and the load is so light that the extra heat is negligible.
 

cederrowe

New member
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?
 

coyoteboy

New member
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 :)
 

MWP

New member
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.
 

klue

New member
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
 

coyoteboy

New member
I've run mine at 15:1 with 40 degrees advance on light load (60-85kpa) and saw no problems with coolant temps, any leaner than that and it's starting to get into unhappy territory - it pulses power at about 15.5:1, anything between 35 and 50 degrees, maying cruise impossible, so I'd always keep near stoic (as the stock ECU did, pretty much bang on) by tuning and let the closed-loop keep it happy.

For those saying while making boost you should be nowhere near stoic - this is EXACTLY what the stock ECU did, until about 4-5psi of boost - I've the logs to prove it. Unless the throttle was fairly wide (>25%), even if it was in fairly notable boost it would be closed-loop on the standard narrowband.
 
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