3.2 - How long after cold start should the DME use the O2 Sensor?
 

Hi Team,

Still trying to figure out my low rpm cold start issue on my Cali car. I have now figured out that after the 1 min or so, cold start cycle of 1100 RPM, the O2 sensor is sensing the rich condition and then bringing the rpm down to 600rpm.

My question is, when should the DME actually start using the O2 Sensor? If the CHT sensor and AFM air temp sensor are telling the DME to put a rich mixture in as its still cold, why would the O2 sensor then try to reduce fuel for a stochimetric burn?

Im wondering if my DME is using the O2 sensor data to early, before the car has actually heated up enough to be at its "Normal" Fueling level.

BTW once car temp is warm, she idles fine at 850 rpm.

Any thoughts appreciated.

Why do you think this, i.e. what data have you gathered? Hopefully, you don't have an aftermarket "performance chip"?

Hi Dave,

Thanks for the reply.

No aftermarket Chip, only changes to engine is a Skart Pre Muffler and sport muffler. Otherwise Stock.

What Data:

I know its a narrow band 02 sensor, but when checking the voltage value of the O2 standalone (O2 unplugged from DME) from cold to second temp indicator mark, the Voltage drops from >0.9V to ~0.7V. I am guessing this is correct as the Cylinder head temp sensor would be affecting the mixture value as the engine warms up from Rich to Lean.

Now if I plug the O2 sensor back in while the engine is running but still cold at first temp indicator, the RPM drops it self to 600rpm from 800rpm. During the entire heat cycle from cold to 2nd gauge indicator, the rpm will slowly creep up from 600 to 850.

This is why I am assuming its the O2 that is reducing the fueling and in turn the rpm to try achieve the AFR of 14.7.

Adding to this, I have checked CHT and ICV for correct operation. O2 sensor is also new, and getting 12V to the heating elements.

What I am trying to understand is if the DME controls the cold start fueling by the CHT, is it true that only when the engine is fully warm would I see the correct idle AFR.

If this is true, and its normal for the AFR to eventually lean to ~14.7 when fully warm, why would the O2 sensor kick in so early and pull fueling?

This is just what I have going around my head at night as I fall asleep, and welcome any thoughts as I am very new to 911s.

Thanks.

The idle should not drop below about 800 RPM when initially starting. My '88 3.2 when cold is about 850 to 900 when first started.
After a few minutes, it's about 800. This is the case for the later 28 pin EPROM. The earlier 24 pin chips start at about 750
and increase to about 800. Hopefully your idle switch is functioning and the air screw on the throttle body is not all the way in.
Also, the air screw on the AFM should only be out about three turns. Check your idle valve, i.e. it should hold about 750-800 RPM.

Thanks Dave,

I've been looking at Scarcellers image below, and mine matches this other than RPM.

After aprox 50 seconds, my rpm drops to 600. His seems to smoothly reduce to 900 rpm after about 2.5 min.

According to the graph at around 1.5min the AFR starts to settle. As the value is at 13.5, I dont believe this is running a stock o2 sensor, which would have shown 14.7. However it does give me an idea of the length of the cold start cycle programmed into the DME.

I will check how many turns out the Air Valve and AFM are. Thanks for the Tip.

Can I ask if your 3.2 ever gets up to 1100rpm at cold start? I thought the initial rpm at cold start was always ~1100 rpm for the first minute, then it dropped off. My exterior temp is 65F during winter btw.

http://forums.pelicanparts.com/uploa...1681267384.jpg

"I thought the initial rpm at cold start was always ~1100 rpm for the first minute"

Yes it does. My '88 starts at 1100-1200 when ambient temp is low, then drops to 880 after a minute or two.

The startup idle can be setup a little differently by adjusting the throttle body screw.

Why did you set yours so low? Because of Socal always warm weather?

It was the best setting for my warm & cold idle desires. It also minimized the idle undershoot from RPM de-acceleration.

Hey Socal, by throttle body screw, do you mean the screw that adjusts the arm position relative to micro switch? Or the bypass?

Only the small bypass screw.