Open loop equals start up or choke position. Open loop means it is using factory settings to give more gas on start up. Open loop takes no input from the front O2 sensors. Typically I would guess 15-30 seconds before closed loop takes over, thus using the 0-5 volt signal front O2 sensor input. kind of equivalent to the choke opening up on the old cars as far as time frame goes. This sensor is examing exhaust gas in the collector area and sending a signal back to the pcm based on what it sees. The pcm interprets this signal, and adjusts the AFR accordingly through short term (like watching your instantaneous fuel economy) and long term (based on a certain time period of short term input) fuel trim, continually trying to adjust the AFR to 14.7 to 1. This ratio is called stoichiometric and is the optimum burn ration for gasoline efficiency.

Closed loop is continually sensing many parameters including intake ambient temp and front O2 sensor input. If the computer senses a fault, or doesn't get a signal from the front O2, cars can go into what some call the 'safe mode' ...which is open loop and typically a little rich because the 'brain' has no idea what's going on and errs on the conservative or rich side with baseline factory fuel and spark mapping.

Open loop occurs again when the throttle is mashed to about 80% or more. The front O2s are thrown out and pre set factory spark and fuel mapping for max power take over.


Rear O2 sensors have nothing to do with determining ongoing AFR. They are monitoring the convertor output and will throw a light if the values are not correct...again based on a 0-5 volt signal.

 

That's great info Shoman90. Will colder OATs cause it to remain in open loop longer than warmer temps? How about if the RL is garaged and then driven out into very cold weather vs non-garaged in very cold weather.

Thanks!

 

I would guess slightly longer, but once the engine has been running for a short bit, I don't believe it will go back into open loop even if the temperature is drastically different...as in ....the weather normal vehicles see.

Even though a vehicle is running in closed loop and looking for 14.7....the engine needs to get to a good hot temp to work most efficiently. Thus I believe one of the reasons that short cold hops in the winter contribute greatly to poorer MPG. Any vehicle that only sees shorts trips like this in the winter is not getting all the moisture out of the engine and exhaust and should therefore see a good highway run on a regular basis.

 

Also if you start the truck, and take off in about 30 seconds, set the cruise at 30 and watch the fuel mpg meter you can see when it come out of open loop. Of course depending on the out side temperature. The colder the longer but usually no longer the 2 minutes. At least that is what I have seen on mine.

Great write up Shoman90,

 

Great write up Shoman90,

Thanks Ourfarm09. I'm no expert, but I had to bone up on this stuff when I started modifying our hot rod with MIL setting headers/exhaust and the blower package I put on in 2006 which had a piggy back computer to add to the system. I tuned the AFR with the aid of a wide band O2 sensor and laptop software. All brand new stuff for me. I had no engine building/tuning background...it was a challenging installation to say the least!

I have a wide band meter mounted on my steering wheel. As you state above, I can watch my choke AFR of 10 to 11 drop out of open loop into closed loop in about 15 seconds. The meter goes to 13.5 to 14.5, the idle drops off, it runs a tad rough for a bit (the nature of the beast)...and we're off to the races!

 

Great info. Thanks guys.

I've tried to keep an eye on my iMPG function of my ecoRoute HD device after a cold start, but with traffic and hilly terrain, it's a challenge to really see it change in that short a time frame. I don't have the open/closed loop info on my Garmin device.