I have done some searching on this site, but can't find what I'm looking for. I have a 95' F150 5.0L. I want to install an air/fuel ratio gauge. Does anyone know exactly what causing the computer to go into open loop and closed loop? Also what is happening during each, I hear that in open loop the gauge will just swing rich to lean 100's/ minute. Someone must know what the comp. is doing during open loop, I Hope. Thanks

 

Ok, when the comp is running in open loop, the comp runs the engine off of tables. Tables being pre programed sequence of events that at throttle position "x", and mass air meter reading "y", air temp "z", at said rpm, the injectors pulse at that desired amount. Now I know this sounds complicated, but it isn't. Now when it runs in closed loop, the air fuel ratio is being read by the o2 sensor and sending a signal to the comp to richen or lean it per the o2 sensor. Now if you had an air/fuel ratio gauge installed, when in closed loop, the gauges fans violently because it is seeing the swings from rich to lean. hundreds of times a sec. But when the comp is in open loop, and the comp adjusting fuel ratio off the tables instead of the o2 sensor, it will give you a smooth steady reading of air/fuel ratio. I hope this makes sense.

 

Makes allot of sense Hippie, now what causes the computer to go from open to closed? Is the closed loop just a WOT response or something? Oh Ya, thanks agian Hippie.

 

The coolant temperature sensor is what tells the PCM to go from
open loop to closed loop. When the engine is cold, it runs in
open loop, when it gets to operating temperature, it goes to
closed loop.....This is the reason to always use the recommended
thermostat.

 

So the closed loop, being adjusted by the computer, should be the more efficient of the two? Staying too cold and in the open loop hurts gas mileage -- hypothetically speaking?

Thanks for the basic how-it-works info. It's a really valuable thing we can get here.

Chuck

 

You're also in open-loop at wide open throttle (WOT)

Closed loop control in general refers to a control strategy in which the output is fed back into the processor to modify the input signal in order to maintain a desired output. In open loop control there is no feedback, the processor simply drives the output device to a predetermined value. In the case of your truck, the O2 sensors modify the signals generated by the throttle position sensor (TPS), engine coolant temp (ECT), manifold absolute pressure (MAP) sensor, etc to correct the timing and the amount of fuel the injectors provide. This is the normal operating mode "warm cruise" and it is the most efficient and has the lowest emissions. In the cold start mode, the engine runs intentionally rich as it does at WOT. Neither efficiency nor emissions are optimized in this mode.

If you want to get into some detail on this read:
Probst, Charles O. How to Understand, Service and Modify Ford Fuel Injection and Electronic Engine Control Volume I covers 1980-87, Volume II covers 1988-93. Both are available from the publisher Robert Bentley at www.rb.com

 

So in the case of watching the air/fuel ratio gauge, once the vehicle has come to operating temp. it is in closed loop and the gauge is worthless? This means that installing the gauge would be a waste of money because 95 percent of the run time will be in warmed up closed loop which fans the gauge if I am understanding all this correctly. What good would a gauge do? I wouldn't want to tune the vehicle based on cold running temp. which is open loop right?

 

The gauge would provide a means of verifying that the PCM is operating correctly and would give you early warning on O2 sensor failure, other than that no real purpose on a normally aspirated truck. If you add a blower or nitrous a fuel/air ratio meter should be considered a mandatory option.

 

When I first installed my gauge I was disappointed. The light show that comes when in closed loop was distracting...But.... Once I ran it for a while, and learned what to look for, I was glad to have it. It lets me know, when at wot, if the ratio is good, or atleast close. But in adition to that, the comp drops out of closed loop way before wot. If I am cruising at speed, and give it just a bit too much gas, the truck may not drop out of overdrive, or unlock the converter, but it will go out of closed loop. As soon as it does, I know! I can see the gauge stop its fanning and go to a steady state in the slightly rich range, and I know to lift off the gas just a bit. This has let me learn to get better mpg by knowing when I am pushing the comp into open loop. Like I said I am glad to have it now, and would suggest it to others. It is functional in a few ways. Later Hippie.