closed loop, open loop?

I know the above has something to do with how the computer controls the engine (I think), can someone explaine what it is?

In short: Open loop ignores the O2s, Closed loop includes them in it's calculations.

Alec said:

In short: Open loop ignores the O2s, Closed loop includes them in it's calculations.

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Thx Alec
Can you explaine the following:My scan gauge unit tells me when i am in an open and or closed loop mode, at cold startup it reads open for maybe 10 secs, than goes to closed and remains there, except under WOT, it will read open under WOT.
Today, ambiant temp was 104, coolant was 198, Intake air temp was 155, sitting in traffic, readings would go back and fourth between open and closed loop, what where the 02 sensors doing?

spindlecone said:

"except under WOT, it will read open under WOT."


"readings would go back and fourth between open and closed loop,"

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It shouldn't do that, it should be read "Open" till the O2's warm up, then goto "Closed" and stay there.

reads open untill warmup as you say,a t full wot will go to open for maybe two seconds

hmm, I thought some EFI systems ignored emission control stuff during WOT, explaining the possibility of an open loop.

but, it shouldn't switch back and forth while idling at a light.

is not at idle, is during stop and go traffic

open at WOT is normal.

during traffic it should stay in closed loop. If it doesn't it's O2s, wiring, or PCM.

^^^ What he said.

It is normal to go into open loop at WOT, it is actually one of the engine control strategies programmmed into the PCM.


BTW, open loop and closed loop are generic terms used to describe control systems. A closed loop system is also known as a feedback control system.

Pls forgive my ignorance, loops are control systems, correct? if so, what do they control?

spindlecone said:

Pls forgive my ignorance, loops are control systems, correct? if so, what do they control?

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Your Explorer has a specialized type of control system: EEC stands for Electronic Engine Control. The Ford EEC system uses a microprocessor (PCM or Powertrain Control Module) sensors (O2, IAT, ECT, MAF) and actuators (fuel injectors, IAC, EGR) to control engine operation.

There are other types of control systems (e.g. a system used to control an elevator in a multi-story building or a control system to guide a missile to its target). The other types of control systems also use a microprocessor or a PLC (Programmable Logic Controller), sensors and actuators to control their systems.

A closed loop control system is a special type of control system; it uses a sensor (or sensors) to monitor the operation of the system and feeds the sensor data back to the microprocessor so that the operation can be tuned toward the "ideal" process. The closed loop system (if designed correctly) will correct itself for "ideal" operation.

An open loop system does not use a sensor to "feedback" info; therefore open loop systems can drift away from the "ideal" operation with no way to correct itself.

I am oversimplifying this drastically; there are books and college courses devoted to the design of control systems. It is taught as part of Mechanical or Electronic Engineering coursework. There are companies who specialize in designing and manufacturing control systems for all kinds of applications.

Yes, as is the usual case Dogfriend has put it succinctly. At WOT the computer will ignore the feedback system in an EEC-IV or EEC-V system. The high heat input situation I have no clue on. Can I imagine the system falling back to open loop ? I can. Having some hot days here too so maybe I'll hook up my scanner and see what I get in that department.... interesting. I'm not prepared to say "bad PCM" yet....

btw... Open loop utilizes "lookup tables"... and often when things go bad, unplugging a sensor and forcing it to go open loop "lookup table" will tell you a LOT! Someone once told me a lookup table operation was at least as good as a carbureated system fuel wise.

Today, ambiant temp was 104, coolant was 198, Intake air temp was 155, sitting in traffic, readings would go back and fourth between open and closed loop, what where the 02 sensors doing?

Click to expand...

Here's a theory: Since heated air (155* intake) is much less dense than cooler air, maybe the PCM was getting outside it's parameters. This caused it to go into open loop and use the lookup table readings. Instead of the O2 sensors affecting the closed/open loop operation, it would be the IAT (intake air temperature) sensor.

Logic dictates that the sensors and PCM are calibrated to work together in a relatively narrow range of "normal" operation. When the intake air temp exceeded the norm, the PCM defaulted to the more "normal" lookup tables.

Most intresting thread EVER !!!!!!!!!!!!!!!!

Things that affect "loop" mainly: ECT, IAT, O2S. However, once the motor is warmed up, ECT and IAT don't really matter in terms of open vs. closed loop.

The programming of your chip can affect the "Loop". Maybe it has a glitch?????????????

Runnin'OnEmpty said:

Since heated air (155* intake) is much less dense than cooler air, maybe the PCM was getting outside it's parameters. . .

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155* isn't unusual for stop & go traffic, at least for my truck.

Thx all for the explanations

the scanguage manual says open loop when warming up and at full throttle, closed loop otherwise. so far i haven't really used the loop guage (i like tps, lod, mpg, & fia) but i might switch to see what it does during my commute.

Open Loop at WOT

My X goes into open loop at WOT according to my Scan Gauge, also open loop during initial warm-up only. Love my Scan Gauge!

http://www.scangauge.com