reply to post by Dr Expired
Nice one I'm glad I was able to help you make sense of it somewhat.
Yeah the ECU would have required some revving of the engine to store enough actual output from the various sensors to calculate the right idle
settings to keep the engine ticking over.
See the management system operates in two distinct 'modes'.Open loop and Closed loop.
Now under normal driving conditions when the engines at operating temperature it being managed in the Closed loop which means the ECU is basically
determing air/fuel ratio and ignition advance (fueling and timing) from the feedback it's getting from the oxygen sensor/lambda sensor and engine
load (from the MAF or MAP sensor).
By measuring the oxygen in the exhaust system the ECU can sense how lean or rich the engines running and modify the fuel/air mix accordingly so the
engines running under a kind of 'feedback' or closed loop condition
That's ok under normal steady or mild part throttle conditions but no good when the engines cold or under full throttle or heavy load conditions when
the the engine requires a much richer fuel/air mix.
So until your engines up to operating temp or when you start accelerating harder it goes into Open loop and ignores the oxygen sensor and uses the raw
output of the coolant temp,throttle pos sensor etc then calculates from the factory data maps that have been modified over time by the 'learnt' data
in it's RAM.
That's why when you disconnect the battery,wipe the RAM data it stalls and idles rough because its cold and in Open Loop and relies on that learnt
When you rev the engine and it gets enough real time data from the sensors it can start filling in the gaps left by the data it had lost.
Also when you turn on the aircon or use the power steering that initial mechanical and electrical power draw will soak up engine power and the ECU
will increase the revs if it's idleing incase the drop in engine speed makes the engine falter.
Try switching it on or turning the steering wheel when the engines ticking over.The engine speed should increase by 150 or 250 revs then drop back to
normal.Thats the ECU speeding the engine so it doesnt miss or falter from the power draw.
You're absolutely right,many a garage or mechanic may charge an unsuspecting victim a whack for next to nothing.
When I worked at a Ford dealers I seen mechanics remove components,clean them with a power washer,refit them and charge for a new part + labour !!!