What controls cold idle speed?

[TouringMatt]

Cos mine sits ~800rpm regardless of if the engine is warm or cold!

Blue ECU temp sensor in the thermostat housing?

Cheers
Matt

[smithy318i]

Erm... The ICV !? (idle control valve)
well mine does anyway.

[cros]

Yes the ICV is responsible for the constant & steady idle speed. Under ECU control, the revs can be raised up a bit, kind of like a 2nd throttle.

Play with it by getting the integral error to wind up: when warm, slowly let the clutch out with the handbrake on in 1st. The ICV will open more & more to maintain 800rpm (and prevent stall). When you find the limit of what the ICV can do, press the clutch down fully and see how it rises to 1500rpm for a second or two while the control loop readjusts and shuts the ICV.

I think the temperature sensor is to enable overfuelling when cold. I imagine that the ICV is just a closed loop setup based on revs.

[TouringMatt]

Play with it by getting the integral error to wind up: when warm, slowly let the clutch out with the handbrake on in 1st. The ICV will open more & more to maintain 800rpm (and prevent stall). When you find the limit of what the ICV can do, press the clutch down fully and see how it rises to 1500rpm for a second or two while the control loop readjusts and shuts the ICV

Tried this, revs dipped to about 500rpm, clutch down and it just goes back to 800rpm without the 1500rpm rise.

Gonna re-test the IVC but im sure ive done this before!

Cheers
Matt

[Andy_magic]

Yeah the blue temp sensor is for cold starting, if disconnected the ECU assumed its cold.

Carb cleaner and toothbrush time for the ICV I reckon

[A1BMW325iSport]

anyone know how much a new ICV is?

[smithy318i]

Ԛ£85.19 or Ԛ£91.65 depending on engine (eurocarparts)
probably about Ԛ£10 from a scrappy.

[amit325i]

if its sitting around 800rpm its fine, it will be lumpy at idle all motronics are like by now! I wouldn't worry about it.

[TouringMatt]

Did some testing today!

IVC has correct resistance and the internals move nicely, but something is not right with the incoming voltage.

Center of the 3 pins reads 12v as it should, the two outer ones read 4v and 9v where according to the Haynes they should both be about 10v each

ECU snafu'd?

Matt

[Andy_magic]

My take on that problem was that the ECU drives both sides of the ICV solenoid to get a particular amount of opening.

I don't think a standard DVM will be able to read the ECu outputs to the ICV accurately as it's a pulsed signal, no steady voltage. You would probably need an osciloscope to see if all is well.

That, coupled with the fact that the haynes manual is full of errors and the fact I was getting 3v & 11v on my car that idles well hot or cold should point to the ECU being OK.

I also think the motronic ECU's are very reliable and are rarely the cause of a fault, they seem to either work or not.

You really need to isolate any possible mechanical problems such as air leaks, carbon/oil build up on the ICV, all the sensors giving correct readings etc.

My latest E30 ran like a bag of sh1te when I got it, there were 3 main problems with it:

Dodgy thermostat running the engine too cool

Dead blue temp sensor

Wiring from blue temp sensor was dead, so the ecu thought it was always cold.

[A1BMW325iSport]

i was lucky i got a virtually new icv off the touring at the breakers today

[TouringMatt]

all the sensors giving correct readings etc.

Yeah i think thats what i need to do, not entirely convinced i have zero air leaks as when giving it some beans im sure i could hear a high pitch sucking noise today, and i need to take some readings on the blue temp sensor.

Thanks for letting me know you got similar readings off your ECU, the Haynes is a bit hit and miss sometimes

I'll let you know how it goes

Cheers
Matt

[TouringMatt]

Ok, i took some temp sensor readings at cold today.

With the meter set on 20k i got 3.031 is that right?!?!

Checked the wiring from plug to the multiplug that sits kinda under the inlet manifold and its ok

Fired the car up and it still idles at 800rpm at cold and splutters if i try and rev it quickly

Thoughts

Matt

[Andy_magic]

You really need to check the wiring all the way back to the ecu. the easy way to do this is to pull the blue sender wire with the engine hot, if it starts running bad the wiring is ok, if it stays the same you got wiring problems.

Spluttering when going from idle to open throttle can be due to badly adjusted CO, go and bribe your MOT inspector and get it checked out.

This spluttering is common on E30's but it can be sorted.

The engine managment on E30's is pretty basic but for the engine to run 100% everything must be right, it's also very easy to hide one problem by messing about but you can introduce others.

I'm still writing a how to on the E30 6 pot engine managment, I've just got to take a few photos and do a few finishing touches and I'll publish it.