High idle - Sorted - THANK YOU for everyones help

[jimbom30cab]

Wondered if anyone has any advice here

Car is a facelift K reg 318i cab now fitted with M30B35 which is DME 1.3 - been running since June 13

Upon start up the idle is very slightly bumpy for the first 30 minute or so but nothing really to worry about with it sitting at 800rpm after a minute or so (i put this down to the shrick cam)

Soon as the engine warms up, when lifting off the gas the engine speed drops to 1600rpm and pauses there for about 10 seconds

Then slowly drops to about 1100rpm, pauses again a while and then returns back to 800 after about 30 seconds.


I have:

The engine is newly built with new vacuum hoses, everything dead clean etc

I have changed the ICV for the one on my 325i (same part number) which works perfect - the same problem is still there

I changed the ECU for a known good one - the problem is still there

I've also replaced the AFM, TB, Crank/speed sensor, blue temp sensor and ensured the TPS is set correctly so it clicks when touched. The trottle cable is slack on idle and the butterfly valve seats shuts perfectly.

Compression test done - everything perfect

No air ingress or vacuum leaks at all, uses no coolant or oil.

The connector to the ICV looks ok, i've taken back the insulation and it all looks fine.

I've also fitted a brand new OEM TPS

The only thing i can think of is potentially a wiring fault ?

Any help would be fab

thanks

[Speedtouch]

Which ECU and chip are you using?

[jimbom30cab]

M30B35 ecu ending 179 - one ECU has a BBR starchip, the other ECU is standard


I've just checked the voltage readings at the connector

The bently says to check the centre pin to ground for 12v - Pressent

Then check the outerpins to the centre pin, they should be 10v

So the results were: Pin 1 = 10v BUT Pin 3 = 4v

Oh dear, looks like pin 3 is only getting 4v

The wiring diagram looks like it has some bridge rectifier circuit supply these in the ECU so I swapped ECUs and re-tested - it gave exactly the same results

Will do some more reading

[Speedtouch]

Any later six-pot ECU (172/173/179/380/381) can be used, if you fit the M30B35 EPROM, so you may want to try one of you happen to have one lying around...

[jimbom30cab]

cool, i tried a spare M20B25 ECU and the voltages at the ICV connector were the same 4V at one of the outer connectors

I fitted a brand new blue coolant sensor and checked the engine was up to temperature first

Just for testing , i check for continuity from the outer ICV connections to pin 4 and 22 on the ECU - my multimeter beeped all happy

Wonder why i seem to be getting low voltage at one of the outer connectors

I was thinking it would be great if i could measure the voltage at ECU pins 4 and 22 but i guess i will have to take the ECU cover off to measure inside as the harness connector will be in the way otherwise

[Speedtouch]

Unless you prod through the cable insulation with the multimeter probe(s)...

[jimbom30cab]

right, i've just fitted the ECU with the back off (edit'd post got wrong pins first time)

Voltage out on pin 4 was 8.2v

Voltage out on pin 22 was 2.7v

I measured these exact same voltages at the ICV plug outer connectors also

Same voltages at the ICV measured again

Outerpin 3 to center = 4v
Outerpin 1 to center = 10v
Center pin to Ground = 12v

time to have a think.....

[Brianmoooore]

No such thing as "correct" voltages on the control pins. It's a feedback loop and the voltages will be whatever the loop determines they should be.
If the idle speed is high, it can only be excess air getting into the engine by other means than through the ICV (throttle stop, sticking cable, air leaks via crankcase, servo, etc?) or the ICV being held too far open. This can occasionally be an electrical fault, but is much more likely to be TPS or temp. sensor, etc. related.

[jimbom30cab]

i guess the ICV plug outer to inner terminal is then measuring the voltage difference

therefore i should be reading 2v out at pin 4 and 22 which when tested against the centre 12v would give a voltage difference of 10v

looks like pin 22 on the ECU therefore is correct

but Pin 4 on the ECU is giving 8v and should be reading 2v

Kinda running out of ideas now

[jimbom30cab]

Thanks Brian, i'm convinced it is the ICV being held open. It drives like having the choke pulled out on a manual choke car.

Honestly it's not air leaks, sticky cable, throttle stop etc etc, the whole lot is brand new and i've replaced almost everything and testing everthing that can be tested.

When the electrical connection from the ICV is diss-connected it idles fine (but a little low) and drives perfect (except occasionally stalls)

It's not the ICV its self as i fitted the one off my 325 which works perfect

Something is holding the ICV open more momentarily when it should be just idling

what should i test next Brian ?

[jimbom30cab]

I think i might have a lead on this

Took the car out for a drive this eve for dinner

Went about 30 miles and the lights started dimming and the radio turned off, guess what....the alternator wasn't charging.

Had no tools with me to Mr nice AA bloke got me home

So maybe my duff alternator might have been causing the problems, i'll change it for the spare in the morning and see if it helps

[Speedtouch]

Indeed, a low battery voltage would cause the ECU to begin malfunctioning, and a varying idle would be one of the first symptoms of this.

[jimbom30cab]

i've got everything crossed in hope Mr Speedtouch, am praying its the causea as i just cant think of anything else it could be

[jimbom30cab]

Right, i've changed the alternator and this is what still happens

I rev the engine, which revs fine, when the revs fall they stop at 1600rpm, it stays there for a while then drops to 800

So, i think the ECU is getting an incorrect signal from somewhere.

Its definately the ICV holding the idle high until it drops to 800

I've checked the wiring and used 3 different ECUs

So i think the ECU must be getting a duff input from one of the sensors

The TPS is new (OEM) and made no difference
The blue temp sender is new
The ICV is a known good one from my 325
There are no vacuum leaks
Plugs, injectors, all gaskets are all new
The TB and accelerator cable all work perfect
The dizzy cap and rotor arm are used but in my opinion very good condition

I guess that leaves the Crank sensor, pulse sensor and AFM left

Any ideas please anyone?

One thing i did think of is that the vibration damper is powder coated and i wondered if this would adversly effect the hall effect magnetic sensor (CPS)

[Speedtouch]

I doubt the powder coating would affect the CPS, unless it is very thick?

Doubt that the CPS is to blame, else you'd soon know about it, with difficult starting, etc.

Try cleaning out the AFM, and use a multimeter to check that its resistance varies linearly as you open the flap.

Another thing, you say you replaced the AFM, but is it the correct one for the engine?

[jimbom30cab]

ok mate, i'll give the AFM a bit of lovin

I'll probably just take the coating off the teeth of the vibration damper just so i've removed that out of the options.

They are both M30B35 AFMs

[jimbom30cab]

Ok, i tested the output voltage from the Alternator whilst on and it was only 12.6v so not charging. Did a load of tests with the multimeter and there was no continuity from the blue small cable on the alternator connection to the c101 pin1. There was however continuity to the diagnostic plug pin 12

So after that splice there must have been a break in the wire. I ran a new wire and it was even blue - check that out - and my dash now lights up like a xmas tree when i turn the ignition on and quickly all the lights go out. Also i'm now charging at 13.6v so happy days.

Off for a test drive now to see if by some mirical that may have cured the high idle, if not, the car is going straight on the for sale section.

[jimbom30cab]

no change to the idle and it's now really bugging me so

i have dissconnected the ICV and lifted the throttle stop a tiny amount (pre TPS click still) and she idles perfect at 800rpm during a good mixed test drive

see how it starts tomorrow morning nice and cold !!

[spook]

Jim if you need to try another ICV you can try mine from my 24

[jimbom30cab]

Thanks Chris, i've got a couple mate but if i get really stuck i'll pop over and you could help me kick the cr@p out of the turd if you like.

[spook]

I have a stuck shut door on my Transit to sort tomorrow, the tools of destruction will be on hand.

[jimbom30cab]

Right then

I've changed the AFM for a known good one, problem still there

when reducing in revs it hangs up at 1600 for a while and then (sometimes) missfires a beat and drops down

Then i dissconnect the ICV it idles and revs up and down perfect

I've tried 3 known good ECUs - no change

Anyone know where the ECU gets its signal from to determine the idle speed as i'm convinced it is a duff sensor causing the ECU to give out an incorrect output the ICV

Anyone help please?

(hope you are reading this Brian, really need some advice please)

[Speedtouch]

The wiring diagrams are here:

http://www.e30zone.net/e30zonewiki/imag ... ection.pdf

Does your car have air conditioning? Because there is an input from the A/C to the ECU that raises the idle speed when it's switched on.

[jimbom30cab]

no mate, no air con for me (unless i put the roof down)

I've got a new dizzy cap and rotor arm so just fitting them - quite a challenge on an m30 in an e30 - its all a bit tight

then i'm gong to change the CPS

But first i've got to put the kids to bed

[jimbom30cab]

Just thought i'd summerise what i have done so far in the hope someone maybe be able to help

Replaced the following with new or known good:

Checked for vacuum leaks (large intake hose is new as are the 2 small vacuum hoses and servo hose
Dizzy cap (new)
Rotor arm (new)
Crank Position Sensor - and checked gap (known good)
Alternator (known good)
DME relay (known good)
ECU (3 off)
TPS (new)
ICV (known good)
AFM (known good)
Blue coolant sensor (new)
Throttle body (known good)
Coil (looks ok says Bosch on it)


It still does exactly the same, when i lift off the gas, the revs hang up at 1600 momentarily then drop back to 1000-800rpm

If i dissconnect the ICV she idles and drives perfect.

[Speedtouch]

Why don't you just disconnect the ICV and be done with it, then?

Are you sure the throttle cable is adjusted correctly, and allowing the throttle butterfly to fully close against its stop when the pedal is released?

I had a Fiat twin cam with a similar issue, that turned out to be the throttle cable snagging against a return spring. So, I removed the original return spring and fitted one that ensured that the carb butterflies fully closed.

[jimbom30cab]

Are you sure the throttle cable is adjusted correctly, and allowing the throttle butterfly to fully close against its stop when the pedal is released

thanks mate, yer, i've pumped the throttle a few times whilst looking at the butterfly to check....howeverit might be snagging on the bonnet - it's all very very tight in there - i'll have another look

just changed the vacuum pipes again, actually used brand new fuel pipes so they should last a while hey!!

[Speedtouch]

Try running it with the bonnet open and see if it still does it, after blipping the throttle a few times...

[jimbom30cab]

just done it mate, very similar but not the same

in the garage with the bonnet up, it hesitates for a second at 1600 then drops back to 800

on the road it hangs up there ages at 1600 then drops really slow to 1200 then 800 eventually

so i'm thinking now, what could be happening differently in that engine bay when driving ???

answers on a postcard !!

[Speedtouch]

Sounds like it's definitely related to the throttle cable rubbing against the bonnet.

Try disconnecting the throttle cable and lubing the inner cable using some light oil, e.g, 3-in-1, and work it back and forth with the pedal to ensure smooth operation.

[jimbom30cab]

ok, will do, thanks

[jimbom30cab]

Well, i've finished for the night, put it all back together and took it out for a damn good thrash.

I have to say it flippin pulls brilliantly and the BBTB gives and instant shove back in the seat when booting it.

[Speedtouch]

Good stuff! One can forgive a car an idling issue when it really delivers the goods otherwise.

[jimbom30cab]

i'll definately change the throttle cable though as it's still the original m40 one and the radius is a little tight. New e34 535 cable is only £23 from the stealers.

If it is the throttle cable, i wonder why everything is fine with the ICV dissconnected !!

[Speedtouch]

Good thinking. Perhaps disconnecting the ICV just compensates for the excessively high idle created by a snagging throttle cable...