Hunting on start-up.

Post here for mechanical and engine topics such as fuel issues, transmission problems, rough idle, exhaust, electrical issues, etc

Moderators: GRNSHRK, ron, bfons

Post Reply
ron
Platinum Member
Platinum Member
Posts: 4569
Joined: Sat Jul 10, 2004 11:35 am
Location: South Wales U.K.

Hunting on start-up.

Post by ron »

This is an M30 (M90) engine in my E23 735i.

On cold start the engine hunts from 1100rpm down to stalling. It also has a vacuum gauge fitted which fluctuates wildly until the engine warms up. Once warm, everything settles to normal with the idle around 800 rpm.

Any ideas on cause?
They are ALWAYS rustier than you thought!!!!!!
'85 M #228
'87 M #367
'88 High line.
'10 X5
‘84 Alfasud 1.5 ti
User avatar
hornhospital
Platinum Member
Platinum Member
Posts: 2929
Joined: Sun Jul 11, 2010 6:46 pm
Location: Silverhill, AL
Contact:

Re: Hunting on start-up.

Post by hornhospital »

Idle control valve?
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann"; '95 M3 "Ashlyn"
stepher
Posts: 22
Joined: Mon Feb 06, 2017 8:16 pm
Location: Port Moody, BC, Canada

Re: Hunting on start-up.

Post by stepher »

Check your mixture, sounds a little rich. Make sure that thee are no vacuum leaks from around the valve cover, oil filler cap or dipstick.

Thanks, Rick
User avatar
Brucey
6 Series Guru
6 Series Guru
Posts: 10077
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: Hunting on start-up.

Post by Brucey »

there is only one reason why the engine runs at 1100rpm when it should be idling; there is too much air getting into the engine. This in turn means that

1) there is an air leak (in which case the idle is likely to be too high all the time)

or

2) that the idle control valve is faulty (sticky)

or

3) the throttle butterfly is wrongly set so that there is too much or too little air getting into the engine through the TB.

The last of these has a subtle effect in that the ICV is too far open or too far closed when it is meant to be in the middle of the stroke; this means it responds poorly/wrongly to the commands it is given and the idle speed isn't properly stable.

2) can cause troubles too, in that the ICV may driven fully open when the engine is cold only, where it might be sticky; once the engine is warmed up the ICV doesn't have to open as far and it is less likely to stick anyway.

The 'hunting' is easily explained; with the throttle closed and the idle contact engaged, so is the overrun fuel cutoff protocol; anytime the engine is turning more than ~1200 rpm and the throttle is closed, the fuel is cut. It takes about half a second for the fuel to cut back in again, which is almost long enough for the engine to stall. This can cause the engine speed to hunt in cycles lasting about two seconds, between some low speed and about 1200 rpm. In the meantime the ICV may be driven fully open (if the engine speed drops even briefly below the target idle speed) and the cycle repeats.

The cure is usually to reset the TB idle position as per the factory settings (wear causes it to close more and more over time), readjust the TPS if necessary, and to check the ICV isn't sticky.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
sharkfan
Platinum Member
Platinum Member
Posts: 2235
Joined: Tue Sep 28, 2004 3:14 pm
Location: Guildford, U.K.

Re: Hunting on start-up.

Post by sharkfan »

Is this sorted now Ron?
2001 Alpina B10 V8 Touring (1 of 12 rhd)
1997 Alpina B12 5.7 L (1 of 2 rhd)
1995 Alpina B10 4.6 Touring (1 of 1 rhd)
1985 BMW M635CSi (1 of 524 rhd)
1982 BMW 635CSiA (1 of 100's left from the 1000's made and still valiantly fighting against a rusty grave)
User avatar
Brucey
6 Series Guru
6 Series Guru
Posts: 10077
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: Hunting on start-up.

Post by Brucey »

thinking about it, also on an L-jet car there is an air bypass valve that increases idle speed when the water is stone cold. IIRC this is also adjustable (for maximum flow rate) via a screw adjuster somewhere. If this is set wide open you will get the same behaviour, i.e. hunting when stone cold. Upon reflection I'd look into this first, in fact.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
GRNSHRK
Platinum Member
Platinum Member
Posts: 3710
Joined: Thu Apr 08, 2004 3:24 pm
Location: Gilroy (SF Bay Area) CA

Re: Hunting on start-up.

Post by GRNSHRK »

Yes, AAV, auxiliary air valve is the culprit, likely to be anyway :-k

The First Fives website describes the adjustment of this valve, even though the "adjustment" is torque sealed from the factory, don't believe it [-X

You will need to perform the adjustment VERY quickly, because as Brucey duly noted, once the coolant starts to warm up, all bets are off as the valve will no longer provide any function :-({|=

But I should throw out one other tidbit of info, something I was made aware of during PSF this year. One of our "members" who installed an M90 in his '81 US 633 has this issue as well, and claims that he cannot overcome the idle hunting, despite being a master mechanic :cry:

My '80 does not have any of these issues, idles about 1100-1200 during warm-up and settles down nicely within a minute or 2 [-o<

Here's a few screen captures from First Fives, great site for you E12/L-jet guys =D>

:mrgreen:
Attachments
AAV-1.jpg
AAV-1.jpg (28.45 KiB) Viewed 4054 times
AAV-2.jpg
AAV-2.jpg (22.25 KiB) Viewed 4054 times
:mrgreen:
Bobbo
1980 633 CSi Cypress Green/Pearl Beige
2017 X5 M Sport Xdrive 35i Carbon Black/Ivory White
2005 330 Ci ZHP Cabrio Imola Red/Bone/Black
Image
Post Reply