Help Support Bigcoupe.com

Help with high idle

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

Moderators: ron, bfons, GRNSHRK

Post Reply
jpr210
Posts: 69
Joined: Tue Jun 25, 2013 2:27 am
Location: Miami, Fla.

Help with high idle

Post by jpr210 » Sun Jan 21, 2018 5:54 pm

Have high idle condition in a M88, Here is what I've done so far:

Engine is newly rebuilt
New o rings in metering needles
New o rings in bypass screw
Fully flowed injectors with new o rings
Adjusted bypass, throttle blades, bypass screw per manual
Slide valve works well. fully closed at working temp (an still high idle). caped bypass hose and has no effect.
new plenum/intake horn couplers with new clamps
New TPS and it is adjusted properly (click at idle position)and corroborated by terminal ohm readings.
idle stop screws have throttle blades closed on all cylinders.
throttle mechanism push rods are adjusted as per manual.
all gaskets are new, all hoses are new.
Bypass screw makes very little effect, even when all the way in. At factory spec, rpms are within 50 of full closed reading.

Ideas are welcome

TIA

Jpr

User avatar
hornhospital
Bigcoupe supporter
Posts: 2297
Joined: Sun Jul 11, 2010 6:46 pm
Location: Silverhill, AL
Contact:

Re: Help with high idle

Post by hornhospital » Sun Jan 21, 2018 7:09 pm

Like Brucey has said many times, a high idle when all else is adjusted to specs points to only one thing: vacuum leak(s). Adding fuel won't make it rev, but adding air will, every time.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'87 325is "Odette"; '93 325is "Elvira"; '95 M3 "Ashlyn"

User avatar
wattsmonkey
Bigcoupe supporter
Posts: 1408
Joined: Mon Jul 16, 2007 2:58 pm
Location: Cheltenham, U.K.

Re: Help with high idle

Post by wattsmonkey » Sun Jan 21, 2018 7:43 pm

If bypass screw isn't doing anything then you've proved air is getting in elsewhere, otherwise you'd stall.
"Most of it necessary; all of it enjoyable[!]" LJKS
'86 M635CSi #367 Bring the Noise.
'84 635CSi, dogleg fun

jpr210
Posts: 69
Joined: Tue Jun 25, 2013 2:27 am
Location: Miami, Fla.

Re: Help with high idle

Post by jpr210 » Sun Jan 21, 2018 11:32 pm

Agreed, the question is however, give all that has been done, where can that large of a vacuum leak be?

User avatar
hornhospital
Bigcoupe supporter
Posts: 2297
Joined: Sun Jul 11, 2010 6:46 pm
Location: Silverhill, AL
Contact:

Re: Help with high idle

Post by hornhospital » Mon Jan 22, 2018 2:57 am

Smoke/vapor test it. That'll find the leak.

One of many ways to do that:

http://www.popularmechanics.com/cars/ho ... cuum-leak/
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'87 325is "Odette"; '93 325is "Elvira"; '95 M3 "Ashlyn"

jpr210
Posts: 69
Joined: Tue Jun 25, 2013 2:27 am
Location: Miami, Fla.

Re: Help with high idle

Post by jpr210 » Mon Jan 29, 2018 9:01 pm

Took the car for smoke testing. They found no leaks. However, I can still place a plastic bag over the airbox opening (I have removed the snorkel) and the engine still runs!!. Not smoothly as without it but the idle does not go down. Again, Throttle plates are all closed (the stop screws are all withdrawn) bypass screw is closed, air slide valve is open when cold and shut when hot (verified), o rings on needles and bypass screw are new. gaskets to the head are new and verified not leaking. All hoses are new with new clamps. All verified not leaking by smoke test. ???

TIA

Jpr

ron
Senior Member
Posts: 4353
Joined: Sat Jul 10, 2004 11:35 am
Location: South Wales U.K.

Re: Help with high idle

Post by ron » Tue Jan 30, 2018 7:55 am

Have you altered the timing?
They are ALWAYS rustier than you thought!!!!!!
'88 High line.
'85 M #228
'85 M #207
'80 735i

jpr210
Posts: 69
Joined: Tue Jun 25, 2013 2:27 am
Location: Miami, Fla.

Re: Help with high idle

Post by jpr210 » Tue Jan 30, 2018 6:49 pm

No, not yet. Stock timing. could coolant temp sensor affect the idle speed?

jpr210
Posts: 69
Joined: Tue Jun 25, 2013 2:27 am
Location: Miami, Fla.

Re: Help with high idle

Post by jpr210 » Tue Jan 30, 2018 11:33 pm

Replaced the CTS and no effect.....What to try next?

User avatar
hornhospital
Bigcoupe supporter
Posts: 2297
Joined: Sun Jul 11, 2010 6:46 pm
Location: Silverhill, AL
Contact:

Re: Help with high idle

Post by hornhospital » Wed Jan 31, 2018 1:56 am

Regardless of the shop finding no vacuum leaks, there HAS to be one, and a big one at that, since you can cover the air intake and it still runs. There's no other way.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'87 325is "Odette"; '93 325is "Elvira"; '95 M3 "Ashlyn"

Pod
Bigcoupe supporter
Posts: 3892
Joined: Tue Mar 16, 2010 8:32 pm
Location: usually in the barn, Kent, England.

Re: Help with high idle

Post by Pod » Sat Feb 03, 2018 9:03 am

If I were you, I’d take it to another shop!

User avatar
TN_M6
Bigcoupe supporter
Posts: 280
Joined: Mon Apr 21, 2008 6:01 pm
Location: Middle TN USA

Re: Help with high idle

Post by TN_M6 » Mon Feb 05, 2018 7:46 pm

A few things...

First I must admit I'm not as familiar with the earlier cars and idle slide valve control. is it just a slide open valve for higher idle when cold?

Does the M88 use the same procedure to tie Idle, WOT & ground (all three pins) on the TPS together to put the computer in open loop for idle balance and such like my later car?

Have you tried to perform any sort of idle balancing yet?

On later cars I find setting the butterflies on the ITB's properly is difficult at best with measurement procedure... And easily left too far open for the idle to drop down where it should. In this case the bypass screws are no longer effective to adjust idle. I would probably suggest closing the ITB's slightly more to let the bypass screws be more useful for their job. Just a guess even though you say they T/B are closed are they really?

The air leak sounds like it could very well be within the crankcase vent system. Any seal or gasket leaks, even the dipstick tube, can let unmetered air into the system. Try removing the hose from the valve cover to the plenum and plug it to see if it changes idle speed. With 6 intakes essentially there are all the O-rings and seals on the air horns than could be cracked too.

Also, I know on my later M6 you can unplug the AFM and it will still idle. There is a minimum fuel command and if the early car is the same that is probably where you are when you plugged the intake. So some unmetered air with minimum fuel command and it will still run, maybe on air leaks anywhere downstream of the AFM. You might try unplugging the AFM and see if it still idles. If so you are not even registering flow on the AFM yet.

User avatar
Boggie
Bigcoupe supporter
Posts: 160
Joined: Wed Jan 31, 2018 3:57 pm
Location: Buckinghamshire UK

Re: Help with high idle

Post by Boggie » Mon Feb 05, 2018 8:06 pm

hornhospital wrote:
Wed Jan 31, 2018 1:56 am
Regardless of the shop finding no vacuum leaks, there HAS to be one, and a big one at that, since you can cover the air intake and it still runs. There's no other way.
Seconded!
And IF the leak is the source of luminous then the leak has to be downstream of the MAF. I always used carb or brake cleaner to find leaks. A spray can with straw nozzle I can shoot accurately. I also have used mechanics stethoscope Like This to listen closely around pipes, points etc. However, it doesn't take much of a leak to keep an engine idling; think about how small the opening is on your closed throttle bodies. Dipstick seal and other crankcase breathers also have a significant effect on cars that breath through the plenum.

Good luck, they can be a bugger to find...
1985 M635CSI
Classic Car Collector & Restorer.

User avatar
Brucey
6 Series Guru
Posts: 9926
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: Help with high idle

Post by Brucey » Sat Feb 10, 2018 10:08 pm

I agree with the above post. I would suggest that you also try clamping the rubber hose between the slide valve and the intake, just to verify that the slide valve isn't leaking air.

Also, think of

- the fact that the airbox may not be airtight; the air may be getting in where you don't expect it
- is the bypass screw actually fitted to the AFM? (if it is missing it will create an air leak that bypasses the airbox)
- are the seals on the TB butterfly shafts in good order?
- are the TBs all able to close fully? (eg if the idle stop screw is further adjusted).
- is the vacuum hose connected to the FPR OK?
- is the vacuum hose connected to the cabin air temperature sensor OK?

As others have pointed out the only way you can get a high idle is if there is too much air getting in somewhere. It is just a question of finding out where that is.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

User avatar
raykoke
Bigcoupe supporter
Posts: 121
Joined: Sun Mar 02, 2014 11:18 am
Location: Savannah GA USA

Re: Help with high idle

Post by raykoke » Sun Feb 11, 2018 12:44 am

jpr, are you referring to the bypass screw on the throttle body assembly, or the one on the AFM bottom?

GL,

Ray
'85 ///M635

jpr210
Posts: 69
Joined: Tue Jun 25, 2013 2:27 am
Location: Miami, Fla.

Re: Help with high idle

Post by jpr210 » Mon Feb 12, 2018 6:50 pm

I have a Miller MAF kit so I dont have the AFM arrangement anymore. The main bypass screw (the one you adjust 8 turns from fully closed) is the one that, at the moment, makes no change on the idle (no matter the position). I have disconnected the hose from the intake plenum and the idle control slide valve without any success (I capped both ends with plastic to make sure it would not leak. The butterfly idle resting position set screws are backed off to the point that they are not pushing the butterflies open at all. I smoke tested the entire intake track and no leaks were found, no cracked hoses, no loose clams, no open ports and no leaking seals (they are all new anyways). I have no oxygen sensor in my car (grey market m635) Installed a new TPS (adjusted correctly, can feel it click, revs fall and then immediately come back up again to the current idle of 1150 rpm) and have a new CTS.

Jpr

User avatar
raykoke
Bigcoupe supporter
Posts: 121
Joined: Sun Mar 02, 2014 11:18 am
Location: Savannah GA USA

Re: Help with high idle

Post by raykoke » Mon Feb 12, 2018 7:00 pm

Well, then.

Good info.

Call Miller.

Cheers,
'85 ///M635

User avatar
Brucey
6 Series Guru
Posts: 9926
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: Help with high idle

Post by Brucey » Tue Feb 13, 2018 1:21 am

jpr210 wrote:
Mon Feb 12, 2018 6:50 pm
I have a Miller MAF kit so I dont have the AFM arrangement anymore. The main bypass screw (the one you adjust 8 turns from fully closed) is the one that, at the moment, makes no change on the idle (no matter the position). I have disconnected the hose from the intake plenum and the idle control slide valve without any success (I capped both ends with plastic to make sure it would not leak. The butterfly idle resting position set screws are backed off to the point that they are not pushing the butterflies open at all. I smoke tested the entire intake track and no leaks were found, no cracked hoses, no loose clams, no open ports and no leaking seals (they are all new anyways). I have no oxygen sensor in my car (grey market m635) Installed a new TPS (adjusted correctly, can feel it click, revs fall and then immediately come back up again to the current idle of 1150 rpm) and have a new CTS.

Jpr
IMHO, you definitely have a massive air leak on the intake, you just have not found it yet. This problem is nothing to do with the Miller kit; all that does is measure the amount air going into the engine, it is in no way responsible for regulating it.

if there is a massive air leak then the idle speed may not increase above a certain point because if it does, it activates the overrun fuel cutoff.

Some things to check

- check the intake for leaks using a fuel of some kind
- you don't mention the intake vacuum pipes that are connected to the FPR and the cabin temperature sensor. Locate these and make sure that they are not leaking abnormally.
- daft question; have you got O ring seals fitted to the injector bodies, and are they fitted correctly?
- have you physically checked the throttle butterflies (using the MK 1 eyeball with the plenum off) to ensure that they are closing properly? It doesn't matter much what the linkages on the outside appear to do, you need to verify that the butterflies are mechanically synchronised and are able to move to fully closed.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Post Reply

Who is online

Users browsing this forum: scarland and 12 guests