idle speed and A/C - The Unofficial BMW M5 Messageboard (m5board.com)

mottati · 31st August 2007, 23:38

I have and intermittent idle speed issue on my recently acquired 91 3.6. Seems to only happen when hot, and today i discovered only with the a/c on. Occasionally, the idle will hang at about 2k rpm. I assumed it was a throttle cable issue, they are looking a little ragged, and i have a new one waiting to be installed, but in the mean time i have safety wired them to the bracket, similar to how the proper factory clip would hold them (found that via a search).

My question is this; is there some type of solenois that raised the idle speed when the car is hot with A/C on? If i'm idling at 2k at a stop, for instance, and shut off the A/C, the idle speed drops back down to it's normal +/-1k rpm.

Thanks Guys, need to post some fresh pics soon, car is looking so much better after a few grand of TLC (and many late night hours in the garage).

Sebring NL · 1st September 2007, 02:23

2K Rpm idle is WAY too high.

When my AC was still working, I used to have a small raise in idle RPM, but barely noticeable on the rev indicator...

mottati · 1st September 2007, 03:01

not consistantly though. I was going to lunch today, it's hot, about 100F, drove the car about 15 miles over lunch, as i was getting back to the office, it started happening. Most of the time though, idle speed is normal.

Then if i shut the ac off, dropped back down to normal, turn ac back on, idle ok, but then rev up to about 2500-3k, with ac on, and revs 'hang' at 2k again. Must be something that bumps the idle speed up with the ac that is malfunctioning. Will investigate when i pull the plenum to clean the tb's and replace vacuum lines.
(your post motivated me)

raymondw · 1st September 2007, 10:17

Hey Mike,

The T(hrottle) P(osition) S(switch) on the S38B36 tells the DME whether it is operating at:

1: Idle
2: Normal operation
3: Full Throttle

At Full Throtlle (3), the DME ignores the signals from the O2-and MAF sensors and thus enrichenes the fuel-air mixture in order to prevent the engine to operate on a lean mixture.

At Normal operation (2), the DME converts the signals comming from the O2-and MAF sensors and operates in the so calles stoichiometric mode, i.e control Carbon Monoxide emmisions.

At Idle (1), the butterfly valves should be closed. This allows the I(dle) C(ontrol) V(alve) to control the idle speed, ergo keep Carbon Monoxide emmisions within the allowed limits.

I suspect that something is wrong with either the basic adjustment of your throttle bodies or the attached vacuum hoses that after 10++ years have deteriorated. Also possible is a malfunctioning TPS that is often overloked by mechanics who diagnose this wronly and over compensate this by adjusting the throttle bowden cable, which is an incorrect method since this results in the Carbon Monoxide emmision level to sky rocket by slighly opening the butterfly valves and thus overrule the ICV.

1: Check the condition of the vacuum lines and hoses.
2: Check and clean the ICV valve (located underneath the butterfly valves, requires the removal of the intake plenum (15min).
3: Calibrate the basic adjustment of the Throttle bodies, part of which is the cotrrect setting of the TPS. Replace the TPS when necessary.

mottati · 1st September 2007, 19:24

thanks raymond, i planned on removing the plenum to check and replace vacuum lines, and clean out the tb's. When i complained about the idle issue to my friend/parts source at my local dealer, first thing he asked was if the tb's were clean and properly sync'd.
Car passed it's emissions test (required for a registration change) a couple months ago when i bought the car, hc and co were extremely low, co2 was closer to the allowable limits.
thanks again. Plenty to keep me busy on this car (which is good therapy).

jetavdk · 5th September 2007, 06:20

Finally, someone with the same problem as me. I have replaced all the vacuum hoses after removing the plenum as well as the ICV. With all the work involved just throwthat idle valve away instead of trying to clean it, you'll see what I mean when you have the left side of the motor apart. I guess now all I have to do is sync the butterflies and hopefully this problem will finally go away. While you have the plenum off clean all the gunk off of the butterflies. I did mine by bending the plastic tube that comes off of a carb cleaner can at a 90 degree angle enabling me to clean on the other side of the butterfly flaps. On a side note I would order all the hoses before starting this work, I had to wait up to 6 weeks for some to arrive from Germany especially the ones on the side of the block. I will re-post when I sync the butterflies and see if that finally puts this problem to bed.

mottati · 5th September 2007, 06:43

glad to hear that i'm not alone. I guess i should study the realoem pgs to get the required hoses etc., and maybe an icv (or maybe not, since it didn't help you!)

jetavdk · 5th September 2007, 07:43

I would replace the valve, after all it has probably been in there a long time like mine was. I know you will hate to re-visit the area when the valve possibly fails later on. We are dealing with cars that are coming up on their 20 birthday soon.

While we are on the subject does anyone out there know a good source to purchase the gauges and tools required to sync the throttle bodies/butterflies?

jetavdk · 22nd September 2007, 08:01

Well, the throttle switch arrived and it has now been in for about a week and a half. The problem has seemed to go away. Kudos to Mark D'Sylva and Kurt Lehman for recommending I try this. Hopefully this problem is gone for good.

mottati · 22nd September 2007, 19:27

Quote:

Originally Posted by jetavdk

Well, the throttle switch arrived and it has now been in for about a week and a half. The problem has seemed to go away. Kudos to Mark D'Sylva and Kurt Lehman for recommending I try this. Hopefully this problem is gone for good.

throttle switch, ok. If you have a P/N handy i'd appreciate it, but i can look it up if not. Will add that to the list when the plenum comes off for some maintanance (hoses, tb cleaning and sync etc).
Thanks for the follow up!