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Old 07-29-2015, 02:44 AM   #1
Rexii
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Low idle when coming to a stop - weird issue

First and foremost; no codes, checked everything and even put it on diagnostics at mechanic.

Ok so a while back I was complaining about a low idle issue where the idle drops to 550 when coming to a stop and shakes the car a little. It certainly doesn't feel normal.

Many things were checked in the car barring the PCV valve. Car was smoke tested and couldn't see any leaks.

Well today I was at the mechanic because after the DME reflash a few things were not programmed back into my car. I pretty much went in there to get the door locking feature when in motion activated.

He couldn't get in to modify anything. He tried 4 times which took a while. The entire time he had it on a charger to keep power to the battery flowing while the car was off and being used.

Anyway, he told me to come back next week and he'll take a look at it. He's a buddy, so not charging me.

Here's the weird part; got it to operating temp, wheeled it out of the workshop, my car is idling at 850. "What the hell" is what I was thinking. Turned the car off, turned it back on, idle at 850. Tried to use the windows or electrical in the car to see if it would drop the idle as it usually does - it did not do that any more.

Drove around a bit. Perfect smooth silky idle at 750-770. Drove around a bit more, maybe about 20 minutes worth. Idle 720-740. And so on until it finally went back to its old tricks idling at 620, and dropping 550-590 under electrical load or when coming to a stop. Heck, even turning the wheel would drop the idle.

Ok, so; would that not logically mean that either the alternator or the battery is the culprit?
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Old 07-29-2015, 06:39 AM   #2
dmarkle
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To me, this indicates a vacuum leak somewhere that's being exposed by a temperature change. Does your low idle happen when you apply the brake?

These cars often need new brake boosters as they start to leak after 10 years. You may also have a crack or leak in your valve cover or some other part which doesn't show itself until the car is up to temperature. You might have to wait until the car heats up, THEN do a smoke test to see what's going on...
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Old 07-29-2015, 08:06 AM   #3
Rexii
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Originally Posted by dmarkle View Post
To me, this indicates a vacuum leak somewhere that's being exposed by a temperature change. Does your low idle happen when you apply the brake?

These cars often need new brake boosters as they start to leak after 10 years. You may also have a crack or leak in your valve cover or some other part which doesn't show itself until the car is up to temperature. You might have to wait until the car heats up, THEN do a smoke test to see what's going on...
Done and done. Had it smoked tested a couple of weeks ago and a full days diagnosis only to find, nothing...

A vacuum leak would have been picked up straight away in my opinion. If not the alternator or the battery, then PCV valve, maybe?
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Old 07-29-2015, 10:08 AM   #4
cvx5832
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Whenever adaptations are cleared it will always idle higher. Then it settles back down like you described. As far as idle speed, I don't think I've seen it around here, or at least I don't see it discussed often (or at all).

How old is the fuel pump?
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Old 07-29-2015, 10:29 AM   #5
Jools
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Out of curiosity, do you get your idle drop when coming to a stop whenever the car slows down or only when pressing the brakes? Does your idle still drop if you drop to a lower gear and coast? Also, is there any change in idle behavior with the AC on or off?

Checking your alternator and battery is very simple, in any case, so you should be able to rule those out. You can use the diagnostic mode in the gauge cluster to see if the alternator's output seems suspect under the conditions where you're having issues. If anything seems amiss, run over to Autozone or wherever and have the alternator tested.

Last edited by Jools; 07-29-2015 at 10:31 AM.
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Old 07-29-2015, 11:18 AM   #6
Rexii
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Originally Posted by Jools View Post
Out of curiosity, do you get your idle drop when coming to a stop whenever the car slows down or only when pressing the brakes? Does your idle still drop if you drop to a lower gear and coast? Also, is there any change in idle behavior with the AC on or off?

Checking your alternator and battery is very simple, in any case, so you should be able to rule those out. You can use the diagnostic mode in the gauge cluster to see if the alternator's output seems suspect under the conditions where you're having issues. If anything seems amiss, run over to Autozone or wherever and have the alternator tested.
The drop from 650 to 550 happens when I am coming to a stop with the brakes. However, if I coast to a stop with very light brakes, the drop is not violent.

Also I should mention that under electrical load, such as rolling up the window or turning headlights on when idling also drops the revs.

The revs only react on idly when the AC has been activated (electrically) then the revs sit on 650 and idles well.
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Old 07-29-2015, 12:56 PM   #7
Jools
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I'm not an expert, but that definitely sounds like it could be a charging system issue. At the very least, having your alternator and battery checked out should be your first step. If there aren't any obvious problems there, then move on to more in depth diagnostics.

Last edited by Jools; 07-29-2015 at 12:59 PM.
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Old 07-29-2015, 06:30 PM   #8
RockCity333
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I had the same idle issue happen to me and it in fact was a vacuum leak. I checked all of the normal areas and it wasn't readily detected. I couldn't figure it out. It turned out there is a small vacuum hose that had gotten brittle with age and broken in half. It's hidden for most of the length of it and connects directly to the CCV. It was torn in a spot that was hidden from view. I guess many others have had the same issue with the hose breaking as causing a vacuum leak as well. However, I'm not sure if that is it with the problems related to an electrical load but it's a place to start and it doesn't cost anything to check.
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Old 07-31-2015, 05:38 PM   #9
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Quote:
Originally Posted by RockCity333 View Post
I had the same idle issue happen to me and it in fact was a vacuum leak. I checked all of the normal areas and it wasn't readily detected. I couldn't figure it out. It turned out there is a small vacuum hose that had gotten brittle with age and broken in half. It's hidden for most of the length of it and connects directly to the CCV. It was torn in a spot that was hidden from view. I guess many others have had the same issue with the hose breaking as causing a vacuum leak as well. However, I'm not sure if that is it with the problems related to an electrical load but it's a place to start and it doesn't cost anything to check.
Can you at least elaborate on the location and if you have any pics? I have the same exact symptoms, when I come to a stop after accelerating and applying brakes. The car will idle real low, almost stalling. It does this up down up down rpm thing. I've always suspected a vaccuum leak.
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Old 07-31-2015, 08:54 PM   #10
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Brake booster leaks CANNOT typically be picked up with a smoke test for a few reasons.

1. The input check valve on the booster will block smoke from entering the booster.

2. Assuming the input check valve lets smoke into the booster, few people even think about looking under the dash for smoke.

Read this thread - http://forum.e46fanatics.com/showthread.php?t=1057387
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Solve your misfires, lean codes, rough idle - http://forum.e46fanatics.com/showthread.php?t=897616

Fuel pump failures - http://forum.e46fanatics.com/showthread.php?t=929501

Temp Info - http://forum.e46fanatics.com/showthread.php?t=964491

Hidden OBC Menu - Check Voltage, Temp, Fuel Level - http://forum.e46fanatics.com/showthread.php?t=239619

E46/E39 GM5 Door Lock Info - www.bmwgm5.com

Lower hose temp switch O-ring - BMW #13621743299
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Old 07-31-2015, 10:33 PM   #11
325CiBrett
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Pretty sure he doesn't mean to smoke test the brake booster. He's referring to the crank case and other areas of the engine being warm before doing the smoke test. At least that's how I read it.
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