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#4558590 - 10/29/17 05:37 PM '03 Lesabre: runaway idle?
Alex_V Offline


Registered: 04/10/16
Posts: 1155
Loc: Campbellsville, KY
'03 Buick LeSabre, 3800 V6, 43K miles: after being warm for a while, but not necessarily running constinuously, the idle starts coming up and can go above 2,000 RPM in park if given long enough. Sometimes playing with throttle, or shutting down and restarting will resolve it to some degree. A new IAC valve fixed it for a few days but it came back. Sprayed the intake down with starting fluid with no results. The nature of the speed increase seems mechanical or electronic rather than from a vacuum
leak but I could be wrong. No CEL codes.

Any help?
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#4558605 - 10/29/17 05:54 PM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
JamesBond Offline


Registered: 12/13/06
Posts: 843
Loc: midwest
Unplug the icv when cold and also when hot and see how it acts then.

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#4558627 - 10/29/17 06:11 PM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
Merkava_4 Offline


Registered: 01/30/07
Posts: 17501
Loc: Clovis, CA
Mine is an '00 LeSabre. I would recommend you remove the throttle body for a thorough cleaning. I did that to mine but had several thousand more miles on it than yours at the time. The throttle body is warmed with engine coolant to prevent icing up, so it's a good idea to drain the radiator so that you don't have coolant pouring out of the upper intake manifold when you pull the throttle body. You'll need to replace the throttle body gasket. The throttle body is held on by 3 nuts plus 1 nut for the throttle body support bracket. To get to that one support bracket nut, it's easier to remove the crossover exhaust pipe first.

I cleaned mine with Walmart carburetor cleaner and a white scotch-brite pad. The white pads are less aggressive than the green pads. Use a cotton towel to absorb the black liquid carb cleaner before it evaporates. An acid brush and a tooth brush for the small areas like the IAC passage bore. One thing I should mention before I forget is you should remove the sensors attached to the throttle body before spraying cleaner in there. That requires a T20 tamperproof Torx driver.

And before somebody tells you that you don't need to remove the throttle body for cleaning, I'll tell you it's the only way to clean the back side of the throttle plate which will be black from PCV oil fume deposits.

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#4558643 - 10/29/17 06:37 PM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
64bawagon Offline


Registered: 01/10/17
Posts: 167
Loc: Michigan
I would bet its the Throttle position sensor or coolant temp sensor (in that order)
The bad TPS will not set a code.
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#4558645 - 10/29/17 06:37 PM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
Nick1994 Online   content


Registered: 02/19/13
Posts: 10118
Loc: Phoenix, AZ
My mom had an 03' Buick Rendezvous 3.4L that did this one time. We were in a Walmart parking lot and she showed me what it was doing and the idle was high enough that from a stop, not touching the gas pedal, it drove all over the parking lot at 25 mph. The next day it was fine and didn't do it for about 5 years afterwards.
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#4558728 - 10/29/17 08:02 PM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
WyrTwister Offline


Registered: 01/13/13
Posts: 1531
Loc: Texas
I think a sophisticated enough scanner will probably display real time data , including the desired idle speed and actual idle speed . If they both come pretty close to agreeing , then I would think something else is causing the computer to request a high idle speed . If they differ significantly & the desired idle speed is reasonable , then I would say the problem is in the fly by wire system operating the TB . Not sure which component .

I do not posses a scanner that will display that detailed information .

Best of luck to you , :-)
Wyr
God bless
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#4558772 - 10/29/17 08:46 PM Re: '03 Lesabre: runaway idle? [Re: 64bawagon]
eljefino Offline


Registered: 06/15/03
Posts: 32640
Loc: ME
Originally Posted By: 64bawagon
I would bet its the Throttle position sensor or coolant temp sensor (in that order)
The bad TPS will not set a code.


+2, this. The TPS recalibrates itself every time you turn the key on, so if it's flaky, it goes into "dashpot" mode with the ISC wide open. To itself, it's not reading wrong, so no code.

These cars have MAFs so you'll throw a code running it without the intake hose but if you can and cover the idle speed bypass hole at the throttle body (about the size of a dime) with your thumb it should stall or run a real low idle. If it still idles high you have a vacuum leak after the fact... I/M gasket, vacuum booster...

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#4558774 - 10/29/17 08:47 PM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
Alex_V Offline


Registered: 04/10/16
Posts: 1155
Loc: Campbellsville, KY
I suspected the other sensors but didn't want to shotgun the parts at it. I'll clean the TB - but it's not DBW so the only way for the computer to up the idle is via the IAC. I have a Snap-on "brick" scanner but I'm sure its capabilities exceed my skill - though I can watch the live data.

Does the IAC shut once the throttle opens? How exactly does coolant temp. input affect the idle speed the computer wants to see? I was watching the digital dash coolant temp. readout and it appeared to be acting normally, albeit reaching 221* after the idle started getting funny.
_________________________
I like Chevy and Valvoline.

'13 Cruze, 1.4T, 112K
'85 GMC C3500, 454, 130K
'82 Mercedes 300CD, OM617 turbo, 167K
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#4558778 - 10/29/17 08:49 PM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
eljefino Offline


Registered: 06/15/03
Posts: 32640
Loc: ME
Nope, the IAC rides wide open all the time except when idling for "Dashpot duty" to have enough air to blow HCs through if you slam the throttle shut. This is also how you barely touch the gas and the car's doing 30, you think, man, it's torquey. laugh

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#4558789 - 10/29/17 08:59 PM Re: '03 Lesabre: runaway idle? [Re: WyrTwister]
Merkava_4 Offline


Registered: 01/30/07
Posts: 17501
Loc: Clovis, CA
Originally Posted By: WyrTwister
I would say the problem is in the fly by wire system operating the TB. Not sure which component.


It's an old fashion cable drive throttle body. Nice try. grin2

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#4559068 - 10/30/17 08:42 AM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
mk378 Offline


Registered: 09/27/15
Posts: 1420
Loc: USA
In that case you should check if the cable or something else is sticking, when it is idling fast go under the hood and try to pull the throttle lever closed.

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#4559070 - 10/30/17 08:44 AM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
StevieC Offline


Registered: 08/21/08
Posts: 17116
Loc: Ontario, Canada
Check the brake booster. I have seen them act up both only cold, and only when the engine area is warm and then it pulls in unmetered air because it creates a vacuum leak.

Also check the adjustment of the TPS. Make sure it's reading 0 when the throttle-plate is closed. (Scan-tool required)


Edited by StevieC (10/30/17 08:47 AM)
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#4559078 - 10/30/17 08:56 AM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
Alex_V Offline


Registered: 04/10/16
Posts: 1155
Loc: Campbellsville, KY
No binding in cable. I'll see what readings I get with the scanner.

So... if the IAC opens when the throttle comes off idle, if the TPS is bad and it's saying the throttle is something other than closed, it would open the IAC and bring RPM's up?
_________________________
I like Chevy and Valvoline.

'13 Cruze, 1.4T, 112K
'85 GMC C3500, 454, 130K
'82 Mercedes 300CD, OM617 turbo, 167K
'67 Suburban, 350, 331K

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#4559105 - 10/30/17 09:26 AM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
SHOZ Offline


Registered: 06/28/03
Posts: 5750
Loc: Illinois
You should be able to see the IAC duty cycle with simple OBDII sensors. They are not usually 100% open but I only have experience with Fords.

With it idling fast rap the IAC with a screwdriver handle to see if it settles down.

Does it have a set screw for the throttle stop you can adjust?
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#4559122 - 10/30/17 09:47 AM Re: '03 Lesabre: runaway idle? [Re: Alex_V]
Alex_V Offline


Registered: 04/10/16
Posts: 1155
Loc: Campbellsville, KY
Not sure about the set screw but I'll look.

So I plugged it in and watched things. Also hooked into my BIL's '98 Lesabre (uses the same TPS and IAC, and runs well) to get another sample of sensor data to compare.
At idle, the '98 reads TPS: .53V and IAC: 80-100?
However, the '03 in question reads .45V TPS (flip-flops between .45 and .47) and much lower value (around 30) on the IAC.
Basically the same, small discrepancy in requested idle vs. actual speed on both cars. I haven't gotten the '03 to run away again yet so no sensor data while it's happening.

Could the flip-flop in TPS vs. IAC values between cars be an indicator?
_________________________
I like Chevy and Valvoline.

'13 Cruze, 1.4T, 112K
'85 GMC C3500, 454, 130K
'82 Mercedes 300CD, OM617 turbo, 167K
'67 Suburban, 350, 331K

Top
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