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Old 12-13-2007, 11:58 AM   #1
justin1982z28
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High idle: How to check TPS and IAC?

I have a 99 Grand Am SE with 150000 miles. 3400 auto with no mods. I recently had a the SES light up with a sticking EGR. I cleaned it and the light has not returned in two days. See this thread: Cleaning EGR Valve after Service Engine Soon Light

Today after the usual one hour drive on the interstate it started idling around 1500-2000rpm in gear 2500 in neutral. I had to hold the brake to keep it from accelerating. This is a car that has always idled at 650, even when started cold. I am at work right now so I cannot check for vacuum leaks, but I plan to tonight. I just started the car and it now idles at 1000rpm, slightly higher than usual. Even after it warmed up completely, it did not idle higher than 1000rpm. I tried moving the throttle manually, it was not sticking. I also unplugged the wires to the TPS and the IAC (seperately) and nothing changed with either unplugged.

I have read that this problem could be:
vacuum leak
TPS (seems most likely from searching)
IAC
MAF
temp sensor
O2 sensor
EGR

How can I test the TPS and IAC to rule them out? It seems like it would not be a coincidence that I just worked on the EGR and something else is causing the high idle, but I would like to rule out these other possibilities. I do not have access to any scanning tools other than Autozone. Thanks for your input.
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Old 12-13-2007, 12:03 PM   #2
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Quote:
Originally Posted by justin1982z28 View Post
I have a 99 Grand Am SE with 150000 miles. 3400 auto with no mods. I recently had a the SES light up with a sticking EGR. I cleaned it and the light has not returned in two days. See this thread: Cleaning EGR Valve after Service Engine Soon Light

Today after the usual one hour drive on the interstate it started idling around 1500-2000rpm in gear 2500 in neutral. I had to hold the brake to keep it from accelerating. This is a car that has always idled at 650, even when started cold. I am at work right now so I cannot check for vacuum leaks, but I plan to tonight. I just started the car and it now idles at 1000rpm, slightly higher than usual. Even after it warmed up completely, it did not idle higher than 1000rpm. I tried moving the throttle manually, it was not sticking. I also unplugged the wires to the TPS and the IAC (seperately) and nothing changed with either unplugged.

I have read that this problem could be:
vacuum leak
TPS (seems most likely from searching)
IAC
MAF
temp sensor
O2 sensor
EGR

How can I test the TPS and IAC to rule them out? It seems like it would not be a coincidence that I just worked on the EGR and something else is causing the high idle, but I would like to rule out these other possibilities. I do not have access to any scanning tools other than Autozone. Thanks for your input.
I cant really call out how to check the IAC but i did test my TPS using the method below... If the throttle is high... Open hood.. Locate TPS, unplug it... If the idle drops immediately after, its most likely the culprit... With the IAC you could clean it out if the above fix doesnt result in a lower idle... I unscrewed the IAC, grabbed 15-20 Q-tips and some carb cleaner and went to town inside the TB and on the IAC itself... runs like a champ now..
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Old 12-13-2007, 09:56 PM   #3
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Yeah I just did that last night on my car as a matter of fact. My car will often idle extremely high when I start it. Last night, I started it up, then it slowly kept climbing up to 2000 rpm on its own. I went under my hood and unplugged my tps for a few seconds. Once I did, it dropped down to "close to normal idle". The thing is, I just had that tps replaced a few months ago, and it still does it with the ses reading code 22 for the tps. I wonder if I got a bunk tps or what?
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Old 12-14-2007, 07:41 AM   #4
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Update: When I drove the car home from work last night and back to work this morning it drove without the problem recurring at all.

I cleaned the IAC last night using carb cleaner, it didn't seem like anything was wrong with it. When the high idle reappears, I will try unplugging the TPS again to see if that is the problem. Until then, thanks for your help.

jnugg69: It does sound like the replacement TPS is also bad. It seems unlikely, but I know that I have bought new alternators that have turned out to be not working right out of the box, so I guess a new TPS could also be bad. Maybe if you post the problem, someone will be able to help diagnose it. If you could find someone else with the same car and borrow their working TPS as a test, then you could find out if yours is bad without buying a new one.
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Old 12-14-2007, 12:28 PM   #5
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i got the idle speed high code a couple of times before. but ive long since changed and fixed a few things havent seen it since.
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Old 12-14-2007, 04:21 PM   #6
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Yeah tru-dat. My mechanic said he'd hook me up with a new one for no charge if it is the tps itself, but he's hellbent on the idea of it being something else. Now all I have to do is to convince him. He's worried if he replaces it, then come to find out it wasn't broken, he'd have to eat the money for it. I think he just doesn't want to the work again without charging more money. People are just so great sometimes.
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Old 01-26-2012, 12:16 PM   #7
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Man, I'm seeing more acronyms than I did I 23 years in the Army.
It is difficult enough for ME to know how to check the IAC or the TPS, now I have to find out WHAT and where they are.
I tried to find a manual at a decent price but everywhere I look they just laugh.
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Old 01-26-2012, 01:10 PM   #8
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Quote:
Originally Posted by stevenlemon View Post
Man, I'm seeing more acronyms than I did I 23 years in the Army.
It is difficult enough for ME to know how to check the IAC or the TPS, now I have to find out WHAT and where they are.
I tried to find a manual at a decent price but everywhere I look they just laugh.
For the 3400 engine the IAC (idle air control) is on the top right side of the throttle body. The TPS (throttle position sensor) is on the bottom right side of the throttle body.
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Old 01-26-2012, 05:41 PM   #9
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Try checking ebay for service manuals. There are usually a few up there for decent prices.
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Old 01-26-2012, 06:39 PM   #10
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Quote:
Originally Posted by stevenlemon View Post
Man, I'm seeing more acronyms than I did I 23 years in the Army.
It is difficult enough for ME to know how to check the IAC or the TPS, now I have to find out WHAT and where they are.
I tried to find a manual at a decent price but everywhere I look they just laugh.
A primer on the IAC and TPS.

(Here is a picture of a 3400 TB on the left with the IAC and TPS installed)

From the top down, IAC first.

IAC stands for Idle Air Control Valve.

When you put your foot on the gas pedal, the throttle plate opens and lets air into the engine...cool!

When your foot is off the gas...the throttle plate is 100% closed! You don't want the engine to stall do you? Nope! This is where the IAC valve comes in. It bypasses the throttle plate and lets some air get into the engine to let it idle! In the picture you can see the little spring and plunger attached to the black "box" on the outside of the TB. This valve opens and closes to let some air in. Personally, I've found them to be verrry verrrry delicate and you should be very careful with them.

NEVER take the plunger out of the IAC and NEVER remove the IAC from the TB and turn the ignition on. The little plunger and spring will play their own variant of the game "52 pickup" with you and be impossible to find and re-instal (from personal experience.)

Remove the IAC from the TB, LOOK at it, if it is all nice and grungy black, spray it off with some brake clean and also a little bit in the hole it came from. If it isn't idling any better, get a new IAC. JUST DON'T TOUCH THE PLUNGER!...trust me.

Now for the TPS. TPS stands for Throttle Position sensor. It is located below the IAC on the TB inline with the middle of the throttle plate and connected to the rod that actuates the throttle plate. It sends a signal to the computer between 0 and 100 (or 99 depending on what software you use) and lets the car know how open the TB plate is. Cool!

TPS can cause your car to idle erraticly if it is malfucntioning and telling the computer your throttle plate is open. Example:

TPS: "Yo PCM, dat TB is open 9%"
PCM: "Sweet deal TPS, lets dump some more fuel in the engine to keep the fuel/air mixture all groovy"
Engine: "WTF PCM? Why are you giving me so much fuel?"
O2 Sensor : "WTF engine? Why are your fuel trims so whacky, PCM! I need more air!"
PCM: "Well ****, the only way I can give the engine more air is to open the IAC! IAC! LEZ GO!"
IAC: "Yessir! Opening!"


Yeah...it isn't the best way to describe it, but it is pretty much what happens in laymens terms.

So how to you test the TPS? Turn that sukka off. Unplug it. The PCM defaults to thinking the TPS is telling it 0 (IIRC). This will reset the whole process. If your car idles better after unplugging the TPS, then odds are it is the TPS malfunctioning.

If you have a good scan tool or HPT you can read all of these values and manually actuate the IAC to test it.
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Old 02-01-2012, 06:56 PM   #11
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tapping the top of the IAC can pop it out if its dirty and stuck ( like mine was ) and be careful some of those "new" TPS sensors could be older cleaned up ones. i've also heard of the spring snapping inside if your constantly over aggressive with your pedal i think thats what was happening to mine i went through 3 in a summer racing.
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Old 02-02-2012, 08:04 AM   #12
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Hey HOYS, do you think the IAC might be the answer to my problem?

(Car rpms drop and try to shut off car when I get off the gas pedal mostly during hwy driving)

Thread Link: Car cuts off while driving...Help Please!?

Thanks
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Old 02-02-2012, 02:27 PM   #13
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Yes. Very likely. No other idling problems?
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Old 02-02-2012, 03:38 PM   #14
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before the engine start cutting off on me it used to rev high while driving and more when in neutral, then i changed the TPS and that helped some. but then the low reving starting happening more often....i guess its worth a try im hating this car, its always something, smh
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Old 02-04-2012, 07:25 AM   #15
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So i removed my IAC and it looks like that may have been the problem. It was very dirty and a part of it was cracked...we'll see after a couple days if the problem stops
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Old 06-20-2012, 08:17 AM   #16
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My 2000 Grand Am GT is doing the same thing. Although - mine does it when I start the car. It will drive down the road at 70 mph without my foot even on the gas. It is a little scary. If I put it in park to shut it off it revs even faster. It doesn't do it all the time. I took it in and told them what it was doing. Nothing showed up and it didn't do it for them. Sometimes I shut it off a couple times and when I restart it - it will run normally. It is similiar to what my daughters Firebird would do - I thought the idle was set up too high and at a red light you would have to rerally press on the brake - or the car just wanted to take off. That is what mine is doing. Any experts suggestions? Thanks.
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Old 06-20-2012, 08:49 AM   #17
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Quote:
Originally Posted by Caiquemom View Post
My 2000 Grand Am GT is doing the same thing. Although - mine does it when I start the car. It will drive down the road at 70 mph without my foot even on the gas. It is a little scary. If I put it in park to shut it off it revs even faster. It doesn't do it all the time. I took it in and told them what it was doing. Nothing showed up and it didn't do it for them. Sometimes I shut it off a couple times and when I restart it - it will run normally. It is similiar to what my daughters Firebird would do - I thought the idle was set up too high and at a red light you would have to rerally press on the brake - or the car just wanted to take off. That is what mine is doing. Any experts suggestions? Thanks.
Your problem sounds throttle related. Have you look at the throttle plate to be sure it's closing all the way at idle or if the the cable and pedal possibly being stuck somehow.
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