RPM's fluctuating

road rage

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Dec 14, 2003

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R.I.

Apr 4, 2004

#1

• Apr 4, 2004

• #1

When the car sits at idle (I warm it up) I can hear it in my house as it fluctuates. Also when I'm driving, I'll come to a stop and sometimes it will drop own to 500 rpm's and then go to around 700. Any ideas?

 

Dr. Wu 2000

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Feb 16, 2004

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Eastern, Pa.

Apr 4, 2004

#2

• Apr 4, 2004

• #2

road rage said:

When the car sits at idle (I warm it up) I can hear it in my house as it fluctuates. Also when I'm driving, I'll come to a stop and sometimes it will drop own to 500 rpm's and then go to around 700. Any ideas?

Click to expand...

I understand that the BBK throttle bodies have a idle air screw that you can
adjust to fix the surging at idle. I don't know if the MAC has that or not. The
idle air screw is on top of the throttle body and changes the position of the
butterfly when at idle. You may want to check that out if it applies
I am having to do that right now after my custom tune I had done. It may
take a couple times of adjusting and testing it to get it correct. Also, if it
is surging and you give it gas and then quickly brake, do you stall out ? I
have had that happen a couple of times

 

new22003

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Jun 22, 2003

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Missouri

Apr 4, 2004

#3

• Apr 4, 2004

• #3

Could be an IAC, very common problem and I had tthe same symptoms before replacing mine.

 

road rage

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Dec 14, 2003

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Apr 4, 2004

#4

• Apr 4, 2004

• #4

Nope hasnt stalled. I'll try to adjust the tb tomorrow

 

JonJon

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Aug 16, 2002

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back in Marylands

Apr 4, 2004

#5

• Apr 4, 2004

• #5

I had that problem, I made it go away by adjusting the idle screw. The IAC had absolutely nothing to do with it since it died and I replaced it... but the RPM bounce was still there.

It doesn't drop at all anymore EXCEPT when I'm using the AC, but the drop is a lot less than before.

 

D

DrK MrK iV

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Jan 11, 2004

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Charlotte

Apr 4, 2004

#6

• Apr 4, 2004

• #6

I have the MAC 70mm TB too. It also does this. I'll check that screw on the top tomorrow as well. What does IAC stand for?

 

JonJon

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Aug 16, 2002

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back in Marylands

Apr 5, 2004

#7

• Apr 5, 2004

• #7

DrK MrK iV said:

I have the MAC 70mm TB too. It also does this. I'll check that screw on the top tomorrow as well. What does IAC stand for?

Click to expand...

idle air controller

 

D

DrK MrK iV

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Jan 11, 2004

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Charlotte

Apr 5, 2004

#8

• Apr 5, 2004

• #8

So do I adjust it or replace it or what? Im clueless with this.

 

JonJon

Founding Member

Aug 16, 2002

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back in Marylands

Apr 5, 2004

#9

• Apr 5, 2004

• #9

DrK MrK iV said:

So do I adjust it or replace it or what? Im clueless with this.

Click to expand...

Replacing the IAC did NOT fix my idle drop problem. My IAC died and the new IAC still had the idle surge. I got an Accufab 75mm TB last month and adjusting the idle screw last week got rid of my idle drop (when coming to a stop). When I have the AC on it drops a little, but not as bad as before. With the AC off, it hasn't dropped since I adjusted the screw

 

D

DrK MrK iV

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Jan 11, 2004

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Charlotte

Apr 5, 2004

#10

• Apr 5, 2004

• #10

So this is going to happen with just about any aftermarket throttlebody? It isn't going to hurt the engine in any way is it? And... I'm guessing a dynotune couldn't fix it.

 

pony_vert02

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Mar 16, 2004

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AUSTIN,TX

Apr 5, 2004

#11

• Apr 5, 2004

• #11

OR.... you can get a tune up! clean you injectors..check/replace your spark plugs.. do you notice any hesitation or drop in rpm's while driving?

 

stangman16

Active Member

Nov 16, 2004

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San Antonio, TX

Feb 6, 2006

#12

• Feb 6, 2006

• #12

I'm having this same problem with my '99 GT and it's all stock. Once I come to a stop the idle will surge to 400-500 and back up again to 700ish. Sometimes it doesn't do it, but most certainly when the A/C is on. I've replaced the IAC already due to it being faulty - How much do you adjust the Idle screw to fix this? I've tried injector cleaner as well. The car has about 75K miles and it's been doing this for the last 20K or so.

 

deftsound

Please ask me how much my supercharger cost

Apr 6, 2004

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Texas City TX

Feb 6, 2006

#13

• Feb 6, 2006

• #13

i have a 99 and mine startd doing it afte rmy offroad h pipe and straight through mufflers, i think its because the lack of backpressure

 

stangman16

Active Member

Nov 16, 2004

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San Antonio, TX

Feb 6, 2006

#14

• Feb 6, 2006

• #14

Wish I could pinpoint it that easily for mine, but I'm all stock - so I don't know what the deal is.

 

NaplesBill

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Aug 10, 2005

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May 25, 2006

#15

• May 25, 2006

• #15

Well, I have an '03 Explorer with the same motor as the 99-04 GT. I had a problem very much like everyone here is describing. It started a couple of days ago and just kept getting worse. Today I had to hold the gas and brake everytime I stopped to prevent the engine from stalling.

I decided to take a look at it tonight. I have an engine cover and when I removed it I kept hearing this sucking sound. It sounded like it was coming from the intake manifold. It turned out that the breather tube that connects from the passenger side valve cover to the intake plenum had a hole in it. I have no idea how the hole got there but I replace the elbow where the hole was and the problem is fixed.

It's the large breather tubing that elbows right at the intake plenum. There are two small cooling lines that connect at the same area on the plenum. It would stand to reason that a smaller whole or leak would cause less severe but similar symptoms.

 

Stang|ess

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Oct 18, 2003

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Hawaii

May 25, 2006

#16

• May 25, 2006

• #16

is the idle screw the one that controls the butterfly? if it is, don't mess with that.

I would say on a cold engine, start the car and don't touch the gas. let the car warm up, shut her off then restart her.

 

E

eblack99

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May 7, 2006

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Houston TX

May 25, 2006

#17

• May 25, 2006

• #17

my 99 Gt was doing the same exact thing after installing a new plenum intake and throttle body, I replaced the idle air control valve (IAC) and disconnected the battery for a good 45 min. for the computer to reset ,after that works great the rpm hangs a lil bit but used to be worst, i also heard about a copper cap in the IAC tube you might want to look that up.

 

Stang|ess

seeking cyber partner(s)

Oct 18, 2003

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Hawaii

May 25, 2006

#18

• May 25, 2006

• #18

iac restrictor plate works well. on ebay, type "restrictor plate" and you'll see it. $15 shipped to your door.

 

hotdog71

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Dec 8, 2004

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Seattle, Eastside

May 26, 2006

#19

• May 26, 2006

• #19

IMO Important rules when it comes to GOOD quality TB's.

1, Never adjust the butterfly screw or any other screw as they are all factory set for your motor and will most likely not help. Unless you have bought 2nd hand TB and it was messed up before.

2, Never clean off the seal paste inside the TB that seals the butterfly when closed.

So far this has worked for me and many others i am sure.