Low RPM's when putting the car into gear- Need advice !? please

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Cherry99GTvert

New Member

Aug 8, 2010

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Colorado Springs

Aug 14, 2010

#1

• Aug 14, 2010

• #1

So ive had this continuous problem with my 99 gt convertible since i bought it over a month ago, first the idle would surge at a stop and try to power through the brakes but now that has become less of an issue, i didnt get much feedback on it through here so im just gonna wait since it seems to have gotten better. The one thing that hasnt changed is when i put the car into drive the rpms drop to around 200 like its gonna die then it lurches forward or reverse ( whichever gear its in) then it drives fine. Any ideas wht it could be? no i havent replaced the iac yet since im not sure thats the issue because ive never experienced a bad iac through these symptoms. Any feedback would be appreciated thanks in advance.

 

trinity_gt

10 Year Member

Jan 31, 2003

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Canada

Aug 14, 2010

#2

• Aug 14, 2010

• #2

IACs are known to go and you're having idle issues. It'd be one of the first places I'd look.

 

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Cherry99GTvert

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Aug 8, 2010

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Aug 14, 2010

#3

• Aug 14, 2010

• #3

Yea i guess its worth just changing. Are they the main cause of most strange idle problems?

 

joshwfl

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Jan 31, 2010

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Aug 14, 2010

#4

• Aug 14, 2010

• #4

Cherry99GTvert said:

Are they the main cause of most strange idle problems?

Click to expand...

Yup. Clean it first, it'll probably make a big difference and save you some coin.

 

wmburns

SN Certified Technician

Aug 14, 2009

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Aug 15, 2010

#5

• Aug 15, 2010

• #5

How to troubleshoot IAC Idle problems

Have you done any of the tests recommended in the first thread? Knowing the results of previous tests can help narrow down the source of a problem. Saves everyone's time.

What is the idle in nuetral?

For excessivly low idle concerns first look for a vacuum leak between the Intake tube and the IAC bypass line. Again, even a tiny vacuum leak can cause problems.

Check the battery. Is it clean? Are the terminals clean and tight? What about the grounds around the battery clean and tight? If the battery/power supply is weak, this may interrupt power to the PCM. This means that the PCM is constantly having to re-learn the idle trim values.

Next, check the ignition system. Possible it is weak.

Perform a power balance test. If one cyl is weak and not pulling it's share, this could also cause an excessive slow idle.

Finally check out post #3

http://www.stangnet.com/mustang-forums/810729-starts-but-dies-idle-help.html

 

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Cherry99GTvert

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Aug 8, 2010

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Colorado Springs

Aug 15, 2010

#6

• Aug 15, 2010

• #6

I have checked for vaccum leaks, There are none that i could see and me and my buddy examined every hose within view. The battery terminals are good and tight, The battery is new about a month ago or so. One of my ignition coils is missing the contact plate inside the boot but the spring inside the coil over is making contact with the plug but i do plan to put on some msd rapid fire coils in the near future. I will see what the idle is in neutral and perofrm the test you reccomended then post the results on here tomorrow or sooner. Thanks for the feedback wmburns i must have forgot to repond to your feedback last time but i did read the thred you reccomended, i did clean the iac but to no avail it is still the same i will buy a new one tomorrow and see what the change if any their is- thanks again everyone

 

wmburns

SN Certified Technician

Aug 14, 2009

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Aug 15, 2010

#7

• Aug 15, 2010

• #7

Cherry99GTvert said:

One of my ignition coils is missing the contact plate inside the boot but the spring inside the coil over is making contact with the plug but i do plan to put on some msd rapid fire coils in the near future.

Click to expand...

Recall that a weak cylinder was mentioned as a possible cause of of low idle. So you don't think that a damaged COP boot/contact could allow the spark to leak out or make a weak spark?

 

Rejekt

Member

Aug 12, 2010

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Raleigh, NC

Aug 15, 2010

#8

• Aug 15, 2010

• #8

I had a rough idle... surged and then dropped.. Cleaned the IAC and it went away.

 

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Cherry99GTvert

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Aug 8, 2010

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Aug 15, 2010

#9

• Aug 15, 2010

• #9

Thats true, i guess i just didnt think it would only affect the engine when put into gear like its under a load and that one cylinder is going to cause a dramatic idle drop. I say this because a few weeks ago water got under the coils and the car misfired in cylinder 5 due to the plug being fouled out i missed some water around the plug and i now know how a misfire feels but i never thought of a weak spark causing such an intense jump in acceleration after a dramatic drop in rpms when shifting the transmission. Im gonna hook it up to the modis scanner and see what the data readings are.