RPM hanging

waikiki · Jul 13, 2011

Hey guys,
here is my problem: the RPM on my 04 mustang hangs most of the time around 1400 whenever I press the clutch in or put the car in neutral while drivin. whenever i get to a complete stop the RPM will drop back down to 750 after 3-4 seconds. It does it less often once the car is totally warmed up... after 7- 10 minutes of driving.
what could be the probleme?
what do i need to check?

Thanks alot for your help!

ps: sorry if some stuff I say dont make total sense.... I have just been speaking english for 2 years!

wmburns · Jul 14, 2011

troubleshoot IAC idle problems 1996-2004

When was the last time the MAF was cleaned?

When was the last time the PCV value was replaced?

Have you adjusted the throttle body idle set screw in an attempt to fix this? Possible it's set too high.

A hanging high idle can also be caused by a bad TPS, bad retrun spring, binding throttle linkage, vacuum leak after the TB, bad IAC.

Check out:
Is your mustang stumbling, dieing at idle or idling rough? - Page 14 - Ford Mustang Forums

OBTW, a higher idle is NORMAL while the car is moving. The PCM will enter slow idle once the car stops. However, 1400 is a little too fast for the fast idle. I'm telling you this so that after repairs are made, you don't expect the idle to drop to 750 while the car is moving.

waikiki · Jul 14, 2011

I would think if one of those was messing up it would mess up all the time though?
But it just stay high around 1400 once I start the car until the car is totally warmed up.. after that the RPM drops to around 1000 then to 750 once the car is stopped for a few seconds.
Also, i Just realize every once a while when i Start the car the computer has reset itself.. Could that be the issue? why would be causing that?
Thank you!

wmburns · Jul 14, 2011

When the battery becomes disconnected, the PCM must re-learn all idle trim values. It would be expected the idle would be rough/high/low until the PCM has completed the re-learning process.

Sounds like you need to go back to basics. Check the battery and battery cables. Is the battery holding a charge? Are the cables clean, corrosion free, and tight?

Today's cars can not run right without a strong battery and charging system.

waikiki · Jul 14, 2011

alright I just checked... the mechanic didnt tight the battery cable good enough when i got my clutch changed...

If I still have this problem when the computer has realearned everything... Could it be the EGR valve or it has nothing to do with this? Also I looked for the PCV valve but I cant find it .. I thought it was somewhere around the TB? and how can see if the PCV works fine?

I alredy tested and cleaned IAC MAF TB and TPS... they work fine.

Thanks

waikiki · Jul 14, 2011

Nevermind i found the pcv valve and it works fine... I' still wondering about the EGR though?

wmburns · Jul 14, 2011

An EGR problem causes the motor to run poorly. No power. An EGR issue is not responsible for a high idle (with the sole exception of a vacuum leak at the EGR valve itself).

waikiki · Jul 14, 2011

I guess I'm gonna test the TPS again more carefully...
could the TPS cause this problem just when the car is not fully warmed up?

wmburns · Jul 15, 2011

Grasshopper, did you read the post with the information on how the TPS works and the role the TPS plays in closed throttle determination for the 1996-2004 MY's?

Do you fully understand what is NORMAL high/low idle and the role that the car's speed plays?

waikiki · Jul 16, 2011

Hey,
I did read your article..
I tested the MAF sensor and the TPS with a voltmeter and they work fine.
I let my car warm up and idle then disconnected the IAC plug and the car did not die, the RPM just got lower and when I drive the car with it unplugged , the RPM does hang at 1400 anymore...
Does this mean I have a bad IAC? is there anyway I could test it?

wmburns · Jul 16, 2011

If the idle drops when the IAC electrical connector is disconnected, this means that the IAC is working at some level.

No where did it say to drive with the IAC disconnected. Further, it's not clear if idle is or isn't high with the IAC disconnected.

If the idle is high with the IAC disconnected, this says:

The idle stop set screw is too high
there is a vacuum leak after the TB
the throttle return spring is weak/broken
the throttle linkage is binding

Since the idle is high with the IAC disconnected, this rules out the TPS angle. Why? Because the IAC is non functional. Therefore the TPS cann't be affecting the idle.

My take, if you don't want to do the tests to narrow this down, replace the IAC. They go bad fairly often and it would aid further diagnostic working with a known good IAC.

waikiki · Jul 16, 2011

alright, I'm gonna get a new IAC , if it doesnt fix the problem... is there any other sensors i need to check?

wmburns · Jul 17, 2011

All of the information to resolve the problem has been given. No further progess is possible until all of the tests are run, repairs completed, and results posted.

waikiki · Jul 20, 2011

alright i Got a new IAC and it didnt solve the problem..
I noticed the problem occurs when the car is cold or after i have been driving for a long time ( 45-60 min). I do not have the problem when pressing the gas when the car is stopped, It drops back to idle.

MAF sensor , TPS, IAC, PCV ,Throttle body, spring are working properly and there is no vaccum leaks...

Also, i never had the problem until i got my clutch change ( I do not know if it could be realted or not.. maybe O2 sensors?)

Any other sensors or wires or,... i should check?

wmburns · Jul 22, 2011

Early on I asked if you understood how the PCM controls idle based upon the car's speed and engine temperature. I also attempted to expain what is normal behaivor after a battery disconnect.

Did you fix the battery? Are the cables clean and tight? Has the alternator been tested for excessive AC ripple and correct output?

It's also normal for the idle to be high when the motor is cold. The idle should come down as the motor warms.

It's NORMAL for the idle to be higher when the car is moving. The PCM is programmed to do this for SMOG reasons. The high idle should be in the 1200-1400 range. However, it can vary down to 1000 depending upon temperature and the car's speed.

The PCM will not activate the low speed idle until the car is fully warmed up AND the car's speed is ZERO. Low speed idle is in the 600-750 area depending upon automatic or manual.

Changing a clutch should have nothing to do with this problem unless they damaged something.

How do you KNOW the TPS is working? How has it been crossed checked or measured? Note, the only reliable way to test the TPS is by monitoring the TP-MODE PID. It's value should be "C/T" each time you take your foot off of the throttle. If it's value="P/T", the a higher than normal idle is to be expected.

lawless00 · Sep 27, 2011

TPS

Had the same problem on mine. Replaced the TPS valve ($22) and problem fixed.

5SpeedPony · Oct 17, 2011

Mine does the same thing when it is cold out. Pretty sure it just stays at a high idle until you come to a complete stop so the engine doesn't die on you

RPM hanging

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