Car wont Idle!!

Spoolin420 · Jan 28, 2010

i went to leave my friends house drove my car down the street heard a little backfire from the exh.. and now when i try and start it i gotta keep hitting the pedal so the car will stay on.. but if i get it close t0 3k rpms it holds rpm there without me hitting gas and doesnt drop.. does ne1 have ne ideas?

wmburns · Jan 28, 2010

What Model Year? Mods?

If I understand, once you rev above 3k for the 1st time, it will stay at 3k and never drop. If so, this tends to rule against an IAC issue being the primary problem.

To me this sounds like:
* Loose, worn, or damaged throttle linkage.
* Vacuum leak.

Of course this could be two problems. IE, IAC and TB linkage. Or the backfire may have dislodged/broken/burst a vacuum line. The backfire could have also damaged the throttle linkage or throttle body.

Start by performing a visual inspection. Look for ANY loose or damaged vacuum lines, PCV lines, Oil dip stick, Oil filler, TB linkage, upper intake, or any other source for a vacuum leak.

Disconnect the electrical connection to the IAC and report what happens. The idle should drop. If not, suspect an IAC problem or vacuum leak.

Spoolin420 · Jan 28, 2010

its a 98 gt built motor and procharged. i thought it would b a vacuum leak too but i looked everywhere and cant find anything checked all the intercooler pipes the maf sensor the throttle linkage seems fine also.. but it wont idle till mid rpms then it wont down idle.. and i do have one cel and its for obd system rediness

Spoolin420 · Jan 28, 2010

i did unplug it last nite but it did the same thing at low rs i had to keep hitting the gas to try and keep it to stay on.. but then i unplugged my maf and it idled at around 600? but very roughly

wmburns · Jan 28, 2010

SOOOOO......Disconnecting the IAC makes no difference in the symptom...This says that the IAC is not really working at all or the vacuum leak is bigger than what the IAC flows.

I suspect that you have the IAC with the black vent. Suspect that the backfire damaged the IAC internally and now the vent is not working as it should (The vent should be open only for starting and then close).

Can you post the full DTC code?

Confirm if the throttle body butter fly valve is closing all the way. Are the throttle return springs in good condition? TB closing all the way? Has the TB set screw been altered? If the set screw has been adjusted, this could cause a high idle condition.

Roger1402 · Jan 28, 2010

I have a 2003 4.6 comp cams and the engine wouldnt idle all the time. So I cleaned the IAC and put it back on with no change in idle, however there was a vacuum whistle coming from IAC. Replaced the IAC and with no change in idle. Now im getting a trouble code P1504. I checked the connection to the
IAC connector, its good. Havent checked the voltage to the connector yet but I though I would run it
by someone here before I go back to the shop.

wmburns · Jan 28, 2010

IMO, likely wiring fault to the IAC itself. Is there and IAC electrical extension? Suggest reading the voltage to IAC. See below from Ford service CD pinpoint test KE2.

KE2 DTC P0505, P1504, P1507 OR STARTS ONLY AT PART THROTTLE: CHECK VPWR VOLTAGE TO IAC VALVE
Note: If EGR DTC P0402 was output during Self Test, diagnose it first before continuing with this Pinpoint Test.

Disconnect IAC valve.
Key on, engine off.
Measure VPWR circuit voltage at the IAC valve harness connector.
Is voltage greater than 10.5 volts?
Yes No
KEY OFF. GO to KE3 . REPAIR open circuit.

KE3 CHECK IAC VALVE RESISTANCE
IAC valve disconnected.
Measure IAC valve resistance.
Is resistance between 6.0 and 13.0 ohms?
Yes No
GO to KE4 . REPLACE IAC valve.

KE4 CHECK IAC VALVE FOR AN INTERNAL SHORT TO IAC CASE
Measure the resistance from either IAC valve pin to IAC valve case.
Is resistance greater than 10,000 ohms?
Yes No
For DTC P1504:

GO to KE7 .

All others:
GO to KE5 . REPLACE IAC valve.

Roger1402 · Jan 28, 2010

Roger1402 New Member old mechanic

This put things in to perspective for me. I will put this test procedure to use either later tonight or tomarrow. I appreciate the information. I was hoping the PCM/ECM was not damaged. I bought this car in N.C. and drove it home to W.V. I knew it had some issues on the way home. It is my retirement hobby. It is a fun ride.
Again Thanks,
Roger1402

Spoolin420 · Jan 30, 2010

i just replaced the iac and not it idles but around 2400 rpms which isnt rely much btr i dont know what else to do

wmburns · Jan 30, 2010

Did you perform the basic IAC test? What were the results? Any decrease in idle speed?

In all likely hood there is a vacuum leak or an electrical fault to the IAC.

Does the throttle body butterfly valve CLOSE ALL THE WAY? Has the TB set screw been adjusted? The TB must close all the way so that all idle air comes only through the IAC. If the set screw has been altered, adjust the set screw so that the engine will die if the IAC were not in place. This means that the TB must close fully.

Also look for any other air entering the engine AFTER the TB. PCV valve, evaporative purge, loose or worn throttle linkage, leaky gaskets between the TB and intake.

What is the condition of the TB return spring?

Is the EGR system tight?

Is there +12 volts to the IAC red wire (key on/engine off). Measure to a known good ground.

Really suspect this is a vacuum leak. But the possibility of an electrical fault to the IAC itself should not be ignored.

Do you have access to an ODB2 scanner? Can you monitor the throttle position PID for a value of "closed" when the throttle is closed? What is the value of the IAC duty cycle?

nyuk98gt · Jan 30, 2010

Spoolin420:

Adding to what "wmburns" posted: it is possible that the intake has popped and now you have a large vac leak. Where, you ask, is the hole? Near the back and on the underside of the intake. There is a plug that is glued in and several people have posted that their intakes have failed at this spot. What with your car being supercharged, it seems like a good area to check. You'll have to use a mechanic's mirror to get a look. (I don't think there is enough room to get a hand back in there to check.)

HTH,

Chris

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