Gt idling issue... tried everything..

[leamangriffin]

I have a 99 Mustang gt vert. It has an 03 Gt motor in it. Automatic. BBk intake system, shift kit, bbk shortys, bbk o/r x pipe with no cats, and thrush welded mufflers. I had a small idling problem a while back... it would surge and drop 10 times, then return to normal, then it would repeat... so I got to looking and heard about the IAC problem. I bought a new one. replaced it. While I was at it I took off my TB and plenum, cleaned both out to remove all the carbon, cleaned the maf. then reinstalled.. also I disconnected the batt... ran fine for a few days... Then it started acting up... I would press the gas and let off and the rpms would stay at 1.5k- 2k for about 5 seconds, then it would return to normal... It only did it when I was in drive or reverse.. When I put it into park it would idle at 900 rpms and slowly climb to 1k-1.1k...

I replaced the TPS sensor. no help... one day I started it, and it idled fine, then I put it in reverse and the revs went to 1.5K then 400, then 1.5K then 400, then 1.5K then it stalled. I started it back up and no problems... then a day later it started acting up again. So I thought it the IAC i got was bad. so I swapped it out... got the new one in. unplugged the batt... and while it was unplugged, i reset my idle set screw. used a .10 feeler, then tightened 1.5 turns. then i connected my batt, started my car with the iac unplugged. set the idle to 800 then shut it off, plugged in the iac, then restarted. i let it idle for 10 mins, then turned on the ac and let it idle for 10 more. then i gave it some throttle and it hung at 1.5k then died. also I checked my tps voltage and i set it at .98 - .99 .

I thought maybe a vacuum leak, so i started checking, I found a small rip in the pcv hose so I replaced it.. then i noticed that when idling the vacuum sucks the pcv hose flat. and when i turn the car off it returns to round. this still didnt fix my idle problem. so I thought it might be the pcv valve. I took it off and it rattles like normal. I sprayed som carb cleaner in it and reinstalled. I have tried resetting with the battery multiple times and nothing is working. I can get it to idle normal everytime, but then when i try to give it gas it hangs up then dies. I tried to install the IAC plate that reduces idle hang, and now the idle is always at 1200rpms. even when i back the idle set screw all the way off. It is not my throttle cable because when it hangs i push the butterfly closed and it doesnt go down. It seems like when i disconnect the IAC I can get it to rev without hanging, but I havent tried driving it like this. also I disconnected the IAC connector while idling and it still runs at like 600rpms, then i pull the hose off and put my finger over the iac and the idle doesnt change.... but it does create suction on my finger... isnt it supposed to be closed when unplugged? then i plug the connector back in and it idles fine with my finger plugging it, but when i remove my finger the car stalls..... I am at a complete loss.... and help would be great...

 

[MarkF]

You mentioned the PCV goes flat, did you replace it? They sell a new hose with PCV valve for about $25. It includes a hard middle section that can not be sucked flat.

 

[wmburns]

Question: When the IAC electrical connector is disconnected, does it idle at 800 RPM's? If so, this is too high. Try closing the idle set screw in 1/4 turn increments. The IAC disconnected idle should be very LOW if not stalling. You should notice a difference in idle when the IAC electrical connector is re-connected.

Is the throttle return spring strong?

What is the IAC duty cycle percent (access with an ODB2 scanner)?

Did you replace the PCV valve? What about the PCV grommet? Are there any blockage in the PCV valve cover itself?

Is the PCV breather line connected to the left hand valve cover and leak free?

Inspect the EVAP purge line and the VMV valve located in the right hand fender well. Check to see if the lines are leak free. Especially the smaller vacuum reference line.

Consider getting a propane based vacuum leak detector. Search Amazon for "vacuum leak detector" in Automotive. IMO, there is still a possibility there is a vacuum leak. Perhaps in the intake or valve covers.

More information:

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

OBTW, the black vented IAC isn't always going to have vacuum on the input line. Especially if the idle is too high to begin with.

 

[trinity_gt]

Cleaning the throttle body was probably a mistake, depending on how you did it. If you used a solvent or a "throttle body cleaner" you probably cleaned away a thin elastomer seal in the bore that's there from the factory to help seal the plate in the closed position. With a damaged or no seal, you have uncontrolled air leaking around the plate.

The second mistake was monkeying around with the throttle stop screw. These are factory set and should never need touching.

The fact that the PCV hose goes flat at idle suggests that the PCV valve is working properly. You need a new hose to prevent the collapse of course but the PCV valve isn't leaking.

For the TB seal, you might try this:

• Remove the TB from the car
• Obtain some O2-sensor-safe RTV sealer
• Open the TB to the WOT position and use some 1000-grit sandpaper to gently rough up the TB bore surface only in the area where the plate touches it. You just want a haze, don't remove any real material. This will help the RTV adhere.
• Use brake-part cleaner and wipe the area thoroughly clean and allow it to dry.
• Close the throttle plate and use a scribe or pencil to trace where the plate rests in the bore. Re-open the plate to the WOT position
• Smear a very thin coat of the RTV on the bore at the scribe-line. By "very thin" I mean very thin. You want to seal the bore, not stop an oil leak... Go as far as you can up each side to the plate shaft.
• Allow the RTV to cure completely before closing the plate and re-installing on the car

Disconnect the IAC electrical connector. Turn the throttle stop screw so it's not touching the linkage/body at all. Start the car. Adjust the stop screw for an idle in the 500RPM range.

Re-connect the IAC motor.

This basically restores the idle screw setting and the TB seal. The original cause of this idling issue may still be present. You may have fuel pressure issues, O2-sensor issues (slow/lazy sensors can cause goofy idle behaviors...) etc. You might also want to check the car's charging system both for DC output but also for AC ripple. If you don't know what these terms mean or don't have a meter, take the car to an alternator shop and have them put a voltmeter on the car and check both parameters at idle and at 2000RPM.

 

[jonbacon]

Ive got to say man you guys are GOOD!!! I had the same issue and basically did all that you just posted and cleared up my issue, NOT TO STEAL YOUR POST BRO, but just gotta say this site and the guys are freaking awesome...

 

[mustangjoe02]

did u ever figure out the problem. ive got the same thing. maybe its my iac. on my tuner it says its running at like 33-35. anyone know if this is right?

 

[wmburns]

did u ever figure out the problem. ive got the same thing. maybe its my iac. on my tuner it says its running at like 33-35. anyone know if this is right?

IAC duty percent of 33-35 is on the low end of acceptable.

Should be in the 40-60% range. However, if the air is cold, this would push the duty cycle down.

 

[mustangjoe02]

that is at operating temp so could the iac be bad? i know youve gone over this numerous times and its startin to be like "a broken record"

 

[wmburns]

I was referring to the weather at your location not the motor's temperature. Remember that COLD air is denser than warm air. So if the air temperature is very low, it would be expected that the IAC duty percent would drop to compensate.

Regarding if your IAC is bad or not, are you are "parts changer" or a "tester"?

If you are a "parts changer", then your IAC may be bad. Change it and find out.

If you are a "tester", then what were the results of the tests called out in the IAC/Idle trouble shooting thread?

Note, having a low IAC duty percent could be something as simple as the idle set screw being a 1/4 turn too high. This could also mean there is a small vacuum leak somewhere. Impossible for someone out in Internet land to tell you without test results.

 

[Don 95Vert]

If you are using the '03 IAC with the '99 PCM, the IAC transfer function is different between the two of them. A tune would help to negate it...

Don

 

[mustangjoe02]

I live in az and its 95degrees out. Ill try testing it. My throttle body doesnt hit the screw all the way. Is that what u mean by overturned?

 

[mustangjoe02]

Just checked iac again and its at 55%. But I got a cel for 1131 & 1151. Both o2s are brand new

 

[mustangjoe02]

if the header bolts came loose, could this be the problem. but i dont hear an exhaust leak

 

[wmburns]

if the header bolts came loose, could this be the problem. but i dont hear an exhaust leak

Exhaust leak is one of the possible causes. However, both sides would need to be leaking.

Based upon what's been done, my vote is still a vacuum leak after the TB. Intake gasket, valve cover, EVAP purge or some place not so typical.

>>
Electrical:
Short to VPWR in harness or HO2S
Water in harness connector
Open/Shorted HO2S circuit
Corrosion or poor mating terminals and wiring
Damaged HO2S
Damaged PCM
Fuel System:
Excessive fuel pressure
Leaking/contaminated fuel injectors
Leaking fuel pressure regulator
Low fuel pressure or running out of fuel
Vapor recovery system
Induction System:
Air leaks after the MAF
Vacuum Leaks
PCV system
Improperly seated engine oil dipstick
EGR System:
Leaking gasket
Stuck EGR valve
Leaking diaphragm or EVR
Base Engine:
Oil overfill
Cam timing
Cylinder compression
Exhaust leaks before or near the HO2S(s)

 

[mustangjoe02]

what is the egr pipe was bent or something? could this be the problem as well?