Idle problem

[Twitch] · Sep 27, 2006

I've been having problems with my idle recently.

My current mods are:
- VT N/A Stage 1 Cams
- Mac Prochamber O/R
- Diablosport Predator
- GMS Upper Intake Plenum
- Steeda U/D Pulleys
- BBK 75mm Throttle Body
- FRPP 3.73 Gears
- Street tires (Exclaim UHP)
- Bullitt Suspension Kit
- SVT Cobra Front Brakes

The idle will drop close to 500rpm then will jump up to 1200rpm then settle. It will do this 1-2 times in a row trying to set the idle. Now before you say IAC valve, I just changed it 2 hours ago and the problem is still there.

I checked for leaks, nothing.

The car will idle perfectly, but when, at a stop, I give a bit of gas rising the RPM to 1500 and let go, the rpm will drop to 1000rpm stay for 0.5sec then drop to 500RPM. This is where the motor kicks it up to 1200rpm to settle the idle.

The car will idle forever with no problem, it's only coming to a stop when the rpm is going or when parked giving gas. My idle is set at 850rpm

Any clue?

What I've done so far:
- Replace IAC with new Motorcraft IAC
- Checked for vacuum leaks
- Reset ECU

RoadconeTuning · Sep 27, 2006

vacuum leak or you need to reset your ecu... pull the neg cable off the battery and let it sit for abotu 30 minutes to clear its head then start it without touching anything...

importrx7 · Sep 27, 2006

I have the same problem, coming to a stop the car nearly stalls out with the rpms falling to 500 or sometimes even less. I changed my IAC, checked for leaks and it still does it, i'll be interested to see the answer!!!

RoadconeTuning said:
vacuum leak or you need to reset your ecu... pull the neg cable off the battery and let it sit for abotu 30 minutes to clear its head then start it without touching anything...

I've done this and still does it!!

[Twitch] · Sep 27, 2006

RoadconeTuning said:
vacuum leak or you need to reset your ecu... pull the neg cable off the battery and let it sit for abotu 30 minutes to clear its head then start it without touching anything...

I have reset the ECU after I installed the new IAC.

jstreet0204 · Sep 27, 2006

Sometimes people don't realize how useful the ability to datalog can be in diagnosing problems like this. Use you tuner to datalog what is going on. The first thing you should check is tps voltage with the throttle closed. Then datalog your fuel trims when the problem occurs.

Dave0803 · Sep 27, 2006

I also have a problem sort of like this. Mine seems to drop when the cooling fan comes on though. I have replaced the IAC. I have a heard a lot of people saying the dealer has replaced alternators for this issue. What should we be looking for in the TPS voltage and fuel trims? I have a tuner but I'm never really sure what I am looking for when I use the dataloger.

[Twitch] · Sep 27, 2006

jstreet0204 said:
Sometimes people don't realize how useful the ability to datalog can be in diagnosing problems like this. Use you tuner to datalog what is going on. The first thing you should check is tps voltage with the throttle closed. Then datalog your fuel trims when the problem occurs.

TPS voltage : 0.86v while at idle, which is within normal range.

Short Fuel Trim @ Idle fluctuate between -4% to +1%, which is within the range

Long Fuel Trim @ Idle +15% to +20%, it should be 0%, but my car has cams, so I do not know if this is normal condition.

jstreet0204 · Sep 27, 2006

[Twitch] said:
TPS voltage : 0.86v while at idle, which is within normal range.

Short Fuel Trim @ Idle fluctuate between -4% to +1%, which is within the range

Long Fuel Trim @ Idle +15% to +20%, it should be 0%, but my car has cams, so I do not know if this is normal condition.

The cams should not effect the LTFT's. Are they that far out throught the entire rpm range?

I would try cleaning the MAF next if you haven't already done so.
Here is a useful link for some more test you can try.
http://www.motor.com/MAGAZINE/Articles/092006_11.html

[Twitch] · Sep 27, 2006

jstreet0204 said:
The cams should not effect the LTFT's. Are they that far out throught the entire rpm range?

I would try cleaning the MAF next if you haven't already done so.
Here is a useful link for some more test you can try.
http://www.motor.com/MAGAZINE/Articles/092006_11.html

If my MAF is dirty and off, wouldn't it affect all other RPM range and WOT?

jstreet0204 · Sep 27, 2006

[Twitch] said:
If my MAF is dirty and off, wouldn't it affect all other RPM range and WOT?

It can that is why I asked if your fuel trims were that far off through the entire range. Drivabilty will not be as noticable in the higher rpm ranges though. It is easy enough to clean it, so it is worth a try.

stangman16 · Dec 20, 2006

Did cleaning the MAF (or replacing) fix your issue? I'm experiencing the same problem, and I've changed the IAC twice already within 45K miles. Would be great to know how you resolved this. I think I've cleaned the MAF sensor 30K miles ago.

Jsun3701 · Dec 20, 2006

Check the other threads on this subject. Many people have gone to buy a new IAC and said the have gotten a bad one from the store/dealer. Also try the seafoam. Personally I had a similar problem and before I would pay $50 for the new IAC, I will pay $5.99 for a can of seafoam. Cleaned my idle tight up. I put it through the FI system first then the gas tank. Good luck.

the98stang · Dec 20, 2006

I am having the same damn problem. I replaced my IAC but I got it from Autozone (not the Motorcraft). I'm not sure if another new one will fix the problem.

The weird thing is, my car will run perfect after I reset my PCM. But about a week later, it will run like crap again. Maybe I have a bad PCM, or it needs to be updated ?

Have you checked for any leaks in the intake manifold or EGR? If you have an aftermarket air intake, check for loose connections.

02GTstangGT02 · Dec 20, 2006

same here I replaced all four O2's, MAF and IAC and ran fine for a whole day then today the service engine is on again and it runs like crap.

stangman16 · Dec 21, 2006

Yea, I did seafoam this last month. I guess I will try cleaning the MAF again, and test it as well. Havent checked for vacuum leaks yet, not 100% sure how to do that.

Barrier · Dec 21, 2006

I've had the same problem for a while, but now that it's cold out it's getting worse... anyone notice a correlation between the idle problem and the ECT temp? I.E. once it gets to operating temp it stops about all together?

Tape · Dec 21, 2006

this is a very common thing, I have started to believe it's a normal thing. Anyone not have this issue in an '99-'04 GT?

Gearhead91 · Jan 30, 2008

Just installed VT stg1 cams, same problem with idle now.

CobraRed_96_GT · Jan 30, 2008

Gearhead91 said:
Just installed VT stg1 cams, same problem with idle now.

no, you need a tune first - lol. Most the problems here are proly cuz of Autozone IAC's although i've never had my IAC go bad

ceracer33 · Jan 31, 2008

im having a similar issue. when i let off the gas and still rolling but coming to a stop the idle is around 1k, when i stop completely it will drop down to around 500 or so then jump back up and settle down where its suppose too, but sometimes it drops down to 500 then surges up around 1200 and does this about 3 times and then stalls. i've checked for vacuum leaks and the iac is new. i have stage 2 cams and im wondering if this is causing the issue, never noticed the problem before the cams. it rarely dies but sometimes it does. most of the time it just drops down low and then jumps up to normal idle. im thinking most of my problem is the fact that i still have some bugs in the tune.

Idle problem

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