PLEASE HELP, I’ve got a 2005 mustang GT that will drop idle between 350-400rpm and then instantly catch itself by jumping back up to normal idle around 750rpm. I’ve recently replaced several components to help this issue all needed to be replaced anyway so I didn’t mind relaxing. Including MAF, Fuel pump, Fuel filter, Battery, coils, spark plugs, clutch, flywheel, catted h-pipe. Nothing seems to fix this issue I’ve been having. I also cleaned the throttle body and I’ve had no luck. Swapped out pcm relay. Also did a idle relearn.

 

Check your MAF sensor, if there's any oily residue or dirt on it? it can also cause misfire issues. Also check for any potential vacuum and exhaust leaks as well. Finally, make certain your PCM relay is properly connected.

 

it’s weird because it’s not a misfire like it’s just drop rpm like it’s going to stall but doesn’t and then after 20 or so times doing that it will stall. Only at idle nothing else. It’s weird. MAF Looks good I checked it earlier today

 

Then check for any potential vacuum and exhaust leaks along with any possible loose connections with the PCM relay? If there are no leaks/loose connections found? I would then suggest connecting a hand-held scanning device to check for trouble/error codes. Once you determine what the error codes are? you'll then know exactly what is causing your rpm/stalling issues.

 

sadly that’s the other issue 0 codes and I’d wiggled wires around relays and everything nothing seems to immediately cause it it just randomly happens once the car is at Normal operating tempature

 

Does your car have an aftermarket calibration tune or is it 100% stock? If you have an aftermarket tune? I would guess that either a fuel, spark or timing parameter may be off somewhere. Otherwise, I would take your car in for servicing at your local dealership and have them perform a diagnostic test on it

 

Howdy!

Not to stomp on M05 there, he's on a good track for ya, but... I noticed you didn't replace the throttle body itself. If the throttle body motor's tired (and it being 15 years old, it probably is) then it could cause this problem as the idle is controlled by the throttle body directly. There is no more idle air bypass circuit, so there's no more idle air controller. Also, the throttle position sensor could be wonky, while I consider it somewhat unlikely because they'd typically throw a code, and yours isn't. Still. Something to check upon.

What coil on packs (or is it 'coils on pack'?) did you get? It's been said unless you're hardcore racing, the Motorcraft ones are the one to stick with. (Side note: I just had to add 'Motorcraft' to my Google dictionary. How dare they. )

What spark plugs did you use? I'm a stocker guy, and yes, that's with the Motorcraft/Autolite two piece ones... I know all the issues, but if you're not racecar, then those will do fine and you need to just do the nickel anti-sieze to prevent the extraction problems forever more. I've heard issues with other kinds of spark plugs being used to get them one piece and such, the electronics are weird about that.

The rest of your list is interesting, but ultimately would not be the cause, by and large, of your issue.

You may need to take it to a Ford mechanic to have them properly diagnose it with the scan tool that'll let them see the idle, spark, commanded throttle body setting, maybe do the direct testing on the components with their tool, all that so they can determine the real problem and point you the right way, if not actually fix it for ya if you want them to.

My *guess* is the throttle body. It will throw no codes and do stupid things like that sometimes. If it 'fixes' itself then the computer won't throw a code directly, but may store it for later diagnosis or such. It's an oddity on these cars that they did that. Wrench light sometimes, but no Check Engine. Just... weird. Anyway, if it IS the throttle body, do not mess around. Get the real deal Motorcraft/Ford Racing one to replace it. Don't rebuild it/replace the motor or sensors, just get the entire thing. It will come with all the things already new on it. Trust me, I've done the other way, was TOTALLY not worth it. No, lubing the gears won't help either. Just replace it. If it's that.

You can do some checks with a multimeter and such, but you'll have to get the manual or something that'll tell you the ranges of ohms/volts/amps that you'd want to see, and that can be fun to find... Here's the manual though. The pages are not necessarily in order, and finding things in it can be fun, but it's all there:
iihs.net/fsm/?dir=1

Hope that helps. And welcome to the forums!

 

I forgot to ask about the throttle body. Knowing that your GT is an "05" I shouldn't had overlooked it. Meanwhile, definitely follow HouTex's advice, as he's 100% spot on about the throttle body!

 

this fluctuating idle thing is fairly common with these cars, I think it has to do with the tune and the throttle body . . . mine does it too, but does not drop so low as to stall out

sorry if it was mentioned and I missed it -- be sure to clean the throttle body, if you haven't already done that

you might also try a datalog and have a tuner look at it, "Lito" (support@tudyno.com) might be able to help with that, he is very responsive by e-mail so it wouldn't hurt to e-mail him and ask about it

 

this whole thing actually is helpful I generally stick with motor craft OEM parts including sparks and most things to make the engine run I have cleaned the throttle body and other components around it it seemed to have gone away yesterday but I don’t know if it’ll start back up again tomorrow. I’ll check into how much I can get the throttle body assembly for from Ford since I get like an employee discountthrough my dealership but as of right now it’s not doing it. I’ll let you guys know if it acts up agains and if the throttle body replacement fixes it

 

quite possible the tunes being funky, When I first got the car someone tuned the rear o2s off so I couldn’t pass emissions I actually went to a DOmestic only performance and tune shop had it flashed to stock and retuned for power and it was operating amazing for over a year.

 

what changed? did you make any additional modifications?

if no change, then it points to a wear/failure or gunk build-up issue

 

well after the tune and over a year of useage the clutch went bad and replaced that, so while I had the trans out I replaced my cats with high flow since someone dugg the original out. Other than that no modifications have been done. I’ve just been ****ing with wires and cleaning stuff to fix it so as of right now it’s working we will see if anything changes tomorrow when I have to work.

 

hmmmm . . . changing the cats might affect the upstream O2 sensors but I doubt it; and assume that the downstream O2's are turned off, so that probably isn't it
but it could be the wires

 

You mentioned that the tuning shop re-flashed your car back to stock, so are you still running the stock tune? Also, if your running high-flow cats on the stock tune? your rear 02 sensors are automatically turned on which will most definitely cause an air/fuel lean condition. If you are no longer running the stock tune? make certain that your rear 02 sensors are turned off when running any type of high-flow cats, except for maybe Kooks high-flow "Green Cats" lol.

 

actually the high flows I bought don’t throw codes they are p pricey because it’s a cat mid pip combo.
I am no longer running stock tune it got retuned right after being flashed to stock. So all o2s work and have 0 issues I’ve been running it that way for about a year

 

The down stream are on actually I had them keep them on because AZs emissions are dumb. They have 0 issues the cat mid pip I bought are meant to be direct replacements but high flow with 0 codes. My cars got a full exhaust system on it with no o2 codes. It’s just a weird problem that randomly occurred recently. But I’ll see how the car runs and idles tomorrow when I work and get back to this post if it starts doing it again I may go with the throttle body replacement and go from there even tho that’s gonna hurt the wallet.

 

I can assure you that all aftermarket high-flow cats will throw a CE light and will not pass the OBDII emissions test. Unless your tuning shop somehow has some of the catalyst monitors turned off/disabled in the tune? there is no other way for an aftermarket high-flow cat to pass the OBDII emissions test with the rear 02 sensors turned on, and especially with running LT headers. My guess is, the aftermarket catted mid-pipe you have paired with your LT headers are not true high-flow cats, as there are no high-flow OEM direct replacement cats that are either 49 or 50 state emissions legal.

 

yeah it’s completely possible they are not true high flow.

 

As I previously said, unless your tuning shop has somehow either turned off or disabled some of the catalyst monitors in the tune? there is no other way that a true aftermarket high-flow cat can pass the OBDII emissions test, especially when running LT headers.

Also note, that once the cats are placed further downstream from the exhaust manifolds, the catalyst monitors will display either a fail code or not ready code during the OBDII emissions test. This also leads me to suspect that your tuning shop specialist has somehow either turned off or disabled some of the catalyst monitors in the tune while also leaving the rear 02 sensors turned on?

If by chance you happen to have the "Kooks" Green Catted mid-pipe? it may then be possible to pass the OBDII emissions test, as "Kooks" supposedly claims their Green catted mid-pipes are designed to pass OBDII emissions testing and supposedly also 49 state EPA certified