Unique Hanging Revs Issue

[ChuckyRick]

So i was doing some pre-dyno maintenance in preparation for a tune session and i might have run into a little issue. It came up after i cleaned my TB. i'm some glad i did because there was quite the buildup of sludge behind the blades. Basically i have a problem with hanging revs between gears. Nothing extreme, but it seems when i hit the clutch and leave it depressed it hangs at that rpm for around a full second and the come down is quite slow compared to pre-TB Clean. Again nothing super serious like 30 seconds to drop 1000rpm for example. But if i press the clutch at lets say 4000rpm, and after the approx 1 sec of hang, revs drop at a very linear rate of probably 1000rpm per 3/4 second, and it only drops down to about 1000rpm, instead of dropping to idle. Compared to before it would just drop right down to idle within a second, no hang time, nothing. The part i don't fully understand is if i'm sitting in the driveway in neutral, or in gear and clutch depressed- revs drop just like before, right down to idle. Problem only occurs when i'm moving. Now from what i can understand (but i could be wrong) it can't be a TB issue because it would happen all the time whether i was moving or in gear or not. Also i don't think it's any sort of clutch issue because a) there was absolutely NO sign of this before i cleaned the TB, and b) there's no slip at any point. Car isn't throwing any unusual codes, i have 2 codes for slow response 02 sensors or something along those lines, which i'm assuming are just flagging the downstream O2's because i have longtubes installed and no tune as of yet to override them. So really my main question is could this simply be a tune (stock tune) issue? And perhaps i never noticed it before due to my TB being so dirty? should probably mention it was dirty enough to cause the car to idle slightly low, which i thought was just due to having some parts installed and not being tuned accordingly. I installed my heat shield for my intake to cut down on set-up time whenever i come down to the shop, and in order for my stock intake to fit i had to cut off the bottom part of the airbox so that it's just open filter, but i can't see this making a difference as 99% of aftermarket intakes are open element anyways? I searched all the mustang forums and a few people did mention that hanging revs were a tune issue for them,but most of them were running a mail-order tune not stock tune. They just got a revised tune and fixed their issue. Also nobody seemed to have an issue like mine, Hence why i'm making this post. I have no idea what my problem is, i'm just hoping it's a tune issue and nothing that will cause an issue come time to get the car on the rollers. Just wanted to explain my situation to you guys and see if you had any ideas on what it could be. I must say the car is extremely drivable, just i'm not used to the car ever behaving this way. Plugs & Coils were new a month ago (Ford Racing & MSD respectively), fuel filter (oem) was changed only a week ago as was the oil (ford synthetic & oem filter) So all i can come up with is thinking it's a tune issue, but obviously a ford tech here or someone that tunes these would know alot more about that than me. Thanks guys, i'm REALLY hoping someone can help me out here

Note: I don't fully remember what i have wrote down in my sig, and i'm not sure how to change it on mobile. So i'll mention it's a '05 Mustang GT and the only mods i have installed right now are pacesetter LT's, corresponding H-pipe, MBRP axlebacks, UDP's, MSD Coils & a drop-in K&N filter in my (hacked up now) stock airbox, if you consider that a mod. Alot more sitting in a box in my garage but have to wait for dyno day to install!

 

[BruceH]

It's in the tune. These motors don't have an iac valve so off throttle air bleed is done by the tune adjusting the tb. IAC stands for idle air control. There are different settings in the tune for drive, idle, and 4th gear iirc. These setting predict how much air the motor will need at different times when it's not under throttle.

To me it sounds like the drive iac values are putting too much air into the motor. I could be wrong. However, adjusting the different idle air values have negated hanging revs for me.

You have a stock tune but are running an aftermarket cai and long tube headers? Is that correct? Not a good idea no matter what K&N says.

 

[ChuckyRick]

Thanks for the reply Bruce! I actually just talked it over with my tuner too and you guys are giving me confudence it's just a tune issue and nothing major. Basically he said the same thing about how it's probably a tune issue, also mentioned how our cars (and fords in general apperantly) are really slow in learning new idle trims and part of my problem could be it's simply still adjusting to my now clean TB. Also to answer your question, i'm running stock tune but the only mods i have installed right now that would affect tune are the headers/exhaust. (I don't imagine upgraded coils and UDP's would throw off the tune any?) i have a JLT V3, CMDP's, and a few other goodies that are getting installed on the car come dyno day. The K&N i was reffering to was just a stock replacement filter. I installed my JLT heat sheild already in an effort to cut down on down-time at the dyno, doing so i had to cut a portion of my lower stock airbox out in order for it to fit within the heatsheild. Sorry for any confusion!

 

[bujeezus]

correct me if i'm wrong but a "stock replacement k&n" vs a "cai k&n" should still require a tune, no?

 

[BruceH]

correct me if i'm wrong but a "stock replacement k&n" vs a "cai k&n" should still require a tune, no?

No. All he changed was the filter. A tune is required when the air flowing past the maf changes. Something like a larger maf housing.

Just in case you weren't aware of how it works heres a Cliffs note version:

The maf heats a precision wire sensor. Air passing over the wire causes it to cool which in turn means more voltage is needed to keep the wire at a constant temp.

The ecu uses the maf voltage to determine how much air is flowing to the motor. This information is used to predict how much fuel will be needed in closed loop. In open loop it's the only information used for fueling.

The maf transfer table is what's tuned and what the ecu is going to use for fueling. Changing the maf housing diameter will change how much voltage is needed to maintain the wire temp. This is when a different tune is needed, to match the maf xfer function to how much air is flowing.