May have found my problem? Please read my last post

[JDwhite98gt]

Just like the title says. For example, if I'm going on a backroad run, either going balls out or just cruizing and I go to blip the throttle before I downshift, the car will hesitate for a brief moment and I usually have to blip the throttle twice. Its very hard to explain but it feels like the car cant keep up, or as if the throttle cable fell off and then by the second blip it found its own way back onto the throttle body. Now in the past I have posted about this happening at higher speeds and some have said the fuel pump, but I have a fuel gauge and I have watched it durring a high speed test through 4 gears and the needle on the gauge stays where it always is at WOT and it never falls or I never loose preassure.
I've had the car tested by a "reputable mustang mechanic" and he can usually diagnose this stuff and he couldnt figure it out. Now I have replaced a lot of parts trying to track down the issue, Fuel filter, plugs, wires, checked for bad injectors, cleaned maf, iac, replaced pcv, checked tps voltage. I have no codes and this problem has been going on intermitently for the past 3 years and it happens less with colder air. One thing that concerns me is my cold air intake, I think its a bbk but for a new edge, so the tube is real wide like a JLT. It has holes in it for the tube that goes to the driverside and for the tube that goes to the plenum and theres supposed to be gromets on the intake tube and they have long rotted away. Could the car be sucking in unmetered air and cause some weird symptoms? Any other things I could look for?
P.S. I have never changed coil packs and have tried anything from 87 octane to 93 just see if it changed the way the car ran.

 

[JDwhite98gt]

Re: Car stumbles when I try to "rev match" and downshift

Bump....anyone?

 

[1996mustangGT]

Re: Car stumbles when I try to "rev match" and downshift

Try putting some sealant around those holes...that could definately be an issue. Unmetered air = no bueno

 

[JDwhite98gt]

Re: Car stumbles when I try to "rev match" and downshift

well I was actually going to just swap in the stocker and run that...so tired of the chrome steeda POS thats on there now.

 

[1996mustangGT]

Re: Car stumbles when I try to "rev match" and downshift

That would work as well let us know how it goes

 

[Lee12609]

Re: Car stumbles when I try to "rev match" and downshift

wait so you have holes in your CAI between the MAF and throttle body??? if thats the case i'm surprised you dont have a TON of threads asking "WTF is wrong with my car"......

put some tape over the holes temporarily, disconnect battery and turn key on for 10 sec, turn key off, reconnect battery, and go for a drive. should fix it it thats your problem.

 

[JDwhite98gt]

Re: Car stumbles when I try to "rev match" and downshift

Well let me be a little more specific. Since I swapped this motor in (3 yrs ago from my 96...which had no problems) I have had some weird stumbling problems. If I go on an all out run to top of 4th gear and then let the car coast and rev down (while in gear) then go to press back on the gas it will hesitiate or feel like only 4 cylinders are firing. It only happens for a split second and then its as if nothing ever happened. Its also intermittent and hard for me to replicate whenever I feel the need to. Anyways, the cold air kit, which I have ran in the 96 with no problems, has the 2 metal nipples that attach to the vaccum hoses (one goes to plenum, other to drivers side valve cover), thses nipples are supposed to be used with rubber gromets in between the nipple and cold air tube which have long since rotted away. I used almost half a tube of RTV around the nipples but it doesnt stick to chrome very well. I"ve posted about this problem before, just click on my name at the top and look for last topics. I also thought it may be a fuel problem because at idle, my fuel gauge only reads about 32 psi and WOT is maybe 38psi. Ive changed over to a 96 fuel rail and regulator, listened to injectors, done 2 full tune ups including everything from wires, plugs, air filter, fuel filter, oil &filter change, tested coil packs (in the dark and unplugging the wire to watch them ark), cleaned IAC, MAF, TB, plenum anything and everything short of replacing the fuel pump, which according to my gauge, never looses pressure at WOT, which I was told that if it holds pressure than its good. So I'm just running out of tests and dont know what to do. I even tried different octane fuels, running the fuel pressure regulator with no vaccumn...everything.

 

[JDwhite98gt]

Re: Car stumbles when I try to "rev match" and downshift

bump....again

 

[JDwhite98gt]

Re: Car stumbles when I try to "rev match" and downshift

So fixed the CAI issue but problem is still there. Lets recap...
1. Car will hesitate intermittently when going throttle - off throttle - car stumbles on tip-in going back on throttle and will backfire sometimes once the sputter stops.
2. I have a fuel preassure gauge on pillar.
IDLE = 28 PSI
WOT = 40 PSI
3. Never does the fuel preassure drop at any given time. WOT preassure holds, when sputtering occurs, the preassure is still where it should be and holds.
4. problem has been happening a little more frequently with the weather being pretty hot (80+) and a bit humid.
5. Fuel filter was changed 200 miles ago along with full synthetic oil change.
6. Did injector test and upon unplugging each injector the idle suffers accordingly.
7. Switched coil packs to known good ones...no change in anything lol
8. Upon cold start up, the car idles fine, starts fine but "Puutts" a bit through the exhaust. you can hear it pu put pu put but it goes away after running for a minute.
I'm planning on pulling the plugs to see what they look like, but I've done this before and they had all looked normal. I'm at a loss here guys, cant seem to get any good answers from mechanics, normal driveability is great and WOT throttle runs are great too. It's just a painful thorn in my side.

 

[JDwhite98gt]

Please read my last post...maybe i found my problem?

I stumbled upon this while searching the forums. Do you think this might have something to do with my problem?

The screw you have been adjusting is not an idle adjustment screw, it is a throttle stop screw used to set the intial airbleed through the TB, so that the PCM can then control the idle by varying air flow through the Idle Air Control (that's why it's called that).

Here 's the procedure for getting the TB stop screw adjustment back to where it's supposed to be.
http://www.paladinmicro.com/PalMFrame00.htm?wks=tba



The only way to change the idle speed is in the tune, the stop screw being only used to preset the amount of air bleed through the TB when the throttle is closed. It is set so that with the throttle closed the IAC is operating in the lower 15% to 20% of it's range when the PCM is controlling the idle speed to provide some "headroom" when low idle speeds are desired and quite a bit of room to increase idle.

Using it (the stop screw) to attempt to control idle speed is a futile task because the PCM will just increase or reduce flow through the IAC to get back to the programmed idle speeds (there are several programed speeds depending on IAT, ECT, load, etc).

If the stop screw is tightened too much, in an attempt to increase idle speed, then at some point too much air will be bleeding through the TB and the PCM will not be able to control the idle even with the IAC completely closed--this generally results in the idle speed being erratic as heck as the PCM tries its best to maintain control.

If it loosened too much to attempt to reduce the idle, the PCM will increase flow through the IAC to keep the programmed speed. It can do this with reasonable success however the engine is likely to stall when the PCM suddenly stops IAC flow (for deceleration, etc.) and there if far too little air bleed through the TB to keep the engine running. At the extreme the throttle plate will stick closed due to engine vacuum, binding, and stiction...

Disconnecting the battery will clear the KAM (Keep Alive Memory) and force the PCM to relearn the idle air trims needed to maintain the programmed idle speeds, however eventually it will relearn the trims anyway.

 

[Lee12609]

thats pretty common knowledge i thought, not quite sure how it would cause a misfire, hesitation, sputter, or any other problem besides stalling and high idle.

 

[JDwhite98gt]

Yea but I can really call this problem a miss fire. Its so hard to describe it. It is common knowledge, the question was whether or not having the throttle stop adjusted wrong can cause the pcm to act funny when I'm going ON THROTTLE- OFF THROTTLE-ON THROTTLE. The car will never hiccup any other time, even if I'm going full throttle through the gears, cruising..etc.

 

[Lee12609]

adjust it and clear the kam see what happens.

 

[JDwhite98gt]

I'll give it a go later on today.

 

[lordgufi]

My car had the same issues. I never figured it out. however I noticed it more when i had the aftermarket CAI on the car vs the stock unit.

I'm It also NEVER did it when the rear o2 sensors were off on my tune ( then tuner turned them back on with no cats ) I assume the issue is a tune issue and not a mechanical issue.

when you blip the throttle your ecu sees the difference in the throttle position vs where it just went and it tracks this. when you do this it causes a acceleration enrichment and if it goes rich you will bog, if it went lean the car would buck and feel like its dying then catch back up ( those are extremes )

Timing will also cause this issue. when you let off the throttle the ecu increases timing into the 40ish deg area and then when you punch it again depending on what conditions the ecu see's ( IAT, CLT, MAF, o2 sensor if in open loop ) it will increase fuel and decease timing all the way back down to a low of some where in the 15 deg area ( basically wasting your time as this deg is even lower than where our cars idle )


When you did the PI swap did you have the car tuned to a basically stock PI motor or did you leave it with the NPI tune?