Fox vs SN95 decel observations

R100RT

Founding Member
Nov 27, 2000
362
0
16
Sonora, CA
I have owned four Fox mustangs and now I have my first SN95. All of my Foxes, Auto or stick, would decel as soon as you let off the throttle. The SN95 seems to wait a while before the ICS allows the RPMs to drop. Maybe it's the difference in the tunes used by the A9X family vs the Sn95 computers. My '94 vert has no idle issues. I notice the throttle hanging between shifts. When I go 4th to 5th the rpms actualy go up a few hundred RPM when I put in the clutch. When I coast downhill, after about 10 seconds being off the throttle, the engine braking will increase (act normal). I am used to decel as soon as you are off the throttle. Is this normal SN95 wierdness?

Eric
 
Eric

Many things can cause the hanging idle thing :bang:

Lets assume nothing mechanically wrong has happened.

I can't tell you how many peeps who get into Tweecer tuning have chased and chased that problem by changing all kinds of variables within the pcm with no success.

I quickly ask them if they've been hosing around with:

throttle body blade stop screw
throttle body bleed screw
throttle position sensor
factory spark value of 10
fuel pressure

They answer with a ............ Yes!

I tell them to put it all back to stock values and then see if their programming starts to become effective.

None of them have told me ........... it didn't work ;)

With mechanical adjustments like those ..............

Sometimes they can be effective :) ......... Sometimes not :(

The pcm tries its best to give input to help and keep a stable idle based upon the data from various sensors.

Sometimes, our mechanical adjustments help it do that :nice:
and
Sometimes we unknowingly :shrug:
Cause it to freak out when we hose around with that stuff :rlaugh:

In my dealings with self tuning, I've come to find this ..........

With drivability issues ........
Each Stang can be very different
and
What works for one Stanger may not work for another :bang:

Look to see if any of those things above is out of line and causing a problem such as the hanging idle.

btw ... the famous unstable idle Stangs are all too well known for can be caused by the above list of items as well :)

Grady
 
My sn does that, decel as soon as I let off, but I've got the a9l computer with a 93 TB setup, so that may be the reason.

Grady, what position is the stock position for the things you mentioned. How do I know if my bleed stop is off or my blade stop is off? When the car is off, the blade is completely closed. I haven't checked it out to see of it has that small hole, which is said to help idle. I don't have many idle problems, except that my window from releasing the clutch and accelerating is very tiny, if I miss it my car can bog and even die. What do you thinks accounts for this.

DON"T MEAN TO HIJACK, BUT WE"RE ON THE SUBJECT ALREADY.
 
My sn does that, decel as soon as I let off, but I've got the a9l computer with a 93 TB setup, so that may be the reason.

Grady, what position is the stock position for the things you mentioned. How do I know if my bleed stop is off or my blade stop is off? When the car is off, the blade is completely closed. I haven't checked it out to see of it has that small hole, which is said to help idle. I don't have many idle problems, except that my window from releasing the clutch and accelerating is very tiny, if I miss it my car can bog and even die. What do you thinks accounts for this.

DON"T MEAN TO HIJACK, BUT WE"RE ON THE SUBJECT ALREADY.

I'm still learning about the SN95 but this is what I think I know:

The throttle stop function is to prevent the throttle from going to far closed and sticking. It should be adjusted as far closed as you can w/o sticking. To test for sticking, with the engine off, pull the TB open and let is slam shut. Slowly open it up and feel if it pops open. It should open smooth. If it's sticking, you can feel it in the gas pedle. The bleed screw should be adjusted for about 600 RPM, spout out, ISC disconnected, engine warm, and no accesories on. After doing this, clear the KAM. Idle should come back into spec. I'm still unclear if the TPS needs to be .9 - .98 VDC. I'm getting conflicting info on this part. To be safe, I try to adjust to .95 VDC.

My idle is perfect except for the hanging. I have already tried a new ICS. When I get my Tweecer, the first thing I will look at is this problem. The speed sensor plays a role as the idle is designed to increase if the car is moving. On my car, less than 8 MPH will cause the idle to go all the way down to 650 RPM. Higher than that, the idle goes to 1100 RPM.

Eric
 
My sn does that, decel as soon as I let off, but I've got the a9l computer with a 93 TB setup, so that may be the reason.

Grady, what position is the stock position for the things you mentioned. How do I know if my bleed stop is off or my blade stop is off? When the car is off, the blade is completely closed. I haven't checked it out to see of it has that small hole, which is said to help idle. I don't have many idle problems, except that my window from releasing the clutch and accelerating is very tiny, if I miss it my car can bog and even die. What do you thinks accounts for this.

DON"T MEAN TO HIJACK, BUT WE"RE ON THE SUBJECT ALREADY.

Yeah Nacho

Eric is right on about the function of the throttle body blade stop screw :nice:
A lot of times, you see peeps have opened it too far trying to up the idle.

You can find all kinds of methods to setup all that stuff but basically here are a few things I've found with my car and some stuff I've seen working with other 94-95 self tuner peeps.

You don't want the stop screw too far out :notnice:
Just enough to keep the blade from sticking :nice:

I've obtained a stable idle with the bleed screw as far in as 1.5 turns and as far out as 2.5 turns. Its setting is of course effected by how you have other things setup in the pcm such as tb airflow, isc neutral idle airflow points, and the like.

I usually find the most stable idle from having the isc setup where when I pull the plug the car almost dies or does in fact die.

Now about the tps setting thing :)

I've worked with quite a few 94-95 peeps who self tune and I've asked them about setting the tps. They've told me they can find no evidence to support the setting of the tps on our cars like the fox boys do. I've seen it be fairly far below that magical, mystical value of .98-.99 in many dlogs peeps have sent me over the years and none of them had idle issues.

On my 95, I've had my stock stuff, a self ported E7 head combo, and this current combo with two different tb's/tps sensors. I never touched the tps on either of those tb's even though they were below that .98 value. I didn't have an unstable idle or hanging idle on any of those combos. When I ran my oem tb the setting was .93 and my current tb is at .96 with both values being out of the box values.

I've seen some on these Stang boards say it made a difference for them when they set it and I am glad it worked for them :D

I can't see a reason why I should argue against their positive results
but
I've often suspected they were too far out of an acceptable range to begin with due to some mechanical problem.

Hey it worked with the Fox :nice:
so
it must be so for the 94-95 :ban:
I just don't believe it to be so :)

Since this subject has not come up in a good while and we got so many noobs lately, I feel it would be helpful if I repeat this info
It is not gonna hurt anything in the pcm if you wanna set it:)
however
Over the years I've seen many hose up their tb or tps when they tried to set it :shrug:

About the hanging idle :)

Usually its caused from the stop screw being too far out and that in turn can throw the isc totally all out of whack :rlaugh:

I've seen the bleed screw being too far out to cause it.

I've seen the spark being too high cause it as well.

As for fuel pressure :)

If you self tune ..............
You REALLY REALLY don't wanna be hosing around with it :nono:
It will just jack around all your efforts to optimize the pcm values :fuss:

Another thing about a hanging or unstable idle.

How close is the maf curve at and around the idle points.
How stable are the K's at and around the idle rpm.

I can say for certain, my 95 does a bit better if the af ratio is a bit fat at idle.

Each combo can be different :D
so
You just have to get in there and fart around with the values to see what your particular Stang likes for good drivability :nice:

Now .... thats not to say :nono:
When your fartin around :)
You shouldn't use some logical reasoning to do that fartin around ;)

Grady
 
Thanks guys. I had been having problems with driving in high gears at low mph, I cleaned my MAF and readjusted my tps to .987 and it worked out great. I can now drive in 5th @ around 1400 rpm/40 mph and the car won't bog.

On a side note, why do I need to keep readjusting my tps. I adjust it to .987 and a few weeks later, I have to go back and readjusted cause it's now at .95x or not at .987. Is there a reason for that or my TPS going out?

I guess my blade stop is too closed cause it was sticking last week, after I threw some carb cleaner on it, it's working fine. I'll play with it tonight.

I'll try to adjust my idle, hopefully it does something. Let me make sure I've got it right.
1.Warm up engine(no accessories)
2. Remove spout
3. Disconnect IAC
4. Adjust bleed screw fro 600-650 rpm
5. Turn off car and disconnect neg. cable from battery and step on brake to reset PCM
6. Start car and plug everything back after restart of engine.
7. Enjoy steady idle.

Does this cover it pretty well?


And yeah, 94-95 don't have the capability to be adjusted. I'm running 93 TB and that's why I can do this, and apparently, it works.
 
Thanks guys. I had been having problems with driving in high gears at low mph, I cleaned my MAF and readjusted my tps to .987 and it worked out great. I can now drive in 5th @ around 1400 rpm/40 mph and the car won't bog.

On a side note, why do I need to keep readjusting my tps. I adjust it to .987 and a few weeks later, I have to go back and readjusted cause it's now at .95x or not at .987. Is there a reason for that or my TPS going out?

I guess my blade stop is too closed cause it was sticking last week, after I threw some carb cleaner on it, it's working fine. I'll play with it tonight.

I'll try to adjust my idle, hopefully it does something. Let me make sure I've got it right.
1.Warm up engine(no accessories)
2. Remove spout
3. Disconnect IAC
4. Adjust bleed screw fro 600-650 rpm
5. Turn off car and disconnect neg. cable from battery and step on brake to reset PCM
6. Start car and plug everything back after restart of engine.
7. Enjoy steady idle.

Does this cover it pretty well?


And yeah, 94-95 don't have the capability to be adjusted. I'm running 93 TB and that's why I can do this, and apparently, it works.

The only feedback I have for you is that if you that Gradys method pulling the ICS worked on my Foxes but didn't do so well on my Sn95. My SN95 idle was unstable at lower air bleed setting.

One of the tricks often talked about, with the Tweecer crowd, is that you should see ISC duty cycle around 30% when done with idle tuning. That may not apply to mod motors. My TT331 is around 20% duty cycle.
 
Welll the only thing II have to add, is the base idle (IAC) disconnetoed, should be where the car will actually run. Its not a hard and set 600-650. Set it to the min rpm that it will idle at. Then you actual idle rpm that you set in the eec will be 120 rpm higher than that. The bigger the cam the higher the idle will be.

For tuning purposes you will need to keep the idle out of that hunting range. IE you will need the idle at a place where you will get a steady vacuum. Remember vacuum affects fuel pressure. So if you vacuum is surgin, so is your fuel pressure and its going to be hell to try and dial in.
 
Welll the only thing II have to add, is the base idle (IAC) disconnetoed, should be where the car will actually run. Its not a hard and set 600-650. Set it to the min rpm that it will idle at. Then you actual idle rpm that you set in the eec will be 120 rpm higher than that. The bigger the cam the higher the idle will be.

For tuning purposes you will need to keep the idle out of that hunting range. IE you will need the idle at a place where you will get a steady vacuum. Remember vacuum affects fuel pressure. So if you vacuum is surgin, so is your fuel pressure and its going to be hell to try and dial in.

I'm with Nacho :D Good info :nice:

I worked with my idle for a good bit and had several things going on such as:

O2's cooling a bit during idle :(
a very narrow lsa cam ;)
and most likely ... stuff I didn't even know about as well :rlaugh:

I've always found it so interesting that what works for one doesn't necessarily mean it is gonna produce similar results for the next.

Good thing we have access to most things needed to find what works best for our particular combo :banana:

Grady