Engine Base Idle Reset-Accufab

89notchDude

Active Member

Apr 11, 2022

84

42

28

central NJ

Jun 30, 2025

#1

• Jun 30, 2025

• #1

I've been having a surging issue lately causing rpms to drop enough to stall at a light or when i pull into my driveway. Setup is stock 302, gt40 upper lower/Accufab 75 TB with 70 pro-m MAF calibrated for 19lb inj (explorer type). I would like to do a base idle reset but Accufab is adamant about NOT adjusting the idle set screw since it has been calibrated at the factory. Is it advisable to just send it and adjust it or should I leave it alone? TIA

 

AeroCoupe

lube between the nut and the face. I know my lubes

Founding Member

Oct 28, 2001

5,793

3,927

183

Claremore, OK

Jun 30, 2025

#2

• Jun 30, 2025

• #2

Just adjust it and move on. They cannot expect the induction system to be lab perfect especially with the hundreds of ways we can plumb the induction.

Fuel Thread 'Accufab TB Adjustment'

Aug 5, 2019

This weekend I installed an Accufab 70mm TB and Spacer on my Fox. I checked the timing (13 degrees), and then set the TPS (.97).

The car runs great and pulls hard. Idle is right at 650-700rpm. But due to my cam, the motor "'almost" stalls out when idling. I'd like to up the idle to 750-800rpm. But in the Accufab directions it states:

"Technical Notes: Idle speed should only be set by adjusting the throttle position sensor. DO NOT adjust any screws on the throttle body. These screws are pre-set at Accufab."

Sooo...what's my preferred course of action here? Play with the TPS for a...

• My92cashtrap
• Replies: 3
• Forum: Fox 5.0 Mustang Tech

 

89notchDude

Active Member

Apr 11, 2022

84

42

28

central NJ

Jun 30, 2025

#3

• Jun 30, 2025

• #3

Thank you.

 

Reactions: AeroCoupe

gkomo

now i can hopefully expect to receive the shaft

Aug 2, 2024

2,124

1,365

133

San Diego, CA

Jun 30, 2025

#4

• Jun 30, 2025

• #4

Pardon my noobness, but if idle surging starts happening after it had been running right previously, wouldn't something else be suspect to cause it? Like a broken vacuum line or something? Isn't adjusting the TB screw like forcing the engine to stay running with a problem lurking elsewhere? I could be way off, i haven't had to deal with a surging idle or doing the base idle reset personally, yet.

 

Reactions: General karthief

AeroCoupe

lube between the nut and the face. I know my lubes

Founding Member

Oct 28, 2001

5,793

3,927

183

Claremore, OK

Jun 30, 2025

#5

• Jun 30, 2025

• #5

So the computer is looking for the amount of airflow that the original TB was allowing to pass at idle. My thinking would be that if the new TB is allowing less or more then you need to perform a base idle reset. From what I understand adjusting the TPS has zero effect on how the ECU controls idle. All the ECU is looking for is a range from 0.6-1.2 Volts with the TB in the closed position.

 

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

Admin Dude

Jan 4, 1985

42,921

21,097

234

Box behind Walmart

Jul 2, 2025

#6

• Jul 2, 2025

• #6

I would also thoroughly clean the IAC. As with [any] solenoid that does a repetitive thing, grime, dirt, and wear patterns can result.

With the new TB, that solenoid will operate in a different range. It's also super cheap to test this theory

Clean the IAC

I would only try moving that screw as a last resort once I was certain that the [rest] of the Base Idle Reset procedure were correctly performed.

 

foxbodybill89

Active Member

Jan 19, 2020

143

65

38

Sioux Falls, SD

Jul 4, 2025

#7

• Jul 4, 2025

• #7

I have not touched the idle screw on my Accufab and it idles perfectly. From all the reading I've done, the idle screw on an EFI engine with IAC should just be turned in enough that the butterfly doesn't stick shut. There's no adjusting idle (in stock manner), the computer decides through IAC modulation. The butterfly setting open too much will just conflict with what the computer is trying to do. Accufab knows what they're talking about.

 

gkomo

now i can hopefully expect to receive the shaft

Aug 2, 2024

2,124

1,365

133

San Diego, CA

Jul 4, 2025

#8

• Jul 4, 2025

• #8

I’ve watched a couple videos from Brew2L for the base idle reset and he does adjust the idle screw with the TPS/IAC unplugged. Then he shuts the car off and plugs it back in then the computer does its thing… I think.

 

89notchDude

Active Member

Apr 11, 2022

84

42

28

central NJ

Jul 7, 2025

#9

• Jul 7, 2025

• #9

I did the base idle reset and all is good so far. The car wouldn't run after unplugging the IAC initially so I just bumped up the idle screw until it would run, then slowly came to about 650 rpm. TPS was at .81. I had to drill out the screw hole a bit. Got it to .91. No more surging or stalling.

 

2000xp8

SN Certified Technician

Aug 8, 2003

8,015

1,613

194

NJ

Jul 7, 2025

#10

• Jul 7, 2025

• #10

Is your intake opening heavily ported?
You really can't and probably shouldn't run a 75mm throttle body on a gt40 intake that only accepts a 65mm TB.
Even if it is ported, it's still way oversized for a stock headed car.

 

Mustang5L5

That is…until I whipped out my Bissell

Mod Dude

Feb 18, 2001

43,170

17,870

224

Massachusetts

Jul 7, 2025

#11

• Jul 7, 2025

• #11

2000xp8 said:

Is your intake opening heavily ported?
You really can't and probably shouldn't run a 75mm throttle body on a gt40 intake that only accepts a 65mm TB.
Even if it is ported, it's still way oversized for a stock headed car.

Click to expand...

I agree. For the combo (especially being untuned) a 75mm is likely oversized. May have some tip-in issues. I actually believe this is the reason Accufab warns you not to adjust the screw because (since the blade is larger), stop screw adjustments need to be very fine as they will make a far greater impact than a smaller blade.

I have a 65mm currently on a HCI combo with a 70mm accufab on the shelf. Now that I can tune i'll likely put the 70mm on.

 

89notchDude

Active Member

Apr 11, 2022

84

42

28

central NJ

Jul 7, 2025

#12

• Jul 7, 2025

• #12

2000xp8 said:

Is your intake opening heavily ported?
You really can't and probably shouldn't run a 75mm throttle body on a gt40 intake that only accepts a 65mm TB.
Even if it is ported, it's still way oversized for a stock headed car.

Click to expand...

Sorry. I miss-typed. Im running a 75 MAf and 70 TB.

 

2000xp8

SN Certified Technician

Aug 8, 2003

8,015

1,613

194

NJ

Jul 7, 2025

#13

• Jul 7, 2025

• #13

Well, that's sounds better.
Is this a GT40 tubular intake or an Explorer gt40 style intake?

 

89notchDude

Active Member

Apr 11, 2022

84

42

28

central NJ

Jul 7, 2025

#14

• Jul 7, 2025

• #14

2000xp8 said:

Well, that's sounds better.
Is this a GT40 tubular intake or an Explorer gt40 style intake?

Click to expand...

Explorer style. I believe the actual opening is about 67mm on those.

 

AeroCoupe

lube between the nut and the face. I know my lubes

Founding Member

Oct 28, 2001

5,793

3,927

183

Claremore, OK

Jul 7, 2025

#15

• Jul 7, 2025

• #15

If the opening on the intake is smaller than the throttle body you really should port the intake opening to match the throttle body. If you do not it will create turbulence and if substantial enough will cause part throttle tip in or so I have read over the years. Nothing fancy just knock the leading edge down and taper it back into the throat of the intake an inch or so. IF you are not having any issues then I would just leave it alone.

 

Reactions: General karthief

Scrapla347

Active Member

Apr 23, 2020

200

67

38

Florida

Jul 22, 2025

#16

• Jul 22, 2025

• #16

Is there a way to reset an Accufab stop screw back to factory specs without sending it back to them? Can you use a feeler gauge around the blade? I heard they come adjusted with a .010" gap around the edge of the blade which they adjust on a light table. I have one that's been messed with and I'm trying to avoid sending it back to be recalibrated.