Engine Random hanging idle

7

706 Saleen

New Member

Feb 10, 2026

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Denver, Colorado

Feb 11, 2026

#1

• Feb 11, 2026

• #1

I have a 88 Saleen… Cali mass air, rebuilt 347, e303, BBK 70 mm TB with Pro M 77 air meter calibrated to my 30 lb injectors… Bisanni equal length into BBK 2.5 high flow cats, SuperChips chip… engine had 8-10 k miles before this problem started…
While driving random times after start, cold or warm, engine rpm hangs at 2100! Since the issue, removed chip, installed new coolant and air charge sensors, EGR, BBK TB with new TPS, new BBK calibrated mass air meter…. Yes base idle reset, TPS set to .87v…. Timing at still at 13
-If I shut car off, problem goes away for a few minutes.
-if I unplug IAC idle drops to normal, plug it in and idle jumps back up
-if I unplug TPS, idle drops to normal, plug it back in idle stays normal.. drive the car and within a few minutes, idle jumps back up.

 

8

86HO5.0

Active Member

Apr 18, 2023

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Feb 11, 2026

#2

• Feb 11, 2026

• #2

Did you pull codes/DTC's?

 

Mustang5L5

That is…until I whipped out my Bissell

Mod Dude

Feb 18, 2001

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Massachusetts

Feb 11, 2026

#3

• Feb 11, 2026

• #3

86HO5.0 said:

Did you pull codes/DTC's?

Click to expand...

I'd start here as well

Failed VSS would cause a hanging idle while driving as you come to a stop. There would be a code 29

 

General karthief

wonder how much it would cost to ship you a pair

Mod Dude

Aug 25, 2016

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polk county florida

Feb 11, 2026

#4

• Feb 11, 2026

• #4

If the engine was running when you pulled the plug on those sensors you caused a code to set, so checking codes now would be really confusing, you need to clear the codes and drive the car around in various conditions then check codes with engine at operating temp.
Just my opinion.

 

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7

706 Saleen

New Member

Feb 10, 2026

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1

Denver, Colorado

Feb 12, 2026

#5

• Feb 12, 2026

• #5

86HO5.0 said:

Did you pull codes/DTC's?

Click to expand...

Yea… while idle was at the 2k, I received high idle code…. No bad sensor data. After shutting car off and retesting codes… there was nothing noted

 

KRUISR

10 Year Member

Apr 16, 2015

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Feb 12, 2026

#6

• Feb 12, 2026

• #6

706 Saleen said:

-if I unplug IAC idle drops to normal, plug it in and idle jumps back up

Click to expand...

Idle should drop further than normal. Down to around 600 rpm or so. If not, idle should be reset.

706 Saleen said:

Yes base idle reset,

Click to expand...

How did you do the base idle reset? Many don't do it properly and wonder why it doesn't improve things.

 

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AeroCoupe

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

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Oct 28, 2001

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Feb 12, 2026

#7

• Feb 12, 2026

• #7

This is the proper way to do it:

Post in thread 'Help me create the "Surging Idle Checklist"'

Apr 10, 2005

OK, now you have gone through the list and eliminated most of the possible problems. Now your mechanical and electronic problems are fixed, you can set the base idle speed.

Setting the base idle speed:
First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. If you have a wild cam, you may have to raise this figure 100-150 RPM or so. Then the electrical signal through the IAC can vary the airflow through it under computer control. Remember that the IAC can only add air to increase the base idle speed set by the mechanical...

• jrichker

 

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KRUISR

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Apr 16, 2015

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#8

• Feb 12, 2026

• #8

One further detail to that first line - set idle below 600 RPM with IAC and spout connectors disconnected. The lower you can set this and keep car running the better IAC can control idle.

 

7

706 Saleen

New Member

Feb 10, 2026

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Denver, Colorado

Feb 19, 2026

#9

• Feb 19, 2026

• #9

All items listed in above posts , I have done after new BBK TB and TPS. Old TB had a loose sloppy butterfly.
Car still gets the random high 2k idle! Anyone have info about TPS voltage spike above 5vdc? I’ve heard if that happens, ECU will go into a weird program mode(limp mode?)
I tested my BBK TPS… seems to swings nice from .8 to 4.8 at WOT… according to my voltmeter. However putting my Motorcraft TPS back in, car has much better drivability and response! How inaccurate are the BBK TPSensors? Could that have caused a voltage spike to the ECU?

 

8

86HO5.0

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Feb 19, 2026

#10

• Feb 19, 2026

• #10

If the ECU sees TPS voltage high enough for it to react, it should trip a code.

Run the Motorcraft TPS if it seems to work better. Personally, I've always ran MC sensors whenever possible.

 

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