4.6 2v SOHC 2k rpm Idle, help reading engine data


New Member
Jan 5, 2020
Hey guys, I own a 2002 automatic mustang gt with:

flow master CAI
flow master H pipe
just had an engine swap done with a rebuilt
78mm bbk throttle body
SCT/Bama tune

I’ve had about 900 miles since rebuild. Originally when I got the Car back from the engine install it had the same problem, 2k rpm idle in park and neutral. When I shift to reverse or drive it drops to 1.2k rpms. I fixed it before by taking off the new aftermarket iac and putting the motorcraft one on. This brought my idle to 1-1.2k in all gears. The other day when I was driving home on the highway (I pressed the O/D off) because I didn’t know what it did and I cruised On the highway with it, it started these idling problems again. I had codes for iac load over duty, iac circuit malfunction, and camshaft position sensor. Since I swapped the TPS and the IAC I haven’t had the two iac codes pop up. I disconnected my battery, drove it about 40 miles, I will scan today again and see what other codes. When I unplug the iac when the motor is running, the idle does not die (non black vent iac)
My question is this, what vacuum lines must I check, and what is my next step? Here is some engine data from my scanner, I don’t know really how to read it.


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Active Member
Jul 12, 2018
I’ll drop a few things quick in case you still are looking for opinions.
So, this has not been running quite right since it was first fired? 900MI.
Have BAMA‘s “Tunes for life” with your SCT device? If Y, BAMA will Datalog, free, if the issues in the program, they’ll edit it. Have a high & low octane tune? Try both?Tuning/editing is last step, after all other components are ruled out.
But, if you have it- free Datalogging without them modifying the tune is an asset you can use, they may see something useful.
You can call & find where they set your idle for, it’s been an issue since their Tune was added-yes? This is a custom tune, not a canned tune- right? Some SCT Tuners allow you to tweak adjustments, including idle
(Only FYI).
No idea of your Cam/Stall-build, 1,200RPM idle is high, unless, your Cam & Converter-build necessitates this..I’m interested in what you actually have for aftermarket components, will really thin out this vague response.
An engine pic, build info will help answers more specific.
I’d suggest not adding any more parts unless they test as bad, and only OE parts- like your TPS.
Any aftermarket ignition components?
Did you pull your MAF connector while running? Car should almost, if not-stall.
Any change in the idle when you unplug the IAC? No erratic, lower Rpm?

Blast your MAF sensor with some CRC MAF sensor, spray-no contact.
Drop Fuel pressure to 35-37lbs, have a rail pressure gauge?
Hook up a vacuum gauge on the motor, what’s it read, at what idle speed?
Did you set the idle of your TB ,or leave as received from the Mfg? There’s a method to set base idle correctly.
Check and recheck your vacuum line routing ? Have a shop manual for the Car?
Did you install the intake? The stock PI intake, it may help if you retorque to spec’s
Certain the EGR is 100% closed at idle.
Voltage at TPS less than 1 Volt at idle?
Check your Alternators output, 13.5-14.3VDC, Battery Min 12VDC, engine running..
Check your TB for proper opening closing, no binding, Vacuum leaks in/on it, and the CAI at all points.
Your MAF is flanged- correct? If certain aftermarket “bullet” types are used, you can “clock”(rotate) the MAF slightly, sometimes it improved airflow across the elements, improving idle quality..
What MAF Mfg? Was it for the correct injectors being run & the CAI/Filter shape you’re running?(Pro-M).
Vacuum leak locating
Can use a smoke tester, or a Can of non flammable Solvent with the straw inserted to carefully seek vacuum leaks around lines, flanges, gaskets. etc. engine rpm will drop if it’s pulled in. I use a smoke tester, or my micro Aluminum chamber Butane torch to do the same, DONT recommend it as you need to be very cautious.
Test any Diaphragms with Very low vacuum from a Mityvac, verify they hold air..
Anywhere from your MAF to the Heads should be checked for vacuum leaks, the MAF’s job is to meter all air entering the engine & report it to the EEC to set injector pulses for correct A/F ratio.
Every gasket, line, sensor, accessory involved should be checked. A single Valve hanging open slightly can cause the 02 to reflect an overall lean condition in that Bank on a running engine,.valve hangs open & pulls in air when it should be closed-Airtight-
A loose or unseated spark plug is one I’d just learned of is another possible leak path.
Scantool readings show near a 1,600rpm idle, and in open loop, cars not warm enough (ECT, 115F)) to be in closed loop.
The data is extremely useful when streaming, engine running. Freeze frame data records data for a good period before & after when a DTC pops, review it slow afterwards, frame by frame- see what changed, ets easier to picku after Doing it a few times. Drive cycles, rest for intermittent issues that throw no codes.
A Touch on Scantool lingo, Tuning
Read an OBD-2 Ref Book, Seek online tutorials, OBD-2‘s meanings are pretty much ditto for all Mfg’s.
LTFT, STFT, ECT, etc. FT’s are fuel trim percentages, short term, long term, B= bank.
FT represents constant fuel% correction via 02’s & other sensory inputs to programmed EEC values, tweaking injector pulses so AFR’s stay on target. ECT=Engine C Temp (2 sensors exist. 1 Gauge, 1 EEC)
Both #s constantly correcting, e.g., Closed loop, 02 sensor values not swinging back/fro , an issue exists.
Open loop sensors used (MAF, etc), not all, (No 02’s, etc) mostly pre-programmed spark & fuel tables, EEC running closed loop utilizes all sensors*.
* Open loop tables used again @ WOT..
Lot more to this, just examples.
SN has a Digital Tuning forum. Use that resource, great bunch of knowledgeable
& helpful members.Software to make your own tunes on your laptop, to your SCT Tuner for your Car.
Good luck!


Active Member
Dec 3, 2009
Dayton, OH
Sounds like you have a vacuum leak or your throttle body idle screw is screwed in too far since you can unplug the IAC and the idle is still high. I'd also look and see if your bbk throttlebody(junk by the way), has an idle bleed screw. Make sure it is closed and not allowing vacuum to bleed. Good luck. Try returning your tune to stock to see if it is a tune issue or not. If your issues go away with the stock tune, find a better tuner.