Engine Idle problem that I can’t figure out


Sep 24, 2022
I know modified 5.0s are no stranger to idle problems. I’ve read many of the threads and search everything I can think of. So I’m down to making my own post. So here is goes: I have a highly moded 5.0 in a 65 mustang. I build the motor from scratch I started with nothing. The highlights are e303 cam, Trickflow heads, fuel rails, 30lbs injectors, throttle body, BBK MAF, stock computer with a Motes chip custom tune, and an Innovative wideband.

Problem: Motor runs great mid and top end.
After warming up the idle has a “stutter” it seems a lot like a miss but if it is I can’t find it. It will idle around 900/1000rpm then stumble and drop to 500/600rpm. When it does the wideband goes from 15:1 to 20:1 but smells rich. Problem gets worse in gear and way worse with ac on. When you put your foot in it if the idle is good the car is a beast, if the idle is at a stumble it falls on its face until mid range then lights the tires up. And it had no codes other then neutral switch which is not hooked up and speed sensor which is also not hooked up.

I have checked:
For a miss
For a vac leak
Check TPS voltage
Fuel pressure
I’ve swapped O2 sensors
MAP or BAP as it’s used
I sent the MAF off to be re calibrated
I added the LMR throttle body spacer

None of these have changed the issue. What ideas to you have? I’m down to thinking I have a computer issue. I’ve bought a Magasquirt plug and play but haven’t made the leap.
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Off the top of my head I would say a bad connection someplace.
Alternator good, no fluctuations in voltage?
Voltage is good. I’ve tried wiggling any and all connections. The only one that seems suspect is the MAF. That connector is junk. But I’ve tapped it it keep it secure. And it only seems to have issue at idle not a higher RPM.
I’ve tried messing with the MAF connection to see if I can change anything, it does not seem to which makes me believe it’s not that connection. I’m using the dash gauge for voltage, it’s a classic industries gage.
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The neutral switch tells the ECU whats going on.....If you have a tuning chip its best to choose "0" no trans controls Forced neutral state or hook the wires up to the clutch pedal with wires #30 and #46 also the VSS on pins #3 and #6 and allow the ECU to read speed and sense when the clutch is depressed...

Heres why...look at all the parameters for such a seemingly simple and meaningless task

Screenshot 2022-10-12 223355.jpg
Screenshot 2022-10-12 223312.jpg
Screenshot 2022-10-12 223226.jpg

If you choose #1 it still looks for a signal from the neutral switch/clutch pedal and if you hook the wires together the ECU will become confused.

Screenshot 2022-10-12 222424.jpg

The other thing to check out is the TPS circuit wires from the TPS plug to the ECU and the voltage going to the MAF using the red and black then red and battery neutral to see if voltages match. If voltages dont match you have a ground issue...


One thing I like to do is pin out the grounds using a DVOM and set to Continuity test and differentiate the different ground wires from the #20 case ground #49 HEGO pulse regulated ground ,,02 sensor heater grounds and the #40/60 ECU grounds using a DVOM on continuity test mode.

The ECU grounds on #40/60 pins on the ECU plug should be grounded to the cylinder head,,#20 case ground to the inner fender ,,#49 HEGO pulse regulated ground wire goes to the firewall and the oxygen sensor ground wires coming from the sensors itself get grounded to the chassis/frame......

If you have an aftermarket harness and the grounds arent seperated I'd pull the harness and address the issues,,

In the world of floating ground electronics there are no simple grounds..

They say to always use the Star Point grounding method....Its foolproof and stops ground loops which can become redundant..

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