IAC Voltage

[JerrySab]

Hi all!

I'm trying to zero in on (yet another) slightly high idle issue. 88 2.9l idles at 950-1100 when warm. Every so often, infrequently, it'll drop down to 800/850, which I believe is the target idle when warm. I'm curious if there's a break along the ECM wire going to IAC, or something that would be causing higher resistance or otherwise messing with the data stream that's preventing it from fully closing. Also, wow the gas mileage is terrible. SO I suppose the technical question is: does a higher voltage signal cause IAC to open further (and thus raise idle)? Or is it the other way, a lower voltage signal would cause IAC to open further?

My guess is a full 12v would open IAC widest, so no resistance. Further, a closed IAC would receive either low or lessened voltage. Or none at all? It's really not a super significant issue, other than I'm super motivated to get better MPG now that gas is a pound of flesh/gallon.

I'm 99% sure it's not a vac leak. Truck idles at wherever throttle screw is set when IAC is unplugged. I have 18"+ compression, steady. IAC is Motorcraft. Only front of mind culprit could be bad alternator/voltage regulator. It's very squeaky, which could also just be bearings. I have a MCR upgrade that I haven't installed yet.

Thanks friends!

 

[RonD]

18" of vacuum

Could be dirty PCV Valve and its sticking now and then

Computer sets IAC Valve voltage/RPM based on engine coolant temp, so ECT sensor
If ECT temp drops from 180degF to 120degF computer would raise RPM to match temp

Yes, warm engine(above 150degF) target idle for automatic is 800 in Park

"Throttle screw" is called the anti-diesel screw on fuel injected engine
And you need to set and never touch it again
Warm up engine
unplug IAC Valve
Turn the screw until idle is under 600rpm, so barely running, this also sets the TPS(throttle sensor) hopefully, under 1 volt
Which could be your issue


IAC Valve gets 12volts on the red wire, key on full time, 14volt with engine running
The other wire, computer wire, is the Ground, computer pulses the ground which in effect changes the voltage in the IAC solenoid
More pulses is lower "voltage" and lower RPMs, less pulses is higher RPMs

Key on engine off you should see about 11volts
Use a sewing needle a pierce the computer wire, put volt meter on battery positive and the sewing needle and you will see IAC voltage engine running, test on cold engine then warmed up engine

IAC Valve is a monitored circuit, computer "learns" what voltage sets what RPM, so you would usually get a code if RPM didn't match voltage at IAC Valve


Yes, test your system for AC Voltage, shouldn't be any of course
An "Alternator" makes AC Volts, but its changed to DC volts by Diodes in the alternator, if a diode fails then AC can "leak in", causing random issues with computers, sensors and controls(like IAC Valve)

With engine running set volt meter to AC Volts, put 1 meter probe on alternators metal case and the other probe on the "B+" terminal, the stud/nut terminal, should show under 0.4v AC

 

[JerrySab]

Ok, legendary wisdom once again, Ron. Thank you. In response to the below:

IAC Valve gets 12volts on the red wire full time
The other wire, computer wire, is the Ground, computer pulses the ground which in effect changes the voltage in the IAC solenoid
More pulses is lower "voltage" and lower RPMs, less pulses is higher RPMs

If this is true, then a bad ground wire could conceivably cause a higher RPM, right? I'm going to explore all of your suggestions, but also just wondering if my train of thought is accurate. Poor old truck sat out in the hot heat, seems to have some dry brittle insulation.

 

[RonD]

Yes, if computers IAC Valve wire was shorting to ground then RPMs would go up, but way up, 2,000rpm
950-1,100 is "normal" Cold Engine idle which is why ECT sensor came to mind

 

[JerrySab]

Ok great. Would out of spec thermostat contribute to this? I replaced ECT not too long ago, and made sure not to get the foreign knockoff version. PO replaced thermostat but I'm suspicious it's opening at the right temp.

Also, does temp sensor for dashboard have any effect on engine ops? I noticed the insulation is badly decayed, with wire exposed for the lower sensor, but my understanding was that didn't have any dialogue with cpu

 

[RonD]

You are correct, the dash temp gauge uses a 12volt Sender, no connection to the 5volt ECT sensor used by the computer