Aloha all, I've got a strange intermittent issue that I'm unable to diagnose.
2004 4.0l sohc manual 4x4

Normally if I am driving and come to a stop(braking with trans in neutral), the rpm will slowly drop to idle as I reach a stop, settling at ~765rpm.

So here's the symptom: As I approach a stop, the rpm will drop as I slow down. Once I come to a stop, the rpm hits 765 then begins to steadily climb to 1800 over the course of 1-2 seconds. It will do this at every stop, and will hold 1800 rpm as long as I am at a stop. While driving, it operates normally, rpm drops between shifts.

Some notes:
If I restart the truck the issue goes away.

Symptom has occurred 4 times in the past 3 months

Scanner says iac valve is in preposition while exhibiting this symptom. Normally it enters ctrl learn at idle.

Iac valve is known good, however scanner is showing 70ish% open during symptom, usually 40ish% at idle.

I got out and looked at the throttle body and it's completely closed.

Egr and vacuum systems are solid. No leaks.

Any help would be greatly appreciated, mahalo!

 

Sounds like a bad IACValve.
Have you tried removing+cleaning the IACValve with CarbThrottleSpray? If that doesn't help then replace it.
Hope this helps. More info on the IACValve at
http://2001fordexplorersporttrac.fan...toControlValve

 

Thanks Dillard, I thought the same thing. However I have experienced this symptom with both the original ford IAC as well as a new ford IAC. When I did the timing and top end rebuild I replaced all the sensors I could get OEM at a reasonable price and the IAC was one of them. Seems like a longshot that both an old working IAC and a new IAC would exhibit the same issue. Since the issue "fixes itsself" when I restart the truck I'm leaning toward "it's a software thing". I'm checking today to see if there's a PCM update that the previous owner never had installed.

I do like the idea of lowering the startup surge rpm. Do you have any experience with modding the IAC as in your link?

 

Check engine/coolant temp with scanner when RPMs are going up at idle, could be computer is seeing a higher temp so is increasing water pump speed/circulation to lower it
That could be from bad ECT sensor or failing fan clutch

 

Thanks Ron, engine coolant temp is around 190 on the scanner, middle of meter on dash. Fan clutch is new. Also new thermostat, housing, and ECT sensors went in when I did the timing. Is there a test I can do to see if the fan clutch is malfunctioning?

 

Yes, did that IACValve tune on my 2001 ExST; it is still working well currently, so I'm happy with it.

PCM software doesn't change, gotta be another cause?

 

Is that 190deg when idle goes high?
You are looking for an intermittent issue

Yes, you can test fan clutch when engine is stone cold
Pop the hood and try to spin the fan, should be hard to spin, cold Locked
Start engine and count to 20, shut engine off, spin fan again, should be unlocked, easy to spin

After driving for the day(temp gauge at just below 1/2), when you get home shut off the engine and spin fan, should be hard to spin, warm lock

 

Temp was 195-198 during the symptom.

I tried to spin the fan after a long drive and it spun fairly easily. About 5 minutes later it had some resistance but I could spin it with 1 finger. I'll try the cold spin tomorrow morning.

 

Fan clutch's spring is heated by the center of radiator, so radiator needs to have good flow in the center

Fan clutch has a silicone oil inside, when its cold its thick which is a "cold lock"
Running engine for 10-20 seconds thins it out, so unlocked
Unlocked should spin easily, meaning you can spin it and it will continue to spin a few blades more after you release it

When the spring is heated it expands and turns a valve, this closes off passages where the oil can flow freely, as there is more and more restriction of oil flow the blades spin closer and closer to water pump RPMs, locked
So if you try to spin the blades they will not spin, you can move a blade but not spin the fan

If ECT temp is showing a steady warm temp then that's not why computer is raising RPMs, so not fan clutch issue

If IAC Valve was showing 70% at idle with no IAC codes then computer is setting IAC at 70%, so computer issue
P0505 Idle Control System Malfunction
P0506 Idle Control System RPM Lower Than Expected
P0507 Idle Control System RPM Higher Than Expected
And
P1504 Idle Air Control Circuit
P1505 Idle Air Control System At Adaptive Clip
P1506 Idle Air Control Overspeed Error
P1507 Idle Air Control Underspeed Error
P1508 Idle Air Control Circuit Open
P1509 Idle Air Control Circuit Shorted

So computer should set a code if IAC Valve is not responding as it should

Unplug IAC valve and see if you get an IAC "circuit" code

EDIT
Just had another thought
Voltage, the 1,800RPM made me think of that
Watch system voltage, should never fall below 13.5volts with engine running
If you lose a Field on the alternator voltage can drop under 13.5volts at idle
2004 does use "smart charging" so maybe its telling computer to kick up the idle when voltage drops too low
1,800 engine RPMs is when an alternator can output maximum amps/volts

Longshot but not a no shot, lol

 

Thank you Ron, now that I understand what that fan clutch is and how it works a bit better I can confirm it is functioning normally.
I like the voltage idea, I just did a load of electrical work a couple months back. I'll monitor voltage and see if it differs during the symptom.
My scanner has multiple pids for voltage. One from GEM, one from ODBII and one from PCM.
At idle:
PCM reads 14.3 volts,
ODBII reads 14.2 volts and
GEM reads 13.5 volts.
DMM on battery reads 14.3 volts

Battery is 1 year old, alternator 3 months old. I ran fresh grounds with 1/0 welding cable to chassis/block/frame.