97 XLT 4.0 OHV 4x4 automatic
Roughly 144,000 miles

Long time reader, first time posting. Love my ranger but I've been chasing a squirrel for over a year now! This might be a long read, i like to give as much info as possible..
Been having a random stalling issue that's getting progressively worse, along with a slight hiccup in the rpms. First couple of times it would stall at a hard stop. Only happened once in a while, usually started right back up and ran fine. Twice i couldn't get it to crank (seemed like lack of fuel) but once it was towed and parked then would start again just fine. Then it began to stall while idling in gear at a stop light with basically no warning. Also, i would be waiting in a line and noticed rpms start to stutter a bit then stall out. Sometimes i could catch it with a tap on the gas pedal, and crazy as this sounds i noticed if i kicked on the ac to bump up the load it almost seemed to help from stalling.. twice it stalled on cold crank almost immediately. Also, I'll start the truck and let it warm up a good 10-15 min then it sometimes stalls out before ever putting it into gear. All of these instances have never thrown any codes until I've unplugged something to make sure it would throw a code..
Now besides all that, when driving it seems to try and stall while accelerating - hard to explain - like pulsing/ stuttering then catches itself if I'm still above 30mph or so. Unplugging the egr vac line seemed to help initially, but i think that was coincidental.

I've visually checked and/ or replaced the following throughout all of this :

Fuses, relays, connectors, grounds, battery
Vacuum hoses, pcv, brake booster
Evap system
Tps
Ect and intake air temp (verified readings with scanner)
Omitted the inertia switch altogether (connector was questionable as well as the ball inside was a bit corroded - once i did this, the hard stop stalling actually stopped happening)
Throttle body is clean
Maf, iac, egr, ckp all replaced (not at same time)
Fuel pump, filter and injectors have all been replaced
Fuel pressure regulator seems to be okay
Ignition coil, spark plugs and wires were new as of last year

I'm sure there are more things I'm forgetting about.. and there are other random issues that may or may not be related to all of this. There is carbon build up all throughout, i have tried the motorcraft carb cleaner a few times to help burn some off.

At this point i want to pull out the ECM and look for any obvious damage. I'm really thinking the brains are malfunctioning and sending bad signals.

I don't have a multimeter or know how to use it for these applications (could learn easily with some good guidance though). I attempted to test fuel pressure with a loaned gauge, read at 35ish while engine running.

What else can i check at this point?

Thanks for taking the time to read this!

 

Welcome to the forum, well first post on the forum

Idle level is decided by the computer(PCM) using temp data from ECT sensor and RPM data from Crank sensor, and then pulsed voltage to IAC Valve to open it

ECT and Crank sensors are pretty standard sensors not really brand specific
IAC Valve on the other hand needs to be Motorcraft or Hitachi brand as those are the only two that are true solenoid valves, other brands are stepper motor type, so WILL cause issues, period
And the IAC Valve ultimately sets idle, and idle control should be +/-4 rpm so its very very precise, should never "wander" up or down
The computer "learns" IAC Valve voltage for RPMs, so when you change IAC Valves idle may wander a bit but that should go away after 3 drive cycles

Yes, turning on AC should bump up idle by 50-75 rpm, shifting from Park to R or D should do the same

For now try to reset the Anti-Diesel screw on the throttle linkage, it looks like an idle screw
After engine is warmed up and idling. unplug the 2 wire connector on IAC Valve
RPMs should drop to about 500-600, engine may even stall, either is GOOD, it means no vacuum leaks
If engine stalled turn Anti-diesel screw 1 full turn clockwise(tighten)
Plug IAC Valve back in and restart engine
Once its idling again unplug IAC Valve again, see if it stalls, it shouldn't, adjust anti-diesel screw to get stable but low idle, 600rpm is fine
Plug IAC Valve back in and idle should go up to 750-800rpm for automatic, 650 for manual trans

After the adjustment you will want to check Throttle sensor with OBD2 reader
With key on engine OFF
TPS should show 16-19% throttle closed
Press gas pedal down to the floor all the way, TPS should show 90-93%, if lower your throttle cable is stretched, Google: Ranger throttle cable mod

Also watch TPS % as you press gas pedal down slowly, should be a nice steady increase no jumping up or down
The TPS was added as the "accelerator pump" replacement when they changed from carb to fuel injection, it gives the computer an instant "heads up" that driver want to accelerate(or coast), the MAF sensor is way to slow to give an instant heads up
The Coast part is a big fuel saver, the computer will shut off injectors when coasting, TPS under 20%, until RPMs are under about 1,400, then it will restart injectors at idle levels
With a carb you actually sucked in MORE fuel when coasting because of the higher negative vacuum in the intake, the downside of Jets, lol


Long shot for stumbling and stalling is exhaust blockage
If a Cat or muffler has broken apart internally, parts will shift around, higher exhaust flow can push parts "open" but when RPMs drop and flow is less the parts "close up" and engine stumbles and/or stalls

Fuel pressure issue usually shows up at higher demand, i.e. highway speeds and limited power, exhaust blockage can show same symptom as well

EGR system is monitored, so absence of EGR codes would tend to take that off the table

PCV Valve is a controller "vacuum leak", PCV valve should be cleaned or changed every 3 or 4 oil changes

 

First of all i want to thank you for all the advice you've given to me and everyone else !

Funny you mention the IAC -
Part of a back story: my grandmother bought this truck new, and i acquired it in 2014 with only 37,500 miles! So I'm the driver that inevitably broke it in. The iac was actually one of the first components i had to replace within a few months. I belive it was a true motorcraft replacement but can't confirm because someone else ordered the part. I do try to get oem parts and fluids as i know these rangers are finicky with most aftermarket parts.. i also had the leaking lower manifold gaskets causing issues (never stalling tho) and finally tackled that job in 2017.

whilst I've been chasing this stalling issue, i did try the disconnect iac and rpms did drop like they should (no stall) but i don't recall exactly why i decided to change it out again- i think i was hearing unusual air flow or so it seemed..but i did put a delphi brand on it recently and reluctantly. I don't know if that is why I'm still having unpredictable issues at all speeds and hot/cold etc.. esp because since changing that part, i have done so many other things..!

yesterday was when i installed new fuel injectors and when i went to crank it, wouldn't stay on unless i hit the gas because i didn't fully seat the connector back into the iac hah! Once i plugged it in, the truck stayed on. That's also one instance where i then just let it idle about 10-15 min and then it stalled.

I often save parts I've replaced because I'm not 100% certain they actually failed- so for fun just now i put that old iac back on and it has apparently stopped functioning as the truck stalled out immediately unless i hit the gas. So i put the delphi back on. I let it warm up and then unplugged and as expected it did stall.

Before i go turning screws in the throttle body I'm tempted to replace the iac once more with the preferred brand..

I also did the pedal check on tps. Steady 18.8% closed and goes to 90% pedal to the floor. Stays consistent with pedal motion- no jumping or erratic fluctuation.

This truck has been kept fairly pristine and was parked in a garage it's whole life until 2020 when i moved to Florida. I've done most of the work on it myself (super proud lol) and would like to keep it going for as long as i can!

 

Engine needs to be fully warmed up to test IAC valve

When you start a cold fuel injected engine without touching gas pedal, it should REV to about 1,500 then drop to 1,000rpm
The REV is because computer has opened IAC Valve all the way for start up, should do that every time it starts, then it starts to close it to set target idle
Then as engine warms up idle will start dropping slowly until its at 750-800 for automatic

That's the computer, ECT sensor and IAC Valve all doing what they should
ECT sensor is the "choke" sensor, it tells the computer that the engine has been off for awhile, so cold, and needs to be run Rich with high idle(choke) until it warms up
Any temp under 120degF requires choke, the colder it is the more choke is needed

 

I should not have said i expected it to stall- but I'll be a bit more specific just in case..i wanted to edit that response a few times lol. Changed the valve in December to the delphi and that's actually when stalling got more frequent, random and progressively worse. Hmmmm..! Had pulled connector off a few times since then (fully warm) and it bogged but never stalled before i reconnected it.
Today i hooked up the old iac before a cold crank and truck immediately died unless i hit the gas. I would assume the part is definitely bad now based on that..? So i reinstalled the delphi valve I've been using since December - started just fine. I let the truck warm up about 10 min before removing the connector. Maybe i could have let it idle a few more min but my dash gauge was basically where it always sits when warm. It didn't stall immediately after unplugging- it bogged down like all of the other times I've done this, but i left it disconnected longer this time - maybe 20 seconds before it finally stalled vs other times were less than 5 seconds when i plugged it back in.

Cold crank rmp- I've noticed a range from 2-10 and up to 30 full seconds (usually under 3) where it seems the rpms will be ever so slightly lower than what it needs to be at- like there's a delay in communication and then it adjusts itself. I haven't tried to capture this on my scanner live data to see how much it truly fluctuates but maybe i will try that tomorrow.

So is it possible that the delphi valve isn't "defective" so much as just won't work well with my engine? I wanted to get oem originally and don't mean to just keep throwing parts at it.. but I'd be willing to try another iac before paying for full diagnostics somewhere

 

If you turn on the key, any time, warm cold doesn't matter, and plug in any IAC valve to the 2 wire connector you should see the valve move in about 3/8", then unplug it and it should move back out, repeat as much a you want
Key on engine off opens IAC valve all the way
You can do that with the older IAC valve to see if its moving

3rd party IAC valves are not "defective" or bad they are just not solenoid valves, Motorcraft or Hitachi brands are solenoid valves
3rd party valve might be the problem but a motorcraft or hitachi valve for sure won't be the problem if there is more stalling

Computer should react instantly if RPMs drop under 750, and it is odd its not setting a code
P0505 Idle Control System Malfunction
P0506 Idle Control System RPM Lower Than Expected
P0507 Idle Control System RPM Higher Than Expected
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

Its like the computer doesn't "see" anything wrong
I assume code is set after unplugging IAC Valve in previous tests?

 

The only time i got a code for iac was Monday when i didn't fully seat the connector after the 3rd attempt to crank, i believe it was p1504.
i tried what you suggested about key on engine off plugging into iac with both old and new valves. Neither of them moved when i did this.. what might that indicate?

 

Also, my rpms in gear fully warm are usually a bit lower than 750 from what I've noticed. More so between 680-730 ish.
It does Idle around 900 in park. (*edit* i had to verify these numbers)

I only recently got the scanner to show me the values, so I'm not sure what the numbers might have been before installing the delphi iac- i can't exactly recall when the Idle started seeming low

 

Out of curiosity, how does the thermostat play in to all of this? I've noticed when the surge/ stall happens when i simply idle in park after basically warmed up, that the temp gauge drops a bit.
i know that was another part that needs to be a specific rating (opens at 180 or 190 degrees?) or brand..
perhaps it isn't opening fully or at the correct temp indication?

 

Only when temp is under 150degF would it effect idle RPMs, i.e. choke mode

If you have a volt meter set it to DC Volts and test the 2 IAC Valve wires, red one is positive, should see about 11-12v key on/engine off

Have seen working IAC Valve's not move on that test, its lack of vacuum, valve should have high vacuum pulling on it in normal operation
Try pressing finger or screw driver against it when powered and off

 

I have acquired a multimeter.
11v at connector

there is no change in pressure when i manually move the valve - i moved it while unhooked completely, connected with key off, and connected with key on engine off. It moved very easily with same consistency in all instances

 

That is odd, with 11v applied it should hold valve in one position, open a bit

They are fairly simple solenoids, seen here: https://www.ford-trucks.com/forums/a...?postid=848276

The amount of voltage creates a magnetic field that pulls a metal rod connected to the valve, not rocket science, lol

 

I tried once more just in case to see if the valve moved when connecting/ disconnecting while koeo and still did not move on its own, and no pressure change in the solenoid when i manually moved it while connected. Stays fully closed

Looking at the old iac again, it bounces back to slightly open (not connected to anything) when i move it, so if it never fully closes is maybe why it failed..?

What else can i check? Now i want to get another iac just to see if it moves when i connect it lol. Maybe that's the "delay" i tried to describe when it takes a few seconds to even rpms out when i cold crank?
but you were saying it has to be open initially for the truck to start at all..?

 

i used this super helpful video to verify voltage accuracy to the tps



Now that i have the multimeter i can test some ground wires..i checked the main battery ground and seems good. When i check my tps connector, it appears i have resistance! I'm still learning how to test for this to be sure.

 

If you have an OBD2 reader you can see what TPS is doing with that
Should show 16-19% with throttle closed
Then slowly go up to 90-92% with throttle wide open
No jumping or dropping

Its actually better to test this way since you are seeing what the computer sees

TPS is a 5 volt sensor
It should show just under 1 volt throttle closed, 1volt is 20% of 5volts, so just under 20% is what to expect
Wide open throttle should be 4.5volts, 4.5v is 90% of 5 volts
TPS should never show 0% or 100%

 

I used the scanner to verify tps% like suggested before, 18.8% closed and goes steady and smooth to 90% @ wot and back down to closed.
double checked for resistance among some of the connectors and grounds all seem good - tps was in fact 0 ohms and iac, egr solenoid, & dpfe were around 1 ohm or just under with koeo (this is acceptable, yes?) I tested those others because they shared the split off from the main harness..

I did pull out the ecm and opened it to look for corrosion or anything suspicious, but was not able to see the underside with the diodes and capacitors etc because i didn't want to break the screws. Could still be a possibility something is compromised, but I'm not buying a new one just for fun lol.

Left the negative battery cable off over night to try to reset. Started the truck today with the intention of letting it idle 15 full min without putting in to gear or turning accessories on, but it stalled out about 7-8 min after cranking. Everything seemed normal as it ever is, rpms had steadily come down to my usual 900ish idle. If i know the stall is coming and i rev it a bit, obviously then it doesn't stall. But it usually happens so quick that by the time i notice starting to bog down, it'll have already died at that point. Still cranks right back up just fine.. sometimes stalls again within a few min, other times I'll let it idle for 20min or more without another stall occurring..

I am constantly looking online for any and every similar experience with the random stalling to see what may or may not have caused/ fixed other's problems. TONS of dead ends and unsolved inquiries.. I finally came across a few articles/ videos that inevitably pointed to recessed valve seats. Other than compression tests, i think i have basically done ALL the things others have tried before tearing the motor open to figure this out. I read about the "dollar on exhaust test" that i have not tried (which is also not guaranteed to be the "ah-ha!" that I'm looking for).

compression testing and better live data (i have a basic obd2 scanner) are things i do not have access to. I suppose the only way to truly verify recessed valves would be to actually look at them.

 

Recessed valve seat issue couldn't cause sudden stall, would cause misfires
And Recessed valve seat issue was 2004-2006 3.0l engine issue, not 4.0l

For a V6 engine stall at least 3 of the cylinders would need to misfire at exactly the same time, and even then it may not stall just stumble

I would set anti-diesel screw to maintain idle at 650-700RPMs so engine never fully stalls, if that works then you will know the issue is IAC Valve not a glitch in spark or fuel

 

Thank you for the clarification, i feel better about that now!

Okay so- Inferior iac valve is still a possibility that i would be willing to replace again. Getting hard to find, but seems rockauto still has the motorcraft avail (dealer near by said back order that could take a month to arrive)

I guess at this point, I'll get that installed and report back whether it fixed the issue or not.

 

I would get one from wrecking yard, motorcraft IAC Valves just didn't fail that often, so pretty safe bet
They were often replaced by DIYers so make sure its the factory one
Same IAC Valve was used across multiple engine models and sizes

But I would reset the anti-diesel screw FIRST and drive it a few days just to be sure that's the problem

 

using your recommendation, i will do this.

How long should i leave it disconnected (before turning screw first time) if it doesn't initially stall after say 10 full seconds..? I'm thinking it will, but asking just in case.

 

Also, once i do this- if i do replace the iac, would i have to make this adjustment again?

 

Turn the screw until idle goes up, engine warm IAC unplugged
Then back if off until idle is at 650-700

Check TPS % should be closer to 20%, if 21% back it off a little more, just a bit

 

After adjusting the screw watching my scanner, the readings came to these-

tps fluctuates between 19.6 & 20.0 % (with no other value displayed between those numbers)

idle in park about 920
range of 890-940

In gear 700
Range of 680-720

Sat in the driveway a good while to see if it tried to stall and it did not. Began to drive and just a bit past the driveway it basically tried to stall (rpms were almost zero) and after i gave it gas came back to normal..so i came back and parked it

This confirms the iac isn't acting as it should?

 

Yes, IAC Valve or computer is dropping the ball, allowing IAC to close too far

And no codes?
There should be codes, "RPM out of range, low" at least

 

No codes for iac except the one time it was actually left disconnected.

I'll be checking out the junkyard for another valve, then hopefully i will no longer have this problem. Thank you soooo much RonD for helping!