Bronco ii stalls

[RonD]

Start the engine, foot OFF the brake, run it for say 30 seconds and shut it off
Press brake pedal down, and let it up, should get 2 or 3 "vacuum assist" presses until pedal gets hard to press
The size of the booster is so a driver can get 2 or 3 power assist braking pushes to get off the road safely after a stall, just FYI on why the booster is so big, lol

Computer has an "idle bump" it uses when transmission is shifted OUT of Park or Neutral, same if you have AC and turn it on
Computer should bump up idle about 75-100RPMs after shifting into gear
It uses IAC Valve to do that
Is it a Motorcraft IAC Valve?
3rd party IAC Valves can work fine then not work fine, then work fine, then..........ect
Its because they are usually solenoid/stepper type "motors"
Rangers need solenoid type only, which is Motorcraft or Hitachi brands

Idle bump is done when Neutral switch no longer shows neutral(ground), on pin 30 of computer

Try replacing PCV Valve, its a controlled vacuum leak, when it gets dirty it can stick so changes vacuum draw, usually computer sees that(drop in RPMs, and can respond fast enough, but not always
And they are cheap and should be changed every 3 oil changes anyway, which no one does, lol


And you can re-adjust the anti-diesel screw to keep idle above 650 or so
After engine is fully warmed up and idling, unplug the IAC valves 2 wire connector
Idle should drop, this is good, no vacuum leaks, lol
But if engine stalls then you can adjust the anti-diesel screw, it looks like an idle screw on throttle linkage, but fuel injection can't use an idle screw, lol
Any way turn it clock wise a bit to increase idle RPMs and restart engine if it stalled, IAC Valve still unplugged
650 RPM is a low but steady idle
Plug back in IAC Valve and RPMs should go up to 750
Unplug it again
You can adjust it so idle is higher but lower than IAC Valve when its plugged in
Then unplug IAC again and shift trans into drive and see if it stalls

That should get you by until problem is found

 

[Cees Klumper]

Thanks RonD - the IAC on there now is a Ford/Motorcraft one (Ford logo on top of it), and could very well be the original one. There are 230K miles on the truck although I don't know if it's the original engine. I replaced the heads with new ones about a month ago and it's been running very well since then (except for this stalling issue, which just became annoying today).
I suppose it's possible that, as an IAC goes bad, it initially works almost all of the time, but then starts to occasionally fail, until it fails more and more / all the time? I.e. do they typically fail 'all at once' or can it be a gradual process? When my fuel pressure regulator failed, it was '100% instantly' for example.
I just did the check of the brake booster you suggested and, with engine off, I do get two (and part of the third) brake presses before the pedal becomes hard to press.
The PCV I replaced about a year ago - it still looks brand new, I checked it today and it 'rattles' / whatever is inside moves freely. I guess if there's no improvement with the IAC I can try that as well, they're super cheap indeed.
I hope the IAC will arrive in a couple of days, it's being shipped from maybe 20 miles away and will update.

 

[Cees Klumper]

Today the new Motorcraft Idle Air Control valve (IAC) arrived. I installed it, went for a test drive and, after about 3 miles warming up, the exact same thing started happening: erratic and dropping idle, until the engine just died.
I checked the various conditions and found it made no difference whether it was in neutral or in drive. At random moments when idling, it would start dropping, recovering and then after a few seconds of that back-and-forth, die.
So, out came the vacuum tester again. Long story short, it seems it was the brake booster check valve - I had checked it when this first started happening and it was ok then (blowing through only possible in one direction) but now it was letting air through fully in both directions.
I picked a new one up (Dorman universal for $6) and that seems to (famous last words) have fixed it. Idle is slightly higher (now at around 750-800 RPM in gear, 900 in neutral) where before it would be around 700 in gear. I went for various test drives during the day, changing conditions, but at no point did the problem re-occur (whereas on Thursday and this morning before installing the new check valve it was doing it pretty much all the time once the engine had warmed up).
So fingers crossed this was it - guess I did not really need the new IAC but it doesn't hurt to have a new one in there of course.
By now I have replaced most widgets on and around the engine: IAC, TPS, MAF, ignition module, spark plug cap, rotor and wires, spark plugs, PCV valve, O2 sensor, fuel pressure regulator, thermostat, coolant temp sensor, vacuum hoses, power steering pump and hoses, and the cylinder heads including the valves, rocker arms and pushrods, exhaust manifolds and the entire exhaust system and cats. When it runs like it should, it runs great. I just hope it stays fixed for a while now!

 

[RonD]

Thanks for the update

Yes, fingers crossed

 

[Cees Klumper]

Final (hopefully) update - installing a new brake booster check valve fixed the stalling at idle issue. But I was still experiencing, from time to time, a rough idle, as if the ignition was missing sometimes. Because I have replaced most ignition components over the past two years, I suspected the coil was on its way out. I got a new one and now the idle has indeed been much smoother. So it may have actually been two issues that, combined, caused the stalling.
I also decided to adjust the screw on the throttle body to have the butterfly valve just-not-bind, i.e. backed out as far as possible, without the flap binding. So all the idle air is now coming from the (new Motorcraft) ICM. Idle in gear is around 700 RPM most of the time; in neutral around 900. I figure that's all ok.

 

[RonD]

Good work

Proper way to adjust the "anti-diesel screw", thats what that screw is called, lol

Warm up engine, leave it running
Unplug IAC Valve's 2 wire connector
Engine RPMs should drop to 500 or so
If engine stalls then no adjustment is needed, but you can adjust it so it won't stall
Turn screw IN a full turn
Restart engine, should now idle
Turn screw OUT until it idle is about 500

Doing this does adjust the TPS as well, so you may want to test its voltage
Key on engine off
Center wire of 3 wire TPS should show 0.69 to 0.99volt
If its higher than 1volt, turn screw OUT until 0.99 is shown
(some TPS are adjustable, if holes are slotted then loosen it bolts and adjust voltage that way)

Open throttle to WOT(wide open) should see 4.5 to 4.8volt
All set

Retest idle with engine warm and no IAC Valve
When you plug back in IAC Valve RPMs should go up to 700-800

 

[Cees Klumper]

Thanks again. Truck had a major stumble again today and also the day before. Just once, in idle when waiting for traffic light. Didn't stall, but clearly had a tough time catching itself almost stalling. Really strange. I just went through the procedure for the throttle plate screw (thanks!) and it did need some adjustment, basically it was screwed in too much so interfering somewhat with the IAC. But that does not seem to explain why * just sometimes * the engine will suddenly 'cough' and drop in rpm. Driving it's perfect, tons of power, no misses anytime, pulls to redline if I want it to (I don't really). It's just at idle. I also checked the TPS: 0.97 volt at idle, 4.7 at WOT. I replaced it 18 months ago with a new Motorcraft one. Also replaced the ignition module around that time.
I am down to my last suspects: loose-ish wire someplace (I will try wiggling/manipulating different wires/connectors with engine idling) or the fuel pressure regulator is starting to go - although I replaced that maybe 18 months ago. Maybe it's the fairly new ignition module (but then why miss only at idle). I even swapped out my ECU just now, since I have a spare one around for this type of situation, but no noticeable change. Sometimes the idle also just starts varying a bit, not serious, just not quite normal (until now, idle was always really stable). To be continued some more ...

 

[Cees Klumper]

Hopefully final update, I decided to put in a new coil, replacing the seemingly original (30 year and 238,000 mile) one. That improved idle and the issue has not come back. I did get a new issue: it would not start on colder (around 68 degrees) temperatures. Fuel pump would not turn on. Tried a few things but it seems swapping the ECU has cured that. Maybe a bad connection inside the ECU or the plug contact not so good. Anyway, hope it stays fixed for a while.

 

[RonD]

Good work

Open up the computer have a look at the circuit board for corrosion, easy to clean off
Also look at the 2 or 3 capacitors, blue or black cylinders, these fail after 20+ years, easy to replace, replace all, under $5 fix

 

[Nelson and Hudson]

" installing a new brake booster check valve fixed the stalling at idle issue "

I am having the exact same issues you had. I will check the brake booster check valve and the voltage on the TPS. I also replaced my coil and a few other electrical components.

 

[Cees Klumper]

Hope you find the culprit. Another possible cause of these symptoms, I learned since this thread, is if the ECU grounds are compromised. They are on the passenger inside fender under the hood, I believe. Poor grounds can cause all kinds of issues.

 

[HungerBunger1213]

I’ve got a 1987 bronco ii. 2,9 v6 fuel injected engine. It’s automatic. When I’m in park, or stopped at a light with the brake on it’s got a rough, heavy idle. When I go from drive to park, the rpms drop real low, some times it dies, sometimes it catches it self, revs up again then idles where it should and won’t stall again. Giving it gas when you put it in park will stop it from stalling. Any ideas or suggestions? Thank you in advance

I’m a few years late to this discussion but I picked up my 86’ Bronco II a couple months ago and had a very similar problem. Hit the highways or roads for a little while, come to a stop for a minute at a light and car died. The way I fixed it was by removing the second fuel pump on the frame rail, running a new line to the fuel pressure regulator, then putting a 90’ fuel pump in the tank (they switched to one pump for the later models). This seems to have solved the majority of my problems, I can drive confidently and stop without worrying the car will die at lights. However my new problem is sometimes while I’m driving the car just turns off, not ideal for highways or crossing intersections, but the silver lining is it just needs to be put back in park and will start right up seconds later, I’m pretty sure this has to do with a switch on the auto trans I read about in the Haynes repair manual, but haven’t put too much energy into fixing it yet.

so to summarize, if you have a 85-89’ Bronco that cuts off at red lights or after driving for a little while, or even just on hot days won’t go for more than a mile down the road;

1) remove the HIGH PRESSURE PUMP on the frame rail
2) run a new line to the fuel pressure regulator from the filter on the frame rail (looks like black hockey puck)
3) put a new high pressure pump in the tank, any Bronco II pump 1990 and on will work (it will cause fuel gauge to stop working they read different ohms but you can work around this if your savy)