Bronco ii stalls

[Randall008]

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

 

[MikeG]

Sounds like a vacuum leak.

 

[Randall008]

Thank you for the help. When I bought the car it had 4 vacuum hoses disconnected, so it’s certainly a possibility that I missed one.. is there a map of the vacuum lines posted anywhere, or do you have a suggestion for where to look?

 

[MikeG]

I don't have one, sorry. Someone here will.

 

[Randall008]

Thanks anyway. Thanks for the advise. It points me in the right direction anyway

 

[RonD]

Does it only happen when your foot is on the brake?
If so power booster on brakes could have a vacuum leak


Other thing is the IAC Valve
Fuel injection engines can't use an "idle screw" they have no Jets
So the computer uses an air valve to set idle RPMs, on Fords its called the Idle Air Control(IAC) valve

Its located next to the throttle plate on the upper intake, looks like a can on its side, has a 2 wire plug in

lets see if its working
Start cold engine, do NOT touch the gas pedal
Engine should REV up and then Idle should be 1,000rpms at least
And then start to drop down after 5 minutes or so to say 750-800rpms, warm engine idle
If this is not happening then IAC Valve could need a good cleaning or is no longer working

Good read here with pictures on cleaning IAC Valve: https://www.explorerforum.com/forums/threads/iac-valve-cleaning-thread-w-pics.84220/

Test IAC Valve
After engine is warmed up and idling
Unplug the 2 wire connector, RPMs should drop to 500 or engine may even stall, either is GOOD, it means it works

IF..........you decide to replace it you can ONLY USE Motorcraft or Hitachi brand IAC Valves

 

[Randall008]

Thank you for all the information. I’ll check tomorrow.. if I turn it on, no gas, it start at either 1000 or 1500, I can’t remember and then over time drops to 750 normally. It doesn’t stall at this point. It stalls when I’ve been driving, I pull into a parking spot, go from drive to park and take my foot off the brake. It idles high for a minute or so, then the rpms drop to about 450 or 500. Sometimes it catches itself, sputters and then idles where it’s supposed to, and sometimes it stalls. If I give it gas when I know it’s about to stall it won’t stall and then idles like normal

 

[Randall008]

I’m not near my truck at the moment or I would look. I think my iac valve is missing one of the bolt that hold it on. Do you think this could be causing a vacuum leak?

 

[RonD]

Yes, could be, it only has the 2 bolts

It does read like you could clean the IAC Valve and get a bolt for it just as general maintenance
The computer "knows" the RPMs and it has full control of the IAC Valve, and it reacts very quickly to keep RPMs within +/-4rpms of target "warm idle"

But if IAC's internal valve is "sticky" computer can't pull it open fast enough

 

[JerryC]

Buy this now. It will prove invaluable. I've fixed more things on my BII with this than with the big service manual (also worth it).
1988 BII vacuum and electrical manual

 

[JerryC]

Dang double post..


Buy this now. It will prove invaluable. I've fixed more things on my BII withis t
1988 BII vacuum and electrical manual

 

[Randall008]

Well thanks for all the advice,
I went out yesterday to work on it. The iac valve was not missing a bolt, it was another sensor. I replaced that anyway. I took the iac off, and cleaned it with maf sensor cleaner. I also noticed that the gasket was in pretty rough shape. I didn’t have another so I used rtv red gasket maker, hopefully that’s all right. Anyways, it no longer stalls, at least so far, when I go from drive to park. I am not sure if it was the sticky valve, or a vacuum leak in the gasket, but the problem is solved... I may still have a vacuum leak somewhere because the idle still feels a bit heavy, but it has definitely smoothed out... this may be completely unrelated, but I also noticed the heater blower was missing a line as well, which I fixed. I don’t see why this would be part of the vacuum system, but I also know they did weird stuff in the 80’s. Thought you’d like to know how it turned out, Thanks again for all the advice

 

[RonD]

Good work

Yes, thanks for the update it WILL helps others when they read this

 

[Cees Klumper]

Reviving this thread, my '90 Bronco II 2.9 V6 automatic has a similar, but slightly different issue.
The issue is that, a few weeks ago, the truck all of a sudden stalled when waiting to turn: in drive, idling, foot on brake. Strange I thought, as it has never done this before. Then it happened again, two weeks later. Then it almost happened: same situation - in drive, foot on brake (e.g. waiting for traffic light). Idle suddenly starts to oscillate, dipping low, catching itself until - stall. So I took off the Idle Air Control Valve, cleaned it some, put it back. Now today, on an errand, it happened three times in 5 minutes. If I quickly put the truck in neutral when coming to a stop, it would stay idling.

The one thing I did notice, as the idle started to falter and 'hunt', is I felt the brake pedal sort of 'move in tandem' with the oscillation - so I am suspecting the brake booster, and happy to see that RonD mentioned that, earlier in this thread, as a possible cause. Because I know, from work on another car, that if the brake booster diaphragm starts to develop a tear, this allows air into the vacuum system (that it is of course connected to) and it will I guess mess up the vacuum signature / computer / idle rpm.

How can I check the brake booster? When cold, the truck starts faithfully at 1,500 rpm and *normally* when warm, has a very steady idle at around 650-700. So I think the IAC is doing its thing. Can it also be that little check valve in the brake booster vacuum hose?

PCV, MAF, TPS, ignition module - all are new in the past 1.5 years and have performed great so far. Truck runs very strong. I scanned for codes after the second time this happened, and there were none. There are no vacuum leaks, I am 99% sure.

 

[Cees Klumper]

Quick update - I checked the brake booster using some online tips: with foot on brake and engine running - turn off engine - pedal should hold steady for at least 30 seconds indicating no leak. Mine held steady for a lot longer than that. Also checked the hose from intake manifold to booster and hose is ok, check valve is ok and so on. So at first glance it does not seem to be the booster/hose/valve.
So I checked symptoms of (going) bad IAC and they all mentioned irregular idle and stalling, so I just now went ahead and ordered a new Motorcraft CX1823 IAC. Even if this is not the cause, it's piece of mind and having replaced so many sensors and done so much work, it seems good to do. When the truck passed smog a few weeks ago, the person testing (oldtimer who said he worked as a mechanic at Ford for a couple of years back in the day) said he'd never seen such a clean and smooth running 2.9 V6, and I intend to keep it for a very long time).