Help please - intermittent stall - now solved (I think!)

[Cees Klumper]

Hey 2.9 community - I need your help with a very elusive intermittent stall. Symptoms:
- without any warning, the car (1990 Bronco II, California ECU) stalls when driving slowly, maybe 5-10 MPH. It restarts fine then runs like nothing happened
- it is progressively getting worse. A few months ago, at idle sometimes (once every 100 miles?) it would start idling poorly, and sometimes the check engine light would come on briefly. Engine would recover and all would be well again for the next 100 miles
- more recently, in the past couple of weeks, it would suddenly stall out while driving at low RPM/speed
- just now, it stalled while pulling into our apartment complex, while going 5-10 MPH. Restarting it for the first time was difficult, and it stalled again repeatedly while looking for a parking spot. This was after at least 30 miles driving around town running errands, with 0 problems.

Troubleshooting I've done:
- codes: only code thrown so far was oxygen sensor. So I replaced that, no change
- I swapped out the ECU with a spare California (same spec) one - no change. I had already replaced the two condensors with new ones
- swapped out the Idle Air Control valve with new Motorcraft - no change
- put in a new Motrocraft ignition control module - no change
- checked the throttle position switch (which is relatively new Motorcraft) - resistance values are fine
- bypassed the fuel cutoff switch in the cabin, passenger footwell - no change
- swapped out the Mass Airflow Sensor - no change
- checked fuel pressure at normal idle - checks out fine. I have not tried checking fuel pressure when the engine stalls, since it happens so infrequently I have not been able to recreate the problem while testing
- replaced the plugs, no change. Old ones looked good and evenly worn, nothing out of the ordinary
- the engine has a relatively new coil and plug wires
- checked the fuel pressure modulator, seems A-OK, holds vacuum. I replaced it with a good new one a few years ago when the old one had ruptured dumping fuel into the intake
- I wiggled all electrical connections/plugs under the hood to see if I could replicate the issue. Yesterday I thought I had it - when I manipulated the connector on the alternator, the engine started idling poorly - I installed a new (aftermarket) alternator last year and the connector doesn't fit all that well, so it was loose and when I touched it it temporarily disturbed the connection causing the engine to falter. I cleaned the connector and reinstalled it with a triewrap around it, so that it is now super-tight. So that doesn't seem like it's it either.
- checked for vacuum leaks, none to be found (and none that could be that dramatic and only so intermittent)
- replaced the EGR valve
- checked the battery, it's in great condition. Starter motor spins like a top so it doesn't seem low-voltage related
- checked the brake booster one-way valve, seems perfectly fine
- the in-tank fuel pump is relatively new, but I have not checked the electrical connections / plug at the pump (no access panel, and it's a pain to drop the tank for this. But, also, since the stalling is so sudden, as if I turn off the ignition, I don't think it's the fuel supply also because when I start it up again, there is a strong smell from the exhaust, as if there's extra fuel being burned off. This to me points more to the ignition than to the fuel being the problem).

So far, it has only happened at idle, or close to idle, not when driving at any speed over say 10 mph - which must be a clue of some sort, I just don't know what to make of it. Also, it is progressively getting worse. But, like today, it was totally absent for the whole day and many short trips to here and there under the exact same conditions that it later decided to stall/difficult restart.
The entire exhaust system is new, less than 5K miles, including the CA compliant catalytic converter. It passed smog with very good values a few months ago.

I am down to the following possibilities, but would very much appreciate any other thoughts:

- could it be the distributor - a bad/loose connection inside? Something that is interrupting the spark? Should I replace the distributor? AFAIK it's the original one, and the engine has 245K miles
- wiring loom - some partially broken wire that under some conditions causes the engine to die? Tough to diagnose since the problem is so intermittent, and only at/slightly above idle
- maybe the capacitor that is attached to the coil?
- possibly it's the ignition switch in the steering column, or the wiring/connector to it?
- bad connection to the fuel pump, at the tank? I can cut out an access panel, am tired of dropping the tank every time I need to replace a hose or e.g. the vent connector I did a few months ago to pass smog. Maybe the fuel pump itself is going bad? It's maybe 5 years old, the previous owner put it in new before he sold the truck to me. But then why is it not happening when driving at speed.
- could it be that both ECUs are bad? But then having the exact same issue, developed at the same time? Seems quite unlikely

I will pull the codes again, retry manipulating that alternator connector; I guess I can try manipulating the big plug that attaches to the ECU in the cabin to see if maybe that replicates the issue. Investigate the ignition switch / wiring in the steering column. This engine runs really well and strong - when it runs which is 90% of the time. I replaced the heads two years ago, replaced the fuel injectors with OEM rebuilts, all vacuum hoses and many other parts. It feels like it is a simple issue, nothing major, just very hard to diagnose.

 

[Cees Klumper]

Update: decided to replace the fuel pump, which took most of today (had just filled up yesterday). New pump did not make a difference - after about 10 miles, and very coincidentally, it stalled while pulling into our parking lot, in pretty much the exact same place as yesterday. It started up easily and kept running until I parked it. So, one more thing off the list.

 

[PetroleumJunkie412]

Investigate TFI at all?

 

[Cees Klumper]

Yes, I replaced the TFI ('ignition control module') with a new Motorcraft one, it made no change. The connector is on tight, wiggling it while engine running makes no change.

 

[rusty ol ranger]

It doesnt misbehave at all under normal driving?

 

[Cees Klumper]

Not so far - strangest thing, only when driving slow (5-10 MPH) or idling. The stall is immediate, as if the ECU shuts down the engine in a spllit second. When driving normally it's just fine.

There's been a few times now that the engine will 'run on' after it stalls as I am coasting to a stop (like a diesel).

I am still thinking ECU / ECU input wiring related, will check codes again and swap out my spare ECU once again, and check that main plug onto the ECU, as well as the wiring in the steering column to the ignition switch. A few years ago the brake lights stopped working, and it turned out to be a loose wire in the steering column, that ultimately powers the brake switch/lights. Plugged it back in and brake lights came back. If a wire into the ignition switch is loose-ish, I guess that could produce the same symptom.

 

[BlackBII]

I didn't see any mentions to the grounds - have you checked and cleaned them all? A single bad ground can cause a ton of intermittent issues, and if you are having a voltage drop at low rpm's, it can cause the ECU to do weird things.

Most of these older trucks have corroded and old battery terminals and grounds, along with the green death in the copper wires, especially close to the battery. I believe your truck will have the ECU grounds as two black wires coming from the power distribution box area and directly to the battery. I have had to strip those wires back an inch or two to get clean copper on more than a couple Ford vehicles.

 

[Cees Klumper]

Many thanks BlackBII, I had not considered the grounds! That's a great tip, will pursue that one for sure especially the ones you mention.

 

[RobbieD]

Good advice on the grounds.

Another thing you may want to try, is a "wiggle test". Engine running (in Park) go through the wiring harnesses, connectors and ground points and wiggling / shaking them, listening and watching for any changes in engine's running.

 

[Cees Klumper]

Thanks RobbieD, I tried this wiggle test twice now, with engine running manipulate all underhood connectors. I thought I had it when I touched the top alternator plug and the idle dropped temporarily, but that turned out to be a 'false positive'. I will report back as I make progress...

 

[rusty ol ranger]

The way its stalling quick dead...i think you can rule out any fuel issue. If its losing fuel generally itll chug to a stop.

Its strange it only does it at low speed. The voltage/wireing might be the correct path to follow.

Also does it tend to stall when the steering wheel is being turned hard and fast....like whipping into a parking spot? If so the column would be the first place i would look.

I had an 89 F250 that would die like that going over bumps....it was in the ignition switch.

 

[Cees Klumper]

Thanks Rusty - it stalls when going straight, after I let off the gas as I coast toward a stop / to slow down - it's never happened while I was accellerating which is another kind of strange 'feature' of this problem. So not when I am turning the wheel.
As for the gas being a possible cause, I thought maybe if the power to the fuel pump gets cut, fairly quickly the pressure in the fuel injection rail drops to where it can't open the injectors anymore.
Have not had a chance to troubleshoot again, but at this point I think the grounds are the most likely culprit. I've certainly had strange issues with poor grounds on other cars before and I've learned how sensitive these Bronco/Ranger ECUs are, so that's my next move.

 

[Cees Klumper]

OK so back on the intermittent stall problem I've been dealing with. Over the weekend tried various things:

- checked, cleaned, tightened various grounds to the PCM/ECU, the two near the ECU itself and one on the firewall near the power distribution box
- cleaned and retightened battery cables
- swapped out the alternator
- tried checking/swapping the three relays underneath the large fuses below the power distribution box particularly the one for the ignition, but I did not have a solid-looking spare
- took the covers off the steering column looking for connectors to the ignition switch. I must not have been looking right because all I saw was the combination switch wiring to the left of the column. Now I understand the actual ignition switch is somewhere at the bottom of the column

All to no avail, truck still randomly stalling out, with or without some run-on, until I turn the ignition key off, then restart it and it will run again as if nothing happened. Yesterday it started for the first time cutting out while driving at speed, on the highway. Before it would only randomly stall while coming to a stop / driving very slowly. It does not seem voltage related.

Next attempt will be to replace the ignition switch. It does seem electrical in nature, as if the ignition is switched off, that's how instantaneously the engine cuts out.

 

[rusty ol ranger]

Just a long shot in the dark but....maybe try bypassing the inertia switch?

 

[Cees Klumper]

Thanks, I did unplug the inertia switch but it made no difference, still random stalling.