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2.9 intermittent spark and stalling

Pix3L8

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Location
Connecticut
Vehicle Year
1989
Make / Model
Bronco ii
Engine Type
2.9 V6
Engine Size
2.9L
Transmission
Manual
2WD / 4WD
4WD
Total Lift
6
Hey guys, I’m having an interesting problem with the old bronco II and ide like to see what you all think. About 8 months or so back I was parked getting gas, when I went to start the truck it just cranked away but didn’t start. So I got it towed and really didn’t mess with it much for a few weeks. When I did get around to it I pulled the dizzy cap, took the plug wires off, inspected and reinstalled everything, boom the truck starts! But when I started it, it cranked 3-4 times then kind of spluttered to life, almost like it was misfiring badly. After about 2-3 seconds of this it idled fine. This continued for a while and sometimes it would just crank with no spark at all.

So now fast forward to last week, the truck has been sitting since the winter and I’ve only moved it around the driveway a few times. I had some spare time so I went out and started messing with it again. I noticed that my connector plug on the coil was a bit sketchy looking. I replaced it and what do you know the truck started up like it used to, about 1-1.5 cranks and boom it came to life strong. So I got for a test drive…as you would and when I put in the clutch and let the revs drop the thing dies on me right is I’m going around a sharp corner. No power steering in a lifted truck is fun let me tell you! so I started it up again and just got home. It seems to me that about 75% of the time the truck will start and do that weird spluttery BS for a few seconds, the rest of the time it starts fine. It also only seems to want to die when I start driving it, put a load on the engine for a few minutes then let it idle again. Sometimes it catches itself and doesn’t die, other times it’s out like a light, very odd. I’ve been testing around and can’t find anything blatantly wrong with it, my thoughts are maybe the coil is on its way out or possibly the TFI module, I’ve replaced that before for other reasons and I remember how much of a pain it was. My mysterious “no spark” has gone away for the time being but honestly I haven’t worked on it much lately because I have so much other stuff going on. As a side note I noticed that the engine seems to run rougher that it used to. A bit of a shake and a noticeable shake in the shifter. I used to be able to balance a full beer can on the intake and it wouldn’t move. I was very proud of that after I rebuilt the engine. Lemme know what this sounds like to you.
 


Paulos

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1987 STX
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2WD / 4WD
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Many years ago on my way to work I stopped to put gas in my '86 Ranger. It was driving fine before I stopped, but when I tried starting it after filling up it would turn over, but not start. I managed to find someone that would deliver a new ignition module to me, installed it, and that solved the problem. I haven't had that happen since then, though years later the same truck died at the same exit two days in a row on the way home from work. It was the ignition module. I discovered the hard way that they don't always go out in the same manner, but when they do it's almost always heat related, and they usually work after they cool down.
 

Pix3L8

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1989
Make / Model
Bronco ii
Engine Type
2.9 V6
Engine Size
2.9L
Transmission
Manual
2WD / 4WD
4WD
Total Lift
6
Many years ago on my way to work I stopped to put gas in my '86 Ranger. It was driving fine before I stopped, but when I tried starting it after filling up it would turn over, but not start. I managed to find someone that would deliver a new ignition module to me, installed it, and that solved the problem. I haven't had that happen since then, though years later the same truck died at the same exit two days in a row on the way home from work. It was the ignition module. I discovered the hard way that they don't always go out in the same manner, but when they do it's almost always heat related, and they usually work after they cool down.
I’ve got a box with several bad TFI’s and I’ve been hesitant to add another one to it. Every time one has failed it has had a different symptom. This is the first time I have actually had an intermittent crank no start condition.
 

RonD

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It seems to me that about 75% of the time the truck will start and do that weird spluttery BS for a few seconds
Reads like fuel pump is not starting/priming for the 2 seconds with key on

Almost stalling or stalling when you come to a stop reads like dirty IAC Valve


On any fuel injected engine you shouldn't touch the gas pedal on start up, this can give you a clue if all is well
With key on the computer will boot up and Prime the fuel system
It will open IAC Valve all the way for start up
It will check the ECT sensor to see if engine is cold or warm to see if it needs to run CHOKE MODE(cold engine)

All those things happen within 1 to 2 seconds of key on

Now crank engine, foot OFF the gas pedal, engine should start and REV to 1,500rpms because IAC Valve is open all the way
Then RPMs will drop to 1,100 if cold engine or 700-800 if warm

If its not REVing then you have an IAC Valve issue
If its stumbling then fuel pressure issue most likely, was under 10psi for a few seconds until RPMs were above 400 and fuel pump was turned on full time to get 30-40psi pressure needed
 

Pix3L8

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Location
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Vehicle Year
1989
Make / Model
Bronco ii
Engine Type
2.9 V6
Engine Size
2.9L
Transmission
Manual
2WD / 4WD
4WD
Total Lift
6
Reads like fuel pump is not starting/priming for the 2 seconds with key on

Almost stalling or stalling when you come to a stop reads like dirty IAC Valve


On any fuel injected engine you shouldn't touch the gas pedal on start up, this can give you a clue if all is well
With key on the computer will boot up and Prime the fuel system
It will open IAC Valve all the way for start up
It will check the ECT sensor to see if engine is cold or warm to see if it needs to run CHOKE MODE(cold engine)

All those things happen within 1 to 2 seconds of key on

Now crank engine, foot OFF the gas pedal, engine should start and REV to 1,500rpms because IAC Valve is open all the way
Then RPMs will drop to 1,100 if cold engine or 700-800 if warm

If its not REVing then you have an IAC Valve issue
If its stumbling then fuel pressure issue most likely, was under 10psi for a few seconds until RPMs were above 400 and fuel pump was turned on full time to get 30-40psi pressure needed
I haven’t tested the fuel pressure when the engine is running but I did hook up the gauge and primed the pump a few times last week. I got roughly 40psi which is good. The truck does rev up when I start it, it always rev’d higher than it should, 2500+ for a second but it’s been like that for years. I’ll test the fuel pump while it’s running and get back to you
 

RonD

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What was fuel pressure to start with?
Should be 15-20psi engine off after a few MONTHS

So if its dropping to 0psi after a few minutes or hours then you would only have 10psi with 1 key on, so engine would stumble on startup after REVing
 

Pix3L8

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Location
Connecticut
Vehicle Year
1989
Make / Model
Bronco ii
Engine Type
2.9 V6
Engine Size
2.9L
Transmission
Manual
2WD / 4WD
4WD
Total Lift
6
Ok so KOER pressure is roughly 29-30 psi which is below spec….hmm. I’m going to leave the gauge hooked up over night and see how far the PSI drops in the morning
 

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Pix3L8

Active Member
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Joined
Jun 13, 2016
Messages
134
Reaction score
40
Points
28
Location
Connecticut
Vehicle Year
1989
Make / Model
Bronco ii
Engine Type
2.9 V6
Engine Size
2.9L
Transmission
Manual
2WD / 4WD
4WD
Total Lift
6
Initially when I hooked up the gauge and primed the system I got 10psi BUT the gauge had to purge the air, after a few more cycles of the key I was at 40psi. So I’ll let it sit over night and try priming it again in the morning. I am noticing now watching the gauge with the engine off after it ran that it has crept up to 45 PSI and it sitting there.
 

Pix3L8

Active Member
Supporting Member
Joined
Jun 13, 2016
Messages
134
Reaction score
40
Points
28
Location
Connecticut
Vehicle Year
1989
Make / Model
Bronco ii
Engine Type
2.9 V6
Engine Size
2.9L
Transmission
Manual
2WD / 4WD
4WD
Total Lift
6
Checked the gauge this morning at it was at 0, primed the truck for 3 seconds and it was back at 35psi. Hm wonder if I just need a fuel pump.
 

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