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Old 04/22/04, 06:20 AM   #1
MATT76
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Dies at idle only *Update*

I'm a little stumped on this one - just occurred out of the blue. Bronc starts and drives fine. However, when left to idle for more than a few minutes it will die. Then, for the next five minutes or so it will crank but not start. After about five minutes of intermittent cranking it will start again and run fine, only to die at idle again after a few minutes.

It is getting spark and fuel. Does this sound like a vacuum leak?

Last edited by MATT76; 05/02/04 at 09:23 PM..
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Old 04/22/04, 08:08 AM   #2
67EB_in_619
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I was gonna guess vapor lock or partially clogged fuel filter?

67' - San Diego - 393 Stroker w/Ford EFI
3.5" BC Suspension, 2" body Lift, 35" MTRs
AOD, Built D44 and 9" w/ 4.88 gears
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Old 04/22/04, 09:24 AM   #3
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My guess is vapor lock. Do you run the thick carb gaskit with the plastic inserts for the studs? you need this. also if you have EGR make sure it isnt sticking open. it will wreak havock at idle.
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Old 04/22/04, 12:58 PM   #4
76Broncofromhell
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Try flooring it when you re crank it. If it fires it means it's getting flooded. It sounds like you carb is leaking from the fuel bowl into the butterflies. This happened to my 750, all it took was a simple rebuild and it was fixed.

'70 Bronco - 408W
'45 Ford GPW - 351W
'84 Toyota - 5.0L HO
'84 FJ-60 - 5.0L HO
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Old 04/22/04, 01:02 PM   #5
mustangmarty
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Is the float sticking?

77 Bronco, busted rear window, jacked lift gate, rusted out floorboards, straightened front left wheel, corrected spindle and hardware, newly replaced window regulators, newly replaced brake rotors...
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Old 04/22/04, 08:31 PM   #6
MATT76
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Thanks for the replys. I agree with the vapor lock theory based on the following:

After sitting for the night I started the bronc and took it for a highway run and to get gas. Got home and let it idle and it died within a couple of minutes. Opened the hood and let it cool down. Restarted it within a few minutes and left the hood up. This time it idled for about ten minutes before cutting out. Left the hood up to cool and restarted as before, but this time idled with the hood down - dies within a couple of minutes. Repeated with hood up and it idled for over ten minutes before stopping.

Vapor lock, right? But why now?
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Old 04/22/04, 09:28 PM   #7
76Broncofromhell
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That sounds like the thing is leaking fuel into the manifold too much. Vapor lock is kind of rare these days. You say it goes down the freeway fine right?

How's the mixture? If you can turn the idle screws all the way in and it still runs then I know your problem...it was my previous post.

If it were vapor locked it would still start back up quickly. Your problem sounds like it's flooded.

'70 Bronco - 408W
'45 Ford GPW - 351W
'84 Toyota - 5.0L HO
'84 FJ-60 - 5.0L HO
'13 Shelby GT500 - 5.8L
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Old 04/22/04, 10:07 PM   #8
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If you think it may be suffering from vapor lock, like mine was, try this. Once your truck has "died" from the causes, unhook the fuel line that runs to the carb. Then hook up another fuel line and "gravity" feed some gasoline into the carb and try to give it a start. If it is truly vapor locking, the gravity feed gasoline should make it start right up.

Mine was vapor locking, i took the manual fuel pump out of the picture and went with an electric fuel pump.
 
Old 04/23/04, 05:43 AM   #9
mustangmarty
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Come to think of it, my '83 Toy 4X4 was doing the same thing not long ago. Do you run an electric fuel pump? Turned out my electric fuel pump was getting hot and would stop pumping fuel, even though it was still running. Opened the hood for a few minutes and it would start right up. I need to move the pump to somewhere cooler on the truck.

77 Bronco, busted rear window, jacked lift gate, rusted out floorboards, straightened front left wheel, corrected spindle and hardware, newly replaced window regulators, newly replaced brake rotors...
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Old 05/02/04, 09:35 PM   #10
MATT76
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Thanks to all that responded. Since I last posted, I have made a few changes:

1. New carb - old one needed a rebuild and if it was the culprit of any flooding this would be taken care of.

2. Electric fuel pump - wanted to make this switch anyway and if vapor lock was the issue, this should solve the problem.

Well, even with the changes the bronc stalls at idle after reaching about 185 on the temp guage. After it stalls, it will just crank as before. After a little while (4-5 minutes) it will start up again to idle fine for 5-7 minutes only to stall again. No drivability issues.

Only thing I can think of now is timing. I timed it by ear and TDC, but was unable to find the sweet spot to run the vacuum advance. Even with no vac advance it runs very well.

Could timing be the culprit?
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Old 05/02/04, 10:59 PM   #11
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If it is getting fuel to the carb while it is stalled and you're cranking it, you do not have vapor lock.

Electronic ignition? The modules become temp sensitive shortly before they fail completely.

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Old 05/03/04, 12:44 AM   #12
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You didn't say where you mounted your new pump. Vapor lock usually occurs between the pump and the tank. Gas boils at a lower temperature when it is under vacuum. Your fuel line sounds like it is too close to a heat source such as the exhaust system. Looks for places the fuel line is close the exhaust system like near the clutch equalizer bar/out put of the exhaust manifold or header, or at the back where the exhaust goes under gas line in the rear. With the hood open the air around the exhaust manifolds is at least 50F cooler which is enough to keep it from vapor locking.
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Old 05/03/04, 02:25 AM   #13
76Broncofromhell
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My friend is having this exact same issue in a Blazer. Everytime it's left to idle, it dies. We rebuild the carb twice and still, it would drive around town fine, but idling afterwords would cause it to die.

It turns out the electric fuel pump is making so much pressure that when the fuel demand gets greater and the floats open, the pressure of the fuel will overcome the needle and seat causing it to flood. Carburetors don't like to run much over 6 psi on the street and most electrics sold will push over 10 psi. I'd say throw on a fuel pressure regulator and the problems will go away

'70 Bronco - 408W
'45 Ford GPW - 351W
'84 Toyota - 5.0L HO
'84 FJ-60 - 5.0L HO
'13 Shelby GT500 - 5.8L
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Old 05/03/04, 05:37 AM   #14
mustangmarty
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I wanna go with the fuel line too close to a heat source. While driving the aire is moving a lot faster around the fuel lines. However, while idling, I would think the air is becoming more stagnant causing the heat to rise around the fuel line.

77 Bronco, busted rear window, jacked lift gate, rusted out floorboards, straightened front left wheel, corrected spindle and hardware, newly replaced window regulators, newly replaced brake rotors...
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Old 05/03/04, 07:48 AM   #15
MATT76
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I'm running a new module (electric ignition) and I routed the fuel lines outside of the passenger frame rail when I installed the new electric pump yesterday - the fuel lines are as far from a direct heat source as possible. I'll try the pressure regulator - I'm running a new Carter electric pump #4070 rated at 4-6 PSI. I mounted it at the rear outside passenger frame rail by the tank (away from exhaust).

After having ruled out possible causes with new parts, the only thing left is the timing - I just can't reason out why this would be the cause........can any of the mechanical gurus out there validify this?
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