• Just a reminder that you won't be able to start new posts or reply to existings posts in the Archive forum.

    This is where all the old posts go so they can still be used for reference and searched.
  • Welcome to ClassicBroncos! - You are currently viewing the forums as a GUEST. To take advantage of all the site features, please take a moment to register. It's fast, simple and absolutely free. So please join our community today!
    If you have problems registering or can't log into your account, please contact Admin.

Dies at idle only *Update*

MATT76

Jr. Member
Joined
Oct 24, 2001
Messages
79
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:

Rustytruck

Bronco Guru
Joined
Feb 24, 2002
Messages
10,875
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.
 

76Broncofromhell

Bronco Totalitarian
Joined
Jul 30, 2001
Messages
4,226
Loc.
Reno, NV
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.
 
OP
OP
M

MATT76

Jr. Member
Joined
Oct 24, 2001
Messages
79
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?
 

76Broncofromhell

Bronco Totalitarian
Joined
Jul 30, 2001
Messages
4,226
Loc.
Reno, NV
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.
 
U

Unregistered

Guest
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.
 

mustangmarty

Bronco Guru
Joined
Apr 1, 2003
Messages
1,229
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.
 
OP
OP
M

MATT76

Jr. Member
Joined
Oct 24, 2001
Messages
79
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?
 

Skuzzlebutt

PhD, Dr. of Broncology
Joined
May 26, 2001
Messages
4,393
Loc.
Honeymoon Bay
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.
 

72Sport

Bronco Guru
Joined
Jul 8, 2002
Messages
2,954
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.
 

76Broncofromhell

Bronco Totalitarian
Joined
Jul 30, 2001
Messages
4,226
Loc.
Reno, NV
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
 

mustangmarty

Bronco Guru
Joined
Apr 1, 2003
Messages
1,229
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.
 
OP
OP
M

MATT76

Jr. Member
Joined
Oct 24, 2001
Messages
79
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?
 

Rustytruck

Bronco Guru
Joined
Feb 24, 2002
Messages
10,875
You've gone far enough to erase the vapor lock issue I would worry about eithor a bad coil or a bad mag pick-up in the Distributer. start it up and as soon as it stalls check the resistance on the coil it may be bad. to check the mag pick-up use a can of compressed air that you use for dusting computers as soon as it stalls pop the cap and freeze the electronics under the cap if the truck starts right away replace the mag pick-up assy.
 

hmaspec

Jr. Member
Joined
Jul 1, 2003
Messages
56
I had a similar problem, lost the use of my bronco for a week. Final guess was maybe the ballast reistor, change that and been running fine since that replacement. I have a Mallory unilite ignition. Hope this helps
 
OP
OP
M

MATT76

Jr. Member
Joined
Oct 24, 2001
Messages
79
Thanks again to all - I swapped the ballast resistor & coil for new, but it had no effect. My next move is the mag pick up in the dist. A mechanic friend also said it might be a bad catalytic converter. Any opinions on that?
 
OP
OP
M

MATT76

Jr. Member
Joined
Oct 24, 2001
Messages
79
It was the mag pickup in the dist! Replaced and all is well - thanks to everyone who responded.
 
Top