I have been having a slight problem with my 84 bronco II. I beleive it is electrical since I have the famous electronic Carb and ignition setup. When my truck is cold and I go to start it up in the morning It will fire up like normal and the computer will begin to warm the truck up, E.G. using a higher rpm. After about a minute to two minutes the truck will begin to run rough and then the idle will begin to fall and then it will cut out. The computer controlled carb tries in vain to keep juicing the throttle to keep it from cutting out. Funny thing is this never happens when its dry outside, only when it has been raining for a few days and the air is very moist. So im sure its electrical but I have tried in vain to search out this problem. Other than this the truck runs amazing. I'm just hoping there might be some other thoughts as to what might be causing this? The truck was from california and most of the wiring is very dry and even in some cases has cracked wiring housing. Also I have a code reader and I have only two codes that have been around forever and that has to do with the EGR system but they have been around since before this problem started.

1984 Bronco II
2.8 2bbl electronic carb. and Igintion
C5 Automoatic 4x4

 

This is a problem I am very familiar with. It's not electrical, it's frost building up in the carb. That's why it's so bad in cool rainy weather. There are several things on the engine to combat this.

First thing to check is the flapper valve in the snorkel of the aircleaner. Make sure when the engine is cold, and you first start it, that the flapper closes. When it closes, it draws warm air up from around a duct and shroud that goes around the exhaust manifold. Make sure the flapper works, the duct hose is in place, and the shroud is in good shape around the exhaust. After it starts, you should be able to grab the duct hose, and feel it start getting warm from the air going though it.

If you get the aircleaner working, that should cure most of it. But some engines also had another warm up device along with the aircleaner. It's a valve located in one of the exhaust pipes near the exhaust manifold. It has a flapper inside that blocks the flow of exhaust, and forces it up though a passage under the carb, that is made into the intake manifold. This warms up the bottom of the carb faster. The valve in the exhaust is usually vacuum controlled, and the vacuum line will go to a sensor mounted in a coolant passage. When the coolant is around 100 degrees or less, vacuum passes to the exhaust valve diaphragm, and makes it block the exhaust. When the coolant gets above 100, then it blocks the vacuum, and lets the exhaust valve open up. The valve is commonly known as a "heat riser valve".

 

I have removed the heater door assembly all together on my truck and installed a higher flow intake setup. I had this same problem when I had the old system installed. I would agree with you that frost would cause this problem but it really is not cold enough to get frost build up yet in the carb. You have me thinking though. maybe since I removed that old system the computer is messing itself up because the intake air sensor is reading it to be to cold of an air mixture of air and this is causing my truck to run like crap and then causes it stalls out. I have a feeling that heater door system was not working when I had it on there before. As far as that exhaust valve is concerned I have never seen one in any of my manuals for my bronco 2. Are you perhaps talking about the air injection system that pumps cold air into the exhaust in order to help burn up hydro carbons for emission purposes?

 

The frost (ICE) problem only happens at about 36*F OAT and above.
Note that a bad choke pulldown diaphragm will also cause it to run this way.