I have a 1981 f150 302/aod 3.25 rear.
I am having some choke issues with it.
The choke housing has a tube going down to the exhaust manifold and an electric wire connected to it as well.
The wire runs as near as I can determine to the stator on the alternator.
What is the purpose of 2 heat sources to the choke housing?
But here is the problem I am having,on warm mornings 65deg and up the choke will "set" the engine will go up on high idle but the choke releases too quickly.
In other words about 1 city block the engine will of course die and hesitate for several more blocks,after that it is fine.
But on very cold mornings <20 deg the choke stays on for what seems like forever,especially if I just start the truck and drive off to work or home,about 15mi one way down the interstate,when I arrive there it is still on fast idle with the choke still partly closed,if I sit there for about 5min after I arrive it will finally kick down.
Does anybody have any solutions?

This is just about driving me crazier than I already am!

Jeff

 

Does your truck still have the fresh air intake hose and heat riser tube on it?

If either one, or both of those are missing, replace them.

On warmer mornings, the fresh air intake draws in air from in front of the truck to help regulate how fast the choke opens up. On colder mornings, the heat riser tube helps by bringing warm air to the carb so that the choke doesn't stay on forever. In the middle of all of this, there's a vacuum operated butterfly valve that also has to be in proper working order.

If your truck is anything like mine, and many other's here. The fresh air intake tube that runs from the end of the arm off the air cleaner assembly to the front of the truck was missing. In my case, the heat riser tube was also missing. I did luck out and found the missing parts at a local wrecker.

 

Thanks for answering,yes,my truck still has the fresh air tube as well as the heat riser tube,as the truck is a one owner truck,I bought it brand new in june of 1981.

 

I kept having problems with my choke so I ditched it for a manual conversion kit from the auto parts store. Cost $10 and now the choke is only on when I want it on for how long I want it on. As simple as pulling a ****.

 

First, I don't think the "Fresh Air / Manifold Heated Air system" has anything to do with choke operation. Below a certain temperature heated air off the exhaust manifold is routed to the carburetor to aid in fuel vaporization in cold weather thus improving drivability. I'm pretty sure the air temperature has to be below about 40 deg. F. for warm air to be supplied to the carburetor.
Secondly, the thermostatically controlled electric choke assist heating element operates only above a specified temperature (about 55 deg. F. or above) to reduce the choke "on" time in warm weather. Power is supplied to it only when the engine is running (alternator charging).
The "hot air" operated portion of the choke is the primary controlling factor. Clean air from the carburetor throat is piped to the exhaust manifold heat tube. Heated air from the manifold tube is "sucked" through the choke housing around the bi-metallic choke coil and into the lower pressure (vacuum) area below the throttle plates thus causing the coil to be warmed and the choke to be released. With the engine running the vacuum operated choke pulldown forces the choke plate to partially open to reduce the fuel air ratio.
I suspect your main problem of the choke staying "on" for a long time has to do with an inadequate supply of heated air from the exhaust manifold. Partially clogged or leaky tubing could be the cause. Look for holes rusted in the pipe at the entry and exit points of the exhaust manifold or cracked rubber hose (if used). Tube might be rusted through internal to the manifold. Could be binding of the choke linkage, if so- spray and free up using carburetor cleaner (never use lubricant). Check the choke pulldown solenoid for proper operation by removing/re-attaching vacuum line while engine is idling; arm should move out/in.

 

I don't quite understand this theory of operation. The choke heating element will heat all the time the engine is running. Are you saying that is doesn't heat to a very high temperature, thus when it's warm outside it doesn't have much effect?

This heating element being burnt out is what I find is the problem most of the time on a choke that hasn't been monkeyed with. You can take an ohmmeter and put one lead on the choke housing terminal, and the other lead on the carb housing, and see if you get a reading. If you don't the element is burnt out.

If you find the element is burnt out, you can buy a new one, and then it sounds like you need to adjust the choke to be a little more aggressive. I guess this goes along with what Piffery1 was saying. The choke being set a little bit more aggressive should make it stay on longer in the summer, and the heating element being repaired should help release the choke better in the winter.

 

Sounds like he has a bad Vacuum PULLDOWN MOTOR, on my 1983 F150 I have had to replace it about four times and it gives those symptoms. It will also put black smoke out the tail pipe until it warms up.

 

Thanks for answering to everyone!
Well, that certainly explains why the choke will release just fine if not a little too quickly when the temps are >60 deg but then takes "forever" to release when the temps are <30 deg.
Don't get me wrong,when the temps are <30 deg it will release but it will take 10-15min of driving/idling before it does.
Last winter I triedan experiment,I unhooked the electric wire going to the choke housing and the choke never released even after driving about 45min on the interstate (terrible effect on gas mileage of course) the choke housing was cold, but after I hooked the electric wire back up,the choke released in about 10-15min.
I will have to look to see if the heat tube is ok,it still does have the metal cover on it.
Thanks again to everyone for the help.

 

Franklin2,
What I'm saying is that below about 55 deg. F. ambient temperature, the electric choke heater is "off" which lets the hot air from the exhaust heater tube control the choke setting which means the choke is "on" for a longer time in colder weather. The electric assist heater is "on" above 55 (with engine running) which heats the choke bi-metallic spring faster than hot air alone thus reducing choke time in warm weather. The choke assist heater also contains a thermostatic switch that governs its operation and I think that it again opens the heater current path (shuts it off) once the choke mechanism reaches a higher temperature (about 75 deg. F.) since by that time the hot air from the manifold heat is keeping the choke fully open. I.e., the electric choke assist heater only operates when its temperature is between about 55 and 75 deg. F. since below 55 a longer choke time is needed and above 75 the choke is already open. If you try to measure the choke assist heater resistance when its temperature is outside the ~55 to ~75 deg. F. range (with an ohmmeter) it will read open, leading you to think it's bad when it's not.

 

Hmmm. I didn't know that. Thanks for the info.


That experiment does prove the element is good doesn't it.

 

With that experiment it sounds like your not getting enough heat to the choke.

On my 1981 there are two heat riser tubes to the carb. One hooks to the choke and the top of the exaust manifold, and another one hooks from under the manifold and hooks with a red rubber hose to the carb air horn. Both help the truck to heat up faster, thus letting the choke be released faster.

For correct choke operation several systems have to be operating or it can set the choke off whack.


First Check the choke plate, and linkage for proper movement.

Second, check the choke housing spring for proper movement and for breaks.

Third check to see if you are getting power from the altenator, to the choke. A faulty altenator could cause choke problems if the wire to the choke doesn't get power. Also check for broken wires or blown fuse links. There is one between the choke and altenator.

Fourth, check the vaccume choke pull down motor. There should be vaccume to the motor all the time, and constant pressure on the choke to open it from this motor. If not replace. The action of the choke being cold and the choke housing spring, puts more tension on the choke to keep it closed, when cold, than the motor has power to keep it open, until the engine warms up, then the oposite is true.

Fifth, The choke heat riser tubes have to be intact, and vaccume at the tubes sucking the warm air. A faulty carberator, or leaking gasket can cause vaccume failure at these tubes.

There are also subsystems all around the truck that can inadvertaintly cause undesirerable choke operation.

#1 There is a "flapper" in the exaust manifold. It flips and controls heat. When warm it is open, when cold, it closes up alowing more heat to build up, therefore warms up the engine sooner. If the flapper is stuck open, it can cause the truck to take longer to warm up, therefore more time for the choke to open.

#2 The heat riser tube and vaccume motor from the exaust manifold to the air cleaner should be in good working condition for proper engine warm up, therfore shorter choke time. There is 1 or more thermal vaccume switchs inside the air cleaner, and constant vaccume to it. When the engine is below a certain temp, the vaccume goes through a switch to a vaccume motor, that controls heated air from the exaust manifold to the carb intake, when cold, or outside air when the engine is warm, or stays on if the tempurature outside is below a set degree. Make sure that the vaccume motor operates properly in the air cleaner. Cold air entering the Carb cools it down, therfore keeps the choke springs colder longer, etc..

#3 The thermactor system, the air pump and exaust manifold intake tubing should be in good operating condition. Air is injected into the exaust manifold from the air pump to help the engine and exaust warm up faster. when the engine warms up, the air is switched downstream to the catalitic converters. Causes hotter exaust which equals to warmer air in the tubes, to go to the choke etc..

#4 The computer tempurature sensors, and computer components. Some of these systems uses a solonoid to activate power to the choke, warming it up, or turning it off when cold.

All these subsystems need to be in good operating condition for the choke to operate properly, or for faster operation when cold. It's like a cascade effect if you follow me. One system affects another etc..

If you check the entire choke system, and if it seems to be working fine, check the subsystems I mentioned above.

Hope this helps,

 

The reason there are two tubes between the carb and manifold is that one supplies cool fresh air to the manifold and the other returns that air (heated) back to the carburetor choke assy.