Bronco 400M missing on load when hot

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DMAN

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Looking for help on 1979 Bronco XLT Ranger with 400M /C6/ Auto.

I was out on a drive and it just started missing and would stall if throttle applied.

Carb rebuilt

New Coil

New factory Ign Control

New Distributor

New Plugs

New Ported Vac Switches

Tested an cleaned EGR - holding vacuum

Tested check valves on vacuum limes

White TVS switch , Not sure *** to test??

Engine will run strong for few miles and after it sits develops bad miss on load in gear.

Will idle fine, but when put in gear and throttle applied it will miss bad. loping like only running on a few cylinders or not getting any fuel

When cooled engine will run fine again???

This truck has been the most reliable vehicle I have owned!! till now.

Any Ideas???

 

Bully Bob

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Hi Dman.., welcome..!

Sounds like the idle circuit is OK ---- but, the main jets may have issues.

Meaning my 1st guess would be the carb.   What kind is it.?

BTW.., what do you mean "hot" ..?

Is it the proper coil for your ignition..?

How long ago did you put in those 6 new parts.?

 
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Seabronc

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Ditto Bob and have you checked the timing when it is hot?  Your vacuum advance circuit could be messed up if one vacuum components, ie. a PVS is bad.

:)>-

 

Rons beast

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Bob and Seabronc have given you the most common culprits. I would check all that first.  You didn't indicate that you changed wires.  Old wires can break down under load. 

Did you use platinum plugs?  I have found that occasionally under load the platinum plugs will not fire correctly and cause a stumble and miss. ( something about "compression suppression" from the engineers)  I had this happen in one of my own cars and encountered it in other customer vehicles too. When the platinum plugs were replaced with "ordinary" plugs the problems did not occur.

Good Luck 

 

Seabronc

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There is one other cause of a symptom like that which happened to me years a go.  It was caused by arching inside the distributor cap and only happened under load.  Look for 1 or more tiny lines along the surface inside the cap.  I think the initial cause was moisture inside the cap but once the damage was done it would crop up during high moisture conditions.

:)>-

 
OP
OP
D

DMAN

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 Thanks for the reply's, I changed out the dizzy and seems to have taking care of the heat soak on the ignition.

Replaced the TVS switch inline going to egr

The carb was redone again and bench flowed and tested

Installed new carb, set timing, now will only run with egr disconnected???  EGR will hold vac and seems to be working ok

With the egr disconnected it will run but l can't get it to idle smooth and have a real bad stumble on mid throttle and it will pop though the carb when I stab it for full throttle. Also no power??

Any ideas???

 

Seabronc

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With the egr disconnected it will run but l can't get it to idle smooth and have a real bad stumble on mid throttle and it will pop though the carb when I stab it for full throttle. Also no power??

Any ideas???
It sounds like a problem with timing perhaps the timing chain or the distributor is off by a tooth.  You need to take a look at the timing under load.  Could be arching wires,  You can check for arching by looking at it in the dark and putting a load on the engine.  

:)>-

 

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