What Could It Be?

[alphabuck]

this problem sounds just like one i had with an 84 bronco years ago.  at any temp under about 60 degrees the carb would ice up.  would run fine at first but the longer you drove the more the ice would build up in the venturis to the point it would just plug them right up.  if i remember right it was a vacuum line to a valve that directed heat to the carb that was at fault.  its hard to envision a fully warmed up engine and 50 degree ambient air temp allowing for ice buildup in your carb but ive seen it with my own eyes.

[Challenger6pak]

Can this computer be checked with an OBD1 reader?

[RusTy/SE]

Parts replaced so far:

computer (used)
fuel filter (new)
distributor cap (new)
rotor button (new)

Drove it yesterday and by the time it reached operating temp the loss of power, spitting and sputtering returned. Made it home, shut it off and attempted to restart right away. Backfire out of the exhaust immediately followed by idle. My ASE certified neighbor came over and we attempted to see where the timing was set but neither my Sears nor his Snap=On timing light would even flash at first. After a 10 minute cooling off, it was difficult to fire up (would turn over but not actually run) but we eventually succeeded; timing showed to be at 35 degrees. That can't be right    Checked to make sure the dist was not 180 degrees and determined the rotor was indeed pointing at #1 spark plug. We decided to shut it down and since i cannot recall when last the coil was replaced we agreed that was where i would look to next. Coils are affected by heat so maybe...

Can this computer be checked with an OBD1 reader?

I haven't looked into that yet...

[Moparal]

Russ, you sure it's not the cat converter as suggested before? Kinda sounds like it

[RusTy/SE]

It sure does but it the cat converter was removed a few years ago. Now stock manifolds into a single exhaust with a flowmaster 40.

[ntstlgl1970]

Wondering if its either rich or ignition primary issue. When the truck wouldn't start when checking the timing, did it have spark? and was it a strong blue spark or 36 degrees with all the vacuum hoses connected for the ignition timing sounds like it is in range for an ESA (electronic spark advance) system according to the manual. If the fuel pressure is good, and the ignition is good, when the issues happen, disconnect the charcoal cannister hose(s) both vent and vacuum and see if it makes a difference (may take a few seconds for the engine to clear out) if that doesn't change anything, maybe the pickup coil in the distributor is going away when the engine gets hot. See if you can duplicate it in your garage or wherever with a hairdryer on the distributor housing and see what happens. If no change, try the same thing on the ignition coil.

My gut feeling is the pickup coil and/or wiring connectors between the pickup, spark controller and ignition coil has issues.

There is little or no computer diagnosis on this truck.

[RusTy/SE]

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[Moparal]

I  had a problem with a reluctor twice on a 440 electronic dist. When it got hot, it went to ground from a crack in the insolator. I am not sure a sm blk your model has one or not. Maybe you should try a point dist and go for a ride.

[JS27N0B]

Could be lack of fuel for sure, have you tried running without the gas cap on yet?
I would pitch that computer in the garbage and switch to a standard electronic ignition set-up.

[RusTy/SE]

  Sorry for the delay in explaining things; this was just a learning experience for me. I was just so happpy to finally clear up my problem. Based on everyone's thoughts and suggestions plus searching similar threads, i've gained knowledge i didn't have before. I better understand the big picture of what i was dealing with and how an engine must function properly in all three areas - air, electricity, and fuel - before each can be optimally tuned.
  Thanks to everyone for not calling me out on my ignorance although i certainly deserved it   Over the years i've allowed too many people to work on my vehicles; they gain the experience and my vehicle is repaired but i've haven't learned anything. One day soon i'll be taking a nice Challenger on the road and it's time for me to be able to learn how to fix things myself. I've profusely thanked my next door neighbor, Bob, for dumbing down his explanations to my level of understanding so that things do make sense now. He told me i get a gold star for every area of a vehicle that i can understand to the point of fixing on my own - tuning this time Also, for some reason, Imncarn82's words from a post in the 'Fender - Repair or Replace?' thread stuck in my thoughts: "Life begins at the end of your comfort zone." Lesson learned
  Ok so you're all thinking get to the point man, what was it??!! I remember Chryco Psycho responded in part to a thread one time that when tuning, one of the most often overlooked components is the coil. So true in this case. When Bob made mention of how greasy and old looking my coil was i decided that would be the next component to be replaced. Well, turns out that it was the original piece Even date coded 246 (24th week 1986). I walked a couple of miles to the local NAPA for a replacement - did i mention the new diet plan i discovered? when your vehicle isn't running, either eat what's in the house or walk everywhere - and as soon as i turned the key it was running the smoothest i can recall both before or after the engine rebuild. All issues vanished and i'm back on the road again

Russ 
 

[Bullitt-]

 .   I'll have to add these symptoms to my vast repitwa of guesses to throw out   

Great going Russ

[Challenger6pak]

It is rewarding to figure it out.  I'm glad you were able to repair it.   

[JS27N0B]

Hope it is the fix for you.
A scope test would have revealed this as a faulty part to begin with. Sometimes it is better to get things done by a proffesional with the proper diagnostic equipment and save your self time and money down the road.

[Moparal]

Hope it is the fix for you.
A scope test would have revealed this as a faulty part to begin with. Sometimes it is better to get things done by a proffesional with the proper diagnostic equipment and save your self time and money down the road.

You are absolutely correct, but sometimes a wallet will not allow such expertise and the help and knowledge of friends and shade tree advice is all we have.

Glad it's up and running Russ

[JS27N0B]

You are absolutely correct, but sometimes a wallet will not allow such expertise and the help and knowledge of friends and shade tree advice is all we have.

you're right Al, but speaking as a tech, without proper test equipment all you can do is throw out educated guesses.