Rough idle when getting hot

My 81 F250 5.8M idles slightly rough - almost like its got a small vacuum leak. Sometimes it almost goes away, other times at a red light its more noticeable. I've given up trying to find out why. Mention this in case it is relevant...

The issue i'm asking about is a really rough idle when the engine heats up. This could be idling in the costco gas line for 10 minutes, stop and go traffic on a 95 degree day, etc. Whenever my engine temps climb it starts idling so poorly I worry it might stall. A couple times it has (think idling in a 4 hour wait in line for compost - don't ask haha). The only way to cure it is to start moving again to drop the engine temps to (presumably) 195 which is my thermostat set point. Its also a heavy duty truck with matching radiator so even on a 100 degree day, if i'm moving it isn't reading any hotter then on a 40 degree day (as long as i'm moving).

What could be the culprit? I have the carb set as lean as it will go when I register in February to pass emissions. I leave it there to maximize MPG. Is it simply too lean in the warm weather that gas starts to vaporize? The EGR is probably ~ 4 years old and always test fine.

Going from easy to difficult --- Is EGR vacuum hook-up correct ? Should be tied to ported vacuum only. Sounds like EGR may potentially be on manifold vacuum.

Of course, there's the vapor lock which is the more difficult one.

I've checked that (and it doesn't run thaaaat rough normally when not too hot. Rougher than i'd like but nothing unreasonable).

The primary issue only surfaces once the temp needle creeps up (but still not overheating/on the far right side).

Consider coil breakdown or the module overheating.

Coolant passing through the engine 'should' eliminate too much variation, but underhood* temps will soar if the truck isn't moving.

My phone is incredibly frustrating when it decides it knows better than me what I'm typing. 😖

hmmmmm that's a good point/question. I don't recall ever replacing the coil. Distributor cap yes, but coil no. I'll take a look at it and any records I may have. Are there any tests you recommend to identify heat related breakdown? Otherwise i'll just google it.

I've never really bothered with the module, i'm not ever sure what its role would be on my truck, it should be pre-computer stuff. Pre -EEE and other acronyms i've been able to ignore for years.

Thanks!

The module is basically just a transistor that's used to cut power to the coil and initiate spark.
The ribbed aluminum case is a big heat sink.

Ignition coils can work fine when cold and start to cut out when they get hot.

This is why I suggest examining them for function, and why I keep "known good" spares,.
As it's easier to swap one and see if the problem goes away than it is to set up a test where you can heat the component and put it under load....

I’ve ordered a motorcraft ignition module and a coil. The coil was clearly replaced at some point (it’s a yellow accel aftermarket version with matching wire). The module must be original.

The wires and connectors that plug into the module look pretty rough/ corroded. Mine have lots of broken plastic and are taped closed. Lots of green corrosion on one wire. I didn’t see a replacement online, but I’ll post a picture - I’d be curious if anyone knows of replacements.

Painless offers a DS-II wiring harness 'kit'
You can ignore the ballast and the retard wires
These are already built into the truck.

Vivek seems to have found a replacement for the elusive distributor connector, the other 2-3 are readily available.

So $$$ for everything, and then some, or you can get creative and build your own.

Dumb question. I ordered a blue streak coil which supposedly is good quality. I'm so irritated with it though. What should have been a 15 job is still incomplete. normal coils, including my old one had threaded  screws to connect the wires to.

The new Blue Streak on just has these weird studs that I can't connect to. It came with no instructions, no help, and when I google this problem nothing pops up. Pretty annoyed at this point that i'm going to have to also change my wires now and also annoyed at being so put-out by what (apparently) everyone else knows about.

....so how do I connect to these brass terminals????

Those are for the horseshoe connector that is a part of every DS-II system!

Obviously someone has butchered your ignition, but the proper connector pigtail is cheap in the aftermarket.
I had to replace mine about a decade ago.

ETA link:  https://www.amazon.com/Dorman-Help-85848-Ignition-Socket/dp/B000C5G8R2

Here's how it looks:

I never found that distributor connector (C-322). Had to modify the one that connects to the DS-II module by cutting off the ears per Jeff's excellent tutorial

https://forum.garysgaragemahal.com/C-322-Duraspark-II-distributor-connector-tp146596.html

Since the Dorman one shows to be out of stock on Amazon, this is the one I used on the coil... shows to be available. The horseshoe connector is so handy.. in case you need to disconnect the coil for safety (running compression tests, etc.. the high voltage disconnects past the coil output are not very reliable as high voltage can 'jump' when close enough... I ran one of trucks once with the main distributor feed voltage wire off slightly for a few weeks.. it rode bad at high rpms and shocked the s**z out of me a few times when I was under the hood before I realized it was off   )...

https://www.amazon.com/Standard-Motor-Products-S583-Pigtail/dp/B000C81PCM/ref=dp_fod_sccl_1/144-9369957-7318344?pd_rd_w=RpLcO&content-id=amzn1.sym.550e945f-c48e-4794-aff0-cc9017996f0a&pf_rd_p=550e945f-c48e-4794-aff0-cc9017996f0a&pf_rd_r=BCHG6D9X27S284H68MF9&pd_rd_wg=5behR&pd_rd_r=f7a54e51-0b68-46c0-ac01-c652ac1759e3&pd_rd_i=B000C81PCM&psc=1

Yeah, but the Dorman # will cross at any parts store

Just bought the horse shoe connector at Napa today and got it installed. It’s starting better and running better before it warms up so that’s a win! However while it isn’t hot enough today to really test it, it started its light “shudder” as it warmed up. When I touch the throttle it runs a lil better but I can still feel the shudder, it’s just masked at higher rpm.

Anything else I should look at? This couldn’t be from being too lean?

If it were an egr issue it should still be present when cold, correct? I’ve checked it before too so it ought to be connected correctly but could double check.

Well just read that the fuel mixture screws have no effect outside of idle, so I guess that’s not the issue if it still shudders at higher rpm.

Disconnected the vacuum line and verified its ported vacuum, no vacuum present at idle. In the past I’ve removed the egr and cleaned it/ check no crud is blocking it open. It’s also not old. Made a video of it running, notice how much the wires shake. Feels like a lopsided idle.

https://youtu.be/3Z0QOipz0EU?si=x7rwIfQpYj_Bstht

EGR should never be active at idle, that's why it's ported and on a delay as well as a thermal switch.

If it shuddering and clears up as you open the throttle it sounds rich to me.

Chris. I practically have the same truck only 2wd. it does seem that the engine has a bit of an irregular stumble. if this were mine, I would start with plugs. then wires then vacuum. I say plugs first because that gives you a chance to "read" the plugs not only inspect them. they can tell you a lot actually. second, I say wires for the obvious. no ignition works without them and in good condition and snapped on well. if headers are used then very often the wires get burned and the spark happens outside the cylinder.
now we get to the vacuum. it seems that you have a down cylinder for one reason or another. commonalities are already checked out. fuel/air and spark. UNLESS you have unmetered air in a single runner. where vacuum lines get connected can be important. pcv for example needs to be at the carb to be evenly drawn by all. if it were hooked to a single runner then you will have a lean cylinder.  same goes for any other vacuum operated device if it were not perfectly sealed. a brake booster with a leaky diaphragm is fairly common. another would be any leak in the factory hvac control circuits. you might start by capping every port and running the engine to see if there is a difference.

If the vibration only happens when hot, I would seriously inspect all the vacuum lines running off the vacuum control valves (VCVs) on the thermostat. Maybe you have a big vacuum leak and only when the temperature hits the setpoint of those VCVs, that big vacuum leak is exposed to the intake manifold...

good point. that's a possible way to narrow the field