Electrical Smoking Wiring Issue After GT40 Head Swap

I recently finished a GT40 head swap on my 87 5.0. The car ran normally before the swap. After putting everything back together, it started and ran for a few seconds, then I heard a small pop the car shut off saw a little smoke and a wire/group of wires in the main harness getting extremely hot. the area is right after the ecc wiring comes out of the firewall on the passenger side ill include a photo although I don't think it's much help. IMPORTANT I think it's only doing that under a load. I have a multimeter and thermal imaging camera, so I'm about to start digging deeper. I'd appreciate any help normally im pretty good with this electrical stuff but im a little overwhelmed and kinda stumped as of right now. I always try to stay away from making posts but I think some collaboration would really help me out today so thanks in advance.

Edit 1: no fuses popped and hardly any wires that ive traced down are actually melted checking continuity at the circular black and tan harness connector seeing interesting readings if anyone knows the pinouts for these connectors that could be a major help.

Mcmahst said:

What on earth is that red wire bundle? That doesn’t appear to be stock. I would unravel that bundle and check each wire. I wouldn’t use any thermal imaging, unless you just want to confirm for the insurance where the fire started lol.

What year is the car, and do you have either a wiring diagram or the EVTM manual for your vehicle? Good luck.

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yeah those red wires weirded me out saw 5 taped around one so i had to investigate I actually belive it could be factory even though it seems odd there are a couple other wires soldered similarly. the car is an 87. currently looking for the pinouts of the circular black and tan connector in the bay checking continuity for shorts

rednotch said:

Probably a short to gound with that rats nest someone's been in the harness before, you probably pinch a wire or one of the exposed wires is touching metal some where and grounding. Would definitely trace those red wires, is it a Maf swap pre 89 car or something? If you heard a pop, you let the magic smoke out of something electrical.

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red wire splices egr solenoid area 3 of the wires go to connectors there and the merge into one goes into the center of the black connector that i added to the post the rest 2 red and 1 orange wire go through the firewall to the ecu it seems. visually none of the wires are hardly burnt, i see no cracks which was my initial idea because when installing heads and intake ive moved the harness a ton to get it out of the way thought maybe they made a connection somewhere under the loom but so far ive almost ripped it all out and haven't seen a thing. sorry if I seem all over the place definitely a pain in my butt not being able to voltage drop. Thanks everyone for sparing your time

nicholase said:

Those red wires are a factory splice. S119. Also in that area there is a fuse link J for your HEGO. It should be taped to, or inside the harness right around there. That could be smoking I guess. Check it. Also check your HEGO harness isn't melted on your exhaust or something.

The AC pressure switch is clearly corroded. Fix stuff as you go. What's the third wire taped to it?

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Thanks for the information im still digging around. been dealing with corroded wires and connectors ever since i got the car the reason I even started the swap was because the ground strap going from head to firewall was so green and nasty and had so much resistance the starter would struggle to get enough amps to turn over. now that ive put a massive ground strap It will turn over just not trying to set a flame to all my work ha.

nicholase said:

The wiring I posted was for a 90. You didn't have the year on your original post. So check the 88 EVTM above.

If the little braided strap from the head to firewall was casuing a starting issue, I'd look at your heavy ground cable at the block. That should be carrying most of the load. Probably a good idea to look up all the grounds in the EVTM, take them apart and clean them up good.

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Important details:
Key ON by itself does not appear to heat the harness. The engine started and ran for a couple seconds before I noticed the smoke/heat. The harness in that area became very hot. Battery-to-engine ground is installed. Ground strap from the back of the head to the body is installed. Small orange EFI ground is attached at the back of the head. Original alternator is still installed/connected. Original engine harness was never removed from the car.
Currently unplugged/deleted:
Both O2/HEGO sensors are currently unplugged. Smog/Thermactor equipment has been removed. Thermactor bypass/diverter solenoids are not being used. EGR has been deleted / EGR vacuum regulator is not being used. Charcoal canister/purge equipment has been removed.

Used the 10 pin salt and pepper connectors checking between each pin for any suspiciously low resistance no signs there (Initially I thought possible dry rotted wires cracked when doing heads and intake) I believe all my grounds are good ive probably touched about every ground within the past few months cleaning making sure I have no resistance. Ive spent nearly all day chasing this thing without even knowing what system its probably about time for the camera to pinpoint what exactly is heating up even though I only wanted to use it as a last resort. I appreciate everyone's input please let me know if there is anymore information I can provide may you all have a wonderful evening.