Finding the Electrical Gremlin. Help!

Good morning, all! I am still fighting with my 1986 F150 w/ EFI 302 in it. Since my last post I have "figured" out the injector issue, but I still have a miss/lurch under load. Its noticeable when you first start the truck and it is revving higher than normal, and when the AOD downshifts. Recently I have noticed that if I am cruising along at 55 and I push the horn in quick succession (I found this out of frustration) it will also create the lurch/miss. This leads me to believe it's an electrical issue. Has anyone else had a similar issue? Not sure how the horn would affect the trucks' ability to run, but maybe someone out here knows! Thanks, in advanced.

This is interesting. I don't have an answer but I'm following this one.

I think you have a ground problem.  (That's my go-to answer when bizarre things happen, and this one is certainly that.)

Looking at the 1986 EVTM I see that the computer (ECA) is only grounded via G208 and G710.  G208 is "near throttle position solenoid" and G710 is at the "left hand side of dash panel".

What's odd is that on the page of the EVTM (Pg 60) showing the EFI wiring it clearly shows that it grounds at G701, but on the page that is purported to show all of the grounds, below, I don't see the ECA.  In fact, the same holds true for the horn - I see it going to G701 on the horn page (Page 108) but not the ground page - although maybe I missed it?

Anyway, if you have a ground problem then honking the horn could easily cause the ECA to do bizarre things.  And it is easy to have a ground problem.  First, there's only one wire grounding the cab - the ground strap from the engine to the firewall.  Second, G701 could have come loose.  And on and on.

So I'd start by checking to make sure the grounds are good.

Well, I was touching the upper and got a zap today randomly. Looked around the motor and couldn’t see a block to firewall ground so I just ran some wire to test it. I can now down shift with no cutting out! It still has a cut when I push the horn…but if I don’t honk then it’s fine. Getting closer!

Progress!  Yes, grounding makes a big difference.  But you may still have a problem with the ground on the firewall near the radio.  And it might not be just the horn since so many other things use that as a ground.  So the next thing I'd do is to check that ground.  Make sure it isn't rusty, and maybe remove the screw, clean both the connector and the firewall with sandpaper, and then coat them with anti-seize compound before reinstalling.

Oh, and check to see that the battery to engine ground connections are good.

Yeah, I believe there is still something with grounds to be figured out. When you first start the truck it sputters a bit and then smooths out and there are not issue after it idles down. No sputtering while driving. Sometimes when I shift to park it will idle way down and then recover. I thought it was maybe the idle air but I changed that part and no luck.