I thought fuel too, and replaced the regulator. It doesn't seem to matter which pump I select.
I have a TFI (ICM) on order. It should be here in a couple days. Mine is not distributor mounted, but remote on the heat sink. I have cheaper Standard one coming for testing. If it makes a difference, I will get another Motorcraft one to replace it and keep it as a spare. If it doesn't then I will have to keep looking.
In the mornings, when cool, seems to be way less severe. When the day is warm, running in closed loop is when is it the worst. I have only tested the EGR valve at idle, by pulling a vacuum on it. It appears to be actuating correctly, and I don't have a code for the position sensor. But what ever it is, is worst in closed loop hot day.
I forgot, does your truck use a MAF sensor or is it SD? Problems that occur in closed-loop operation can be traced to sensors that aren't reading right. The biggest contributors are the MAF and EGO sensors. But if those were bad enough, they would have triggered codes in the EEC self-test.
Just for giggles, (and I was bored) I re-ran fuel pressure test. With the vacuum disconnected, it get 36 aft tank and 38 front tank. 22-24 psi at idle, with vacuum connected.
Still not codes, ran KOEO and KOER tests. This one has a MAP sensor. I had an aftermarket in, to test with, and no improvement. So I put the Motorcraft back in. The EGO is a new Bosch. I changed that about a month ago.
I was thinking about the EGR position sensor. It reads ok with vacuum pump, and didn't appear to have a bad spot, but I am wondering if it is breaking down inside when it is getting hot. But no codes.
The EGR itself seems to test fine, and vacuum at idle is about 17 - 18 inches and nice and steady. I don't think it is leaking by, but it could be hunting back and forth in closed loop. Is there a way to test that, without throwing the computer into failsafe mode while driving? If I disconnect the vacuum line, and plug it, won't the computer pick that up as a fault, and take it out of closed loop? I would like to see if drivability improves with the EGR out of the loop.
The fuel pressure should read at the high end of the range with the vacuum line disconnected, and it looks like yours is in the range, though 36 seems a little low. Is there a chance you can keep watching the fuel pressure while the truck is under load? This will check to see if the pump is able to keep up with fuel delivery.
I don't think I have enough hose or the right fittings to reach that far with my fuel pressure tester. Vacuum is about all I can tell while driving. I found an aftermarket check valve for my fuel pump problem with leakdown, and will get one on order.
I am still leaning toward ignition. when I am at cruise, it seems like someone flips a switch, and I lose a cylinder or two for a second or two, then comes right back. It is just more pronounced when the day is hot. I routed the plug wires per Ford TSB diagram, with recent cap and rotor. New plug wires.
I think I will change the plugs, re-re-route the wires, and wait for my test TFI to come in.
Unless you can think of anything else to try while I am waiting.
Based on your description of how abrupt the power loss is, it does seem like an ignition problem. You can verify that once you do your tests with the new parts.
I once worked on a car that also had a mysterious stumble, which would stall the engine some times. It turned out to be a bad ground connection. One of the bundles of wires that ran through the engine compartment has many individual ground wires coming from sensors and actuators. They meet up at one point in the harness where a small section of insulation is stripped off, and they are pressed together so hard that the copper strands actually cold weld together. At least that's what was supposed to happen. In this car, a couple of the ground wires worked loose, causing the intermittent stumble. That was a difficult find, but I got it by moving and tugging on parts of the wiring harness to try to replicate the condition.
I did go through the TFI harness per Ford TSB, and found no problems. Re-wrapped the foil and harness. I got to thinking yesterday, that the coil that is on there is a cheapo, and about 8 years old. Since this is temperature related, (it is often close to or over 100 here) I got to thinking maybe that is breaking down electrically inside. So, since I have been reduced to throwing parts at it, I am going to grab a coil and some new plugs on the way home tonight from work, and swap them out..
Well, still going... Changed out the coil to a high output Accel, and changed the ICM (TFI) and plugs. I thought the problems was solved, but each day that goes by, it is getting back to it's normal miss. It seemed to run great for the first day or so. I was confused by that, but then thought that I did disconnect the battery while changing the parts out. With the computer reset, it ran good. The longer history it builds, the worse it runs. I don' know why it would do that unless there is still something that is out of limits in the sensors that it is trending, but not throwing a code. Either hard or soft code. Or the computer itself? It is definitely an ignition problem, but the only parts unchanged are the hall effect trigger in the distributor, the distributor itself, and the computer. Timing check shows that the timing is advancing when replacing the spout.