Massive Power Loss when Hot

Hey,
so I have struggled the past 3 weeks to figure out what is going on with my car. It's a 2003 Mustang 3.8L V6.
First she started loosing power when the AC was on, more than usual. Then she started to loose power overall. I cannot figure out why.
I can repeatedly get her back to peak performance when she cools down over night. But even simply sitting in the sun during the day is already too much.
I have datalogs from a quarterhorse with TunerPro available, but I'm a little unsure how to interpret what I am seeing. Some observations I have are the MAF cts are a tad higher than they were before (but that might just be because I need to almost floor her to leave from a stop light), and IPSIBR goes all the way up to 0.5 and my ISCDTY (ISC duty cycle) is almost all the way to the top all the time.
Already posted over at efidynotuning.com to see if someone can make sense of that.

The symptoms I notice are hesitation, sometimes sputtering when I hit the gas too quickly to revv her, and I have heard the occasional mild backfire from the exhaust on decel.
Also hard to start once hot, cranks, fires, but doesn't catch to idle revvs unless I give her a little bit of gas

Things I have replaced in the past 3-4 weeks that did absolutely nothing (not necessarily in this order):
- Spark Plugs
- Spark plug wires
- coil pack
- Fuel filter
- Air filter
- cleaned MAF with MAF cleaner
- Passenger side exhaust manifold (fixed the notorious cracking due to dripping AC condensation)
- recharged AC (I'm never running it though, she wouldn't accelerate or even go up a slight incline with it on)
- Valve stem seals
- Throttle body gasked (had to take it off to access valve covers)
- oil + filter change
- seafoam (spray for intake and fluid for fuel and oil)
- lucas injector cleaner
- 93 octane (just for the heck of it as I was running out of ideas)
- cut a vent hole in the hood (I'm gonna take the hood off later just to see if it does something)


Basically, it feels like she has like maybe 50 hp left once she reaches 200°. And it's been continuously getting worse day by day, except for in the mornings where she's cooled down.

I'm kinda desperate at this point, I don't know what else to do. I'm a little frustrated because I seem to be throwing the wrong parts at her.


EDIT:
For the record, some additional things that have been worked on during the last year (in order, most recent at the top):
- quarterhorse installed to run different strategy on PCM (to make the stick work properly)
- auto to manual swap
- thermostat changed
- water pump gasket changed
- radiator changed (and consequently flushed, duh)
- ball joints, battery terminals, radio, brake pads and disks, other chassis stuff.
- IMRC clips broken

 

Well I'm reading from the Quarterhorse, and it hovers between 200-214 depending on if I am sitting somewhere and fan is on or not, but it seems steady.

 

I can do that tomorrow probably.
Wouldn't it have low compression all the time though? Like, in the mornings when she's cold, she has all the power back...that's what's confusing me...

 

Timing seems to be ok, I have all the datalogs from the quarterhorse. It does retard it significantly though, but only to the borderline knock table values, so that should be fine. And again, she's running perfectly fine in the mornings.

I think before I go do the compression check I will get one of those magic compression restorer magical potions and see if that does anything. I only have this week left before I leave the country for a little while, and the car has to go into storage anyways, so if there is no compression I'm screwed either way. So I'm gonna try that, if that fixes it, great, if not, I'll put the car into storage and worry about that in half a year when I have a little more time.

Sorry to shut down your idea, but time constraints and personal circumstances make it hard for me to justify that time right now. Totally would do it though if I wasn't leaving at the end of this week. I was just hoping to get a nice roadtrip in before that, but I have already given up on that idea.

Also I'll see what happens if I take the hood off, maybe it is a heat issue.

Thank you for the suggestion though! I like this forum, there's actually helpful stuff going on here. Should've consulted here 3 weeks ago...

 

I've been running 87 since I had her (155k, now 168k), ran half a can of sea foam in the tank and the other half in the oil like 300 miles ago, did the spray seafoam stuff at the same time, changed the oil couple days ago, again, split a can of seafoam between oil and fuel, and two days ago put 93 octane out of desperation in her and yesterday dumped a can of lucas injector cleaner in it because I didn't know what to do and it was only $4. So plenty of cleaning stuff in it.

 

Also I tried to see if I have leaky injectors by holding the gas to the floor and cranking it (which shuts off the injectors), and there was absolutely no ignition of any sorts, so that leads me to conclude that the injectors are PROBABLY fine? Or at least not overly leaking...

Well I’m gonna run out later and grab the magic compression cure stuff and see what it does, and if it doesn’t do anything...well...it’s probably more serious and I won’t be able to fix it in time anyways.

 

Wow that's a lot of replies.

@07GtS197 IAT's hover around 90 on a cold day, go up to 130 sitting in traffic. But I had moments where the car was totally fine with IAT's at 95, and moments where she felt like 50hp with IAT's around 90.

@ttocs Texas, so still 100F. I did change the thermostat in April, when I swapped to a manual. Did the water pump gasket too, and consequently changed the coolant. Wouldn't I see rising temps if it was clogged up though? She's sitting steady at 204 at cruise and 210-218 with the fan on in traffic. It was a little brown earlier when I checked it, so that was a little concerning.

@JAD67428 I was hoping that was not the case. Out of desperation I got baited and bought some extreme cleaner for catalytic converters, so I'll see if that does something. There's no CEL for it though and the O2's SEEM to read a working cat. I don't have a thermometer to check though. Would explain the heat thing though, I would assume they close up more with more heat.

@waynenorcross yeah no CEL.


For a moment I was thinking of running open headers to see if that solves it (then it would definetly be the cats), but that seems unwise as the O2's are on the Y-pipe...any other ideas on testing that without buying a new Y-pipe?