Finally fixed; stalling when hot (long)

I have a 93 Jimmy, 4wd, 4.3, VIN W,150,000 miles. I live in Phoenix, AZ (yes, this is a factor). A year ago, I started having a problem where the truck would drastically lose power and want to stall when it got hot. If I pressed on the accelerator it would get worse and if pushed hard enough, backfire through the exhaust. I would have to let off the gas and feather the pedal trying to put as little torque on the engine as possible. If I was lucky, I might be able to maintain speed. Most times I would have to reduce speed and allow the engine to cool. At the time I had to drive it at highway speeds for about an hour to get it to act up. It initially would happen at over 70 mph. It would do the same thing in the city and lower speeds if the truck and the ambient temperature got warm enough. There were no codes or service engine lights produced.

I replaced the fuel filter and fuel pump with no improvement. I took it to my (former) mechanic who diagnosed it as a bad EGR valve. I argued that the EGR would only be acting up at low speeds but he stayed firm. I paid about $100 to de-carbon the engine (probably a Sea Foam job) with no improvement. I then replaced the EGR. There were very little signs of carbon on the EGR and I saw no improvement. My mechanic still said it was definitely the EGR and the intake manifold had to be taken apart and the carbon buildup physically removed. He stated that carbon chunks were blowing through the manifold and lodging in the EGR. I didn’t think this sounded right as that would be an intermittent problem, not a predictable one like I was experiencing. So I did nothing until this past March. I had minimal problems with the truck as the weather was cooler and I did not take it on road trips.

As the weather started to warm up, the problem started occurring more often. So I bit the bullet and decided to clean the carbon from the manifold. There was almost no carbon buildup at all! Here is all of the work that I did:

Replaced fuel injectors (with a set I had lying around from a previous misdiagnosis)
Replaced the Nut Kit
New plugs, wires, distributor cap, rotor, coil
Cleaned upper and lower intake manifolds (was not that bad)

I noticed a significant improvement in the performance of the engine. I then de-carboned the engine with Sea Foam. I saw with no noticeable additional improvement, and possibly a rougher idle after the treatment. I thought I had solved the problem until about six weeks later I started seeing the same symptoms except they would now occur after only about 7 to 10 miles of driving. I even got it to act up while idling in my driveway (after driving from work). I purposely floored the accelerator to get it to act up and I actually made it stall. I immediately got out and smelled the exhaust (no odor), and opened up the hood to looking for odors or odd sounds (none found).

This past weekend I decided to look at the sensors. Here is what I did:

Inspected, cleaned, and/or tested:
MAP Sensor
Intake Air Temperature Sensor
Throttle Position Sensor

Replaced:
Ignition Module (I had it tested and it tested good, I replaced anyway)
Pick-up coil

I also cleaned every electrical connection I could easily find with contact cleaner.

I am fairly certain that I have finally solved the problem. I tested the truck pretty extensively yesterday afternoon in 112-degree heat, driving about 20 miles with hard acceleration and starting and stopping. It also performed great today with my work commute in which it has been acting up on for the past week (at cooler ambient temperatures). I am not sure which of the above actually fixed the problem, but I am suspecting the ignition module. I do notice a difference in how the engine acts, most notably that the clutch fan kicks in more often. Of course the air temp has shot up about 10 degrees over the weekend as well. The idle is also much smoother.

I hope this helps someone out. This board has been instrumental in helping me fix the problem.