Engine misfire troublshooting

[Kudzupatch]

93 Suburban, 170K on very well maintained vehicle. A few weeks back I gave it full tune up, wires, cap, rotor, plugs etc. I took it on trip into the mountains and it started misfiring under light to moderate loads. For example when we hit a bit of a climb and had to step down on the throttle a little bit.

Stop and when it cools down it runs fine till it gets warm again. No indications it is overheating at all. But the warmer it was the worse the problem seemed to be. Sounded like the ignition module so I replaced that and it seemed to fix it.

Then, everyone once in a while I would get a backfire through the TBI. I finally started seeing a pattern. While it only does it occasionally it always happens under acceleration or an increase in throttle and load and only at lower throttle openings. If you step down hard on it doesn't do it.

This week I made a 300 mile road trip on the interstate and it starts doing it again but consistently after 200+ miles. Rough idle, rough running at low to moderate loads with occasional backfires when you increase the load or start up from a red light. I did have a small load in the vehicles and 300 lb trailer behind it. Again, more throttle and it smooths out.

Got another trip coming up in a couple of weeks so I am anxious to find the problem. Fuel pressure gauge is on order. Will start there and report what I find. What else could cause this?