1990 GMC Sierra 5.7 engine miss under load
 

I just got back from a 1000 mile trip towing a heavy trailer. All was well going down to pick up the load, but on the way back, with the heavy load, the engine started to miss badly on hills. When it downshifted to 1st gear, I had to back off on the throttle to keep it from stalling altogether. Idle is still fine. Check engine light came on and off during the return trip. I have never done much in the way of maintenance except oil and filter changes. Plugs looked fine last time I pulled 'em. I have no tool for pulling the codes. Anything to check before I take it to a mechanic? 92000 miles. Auto trans. 4WD.

Update...
 

Tore into the ignition system. Replaced cap, rotor, plugs, wires. All were in really bad shape. No improvement in the high speed miss, though. I'm going to change the fuel filter today, and I added a can of injector cleaner to see if it helps. Plugs were worn, but they were REALLY clean. Not the usual tan color you would expect. Running too lean? That's why I'm now thinking fuel problem. Since there were no replies to my original post, I guess I'm on my own. New to this forum. Maybe I'm expecting too much.

Look into testing your Throttle Position Sensor.

water may be getting into one of your cyine

Fixed....
 

I finally read how to retrieve the trouble codes from the computer. Turn on the ignition, jumper the top right two pins of the connector and the codes will blink out, starting with 12, which means diagnostic mode. Each code blinks three times. Showed a code 44, which is "Lean exhaust". Turned out that the fuel filter was the culprit. Runs great now. Hope this helps someone else.