Looking for help with truck engine losing and gaining power.

Hello, I am trying to help my dad with some truck issues. He lives in another state so I can only help over the phone and inversely can only get feedback over the phone. He has a 100% stock 1988 Chevrolet k2500 light duty. Basically a 1500 with a higher payload. It has a 5.7 tbi and 700r4. The transmission is not that old and is a gm re manufactured unit. The engine is a new gm tbi crate engine. The issue he has is when the engine is put under load say backing up a steep incline or going up a hill on the highway the engine loses power then gains power then loses power then gains power. Its almost like the timing is being retarded. If he gives it a lot of throttle input the engine seems to smooth out and power picks up. His truck is fully loaded with a cap and is filled to the brim with tools so its quite heavy. What are some things that could cause this issue?

There is no magic solution, especially over the phone or internet. Start with the basics. It's easier if the diagnosis can be done in a loaded state that duplicates the problem. Find out if your dad has access to a chassis dyno. Fuel pressure and exhaust back pressure are ideas along with looking at knock retard and TPS signal. Really before you get too crazy, you need to start with the basics. The factory GM manuals had a drivability issue diagram in them. I can not tell you how many times the customers swore they checked everything only to find the problem in the first couple of steps.

He definitely does not have access to a dyno. Last time we pulled the plugs they looked good. I am the only one with access to tunerpro to monitor data while the issue is occuring. He said he will change his cap rotor and wires and see how it goes

I'll post an update when he changes the parts.

So he changed the cap rotor and wires and the issue of the truck gaining and loosing power has disappeared.

replacing the TBI fuel pump with a TPI pump, along with modifying the regulator to run @ 13 to 14 psi made a noticeable difference in power and drivability on my 89 caprice. food for thought especially if truck is high mileage and no history if pump has been replaced.

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So he changed the cap rotor and wires and the issue of the truck gaining and loosing power has disappeared.

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This is the type of problem that's very hard to catch through a written description but takes seconds for an experienced mechanic to diagnose in real life. Putting a load on the engine causes the mixture to lean slightly which causes misfires on one or more cylinders. If the driver continues to open the throttle the ecm will add some fuel for acceleration enrichment and will eventually switch from emissions friendly 14.7:1 afr to roughly 12:1 afr. The richer mixture will help the spark jump the plug gap which will cause all cylinders to fire which, as reported, feels like the engine is producing power again. It was a good guess on your father's part and I would suggest that you or he keep a record of what parts get changed and when it happens as this can be a great tool to help in diagnosing future problms.