I have a question about the powertrain on my 1993 Chevy G20. It has a 5.0L motor with the 4L60-E transmission.

It struggles to maintain speed on gradual grades that you would think would be no problem. As a result, I have to take it out of Overdrive on the slightest of inclines.

My question: Is this an engine problem (as in under powered) or lack of torque in the transmission?

I put new plugs, distributor cap/rotor and nothing improved.

Another issue and I don't know if this is related, but when climbing a hill, I feel cold air out of the defrosters when the heat is on.

 

Check for codes. Throttle position sensor maybe. If it kicking out of gear to soon, then maybe a bad sensor. Could also be a vaccum leak maybe....That 5 litre is only about 190 horse.....
Cold air, maybe the heater control valve is not working right. Do you have air? Is the engine temp right? ( stuck open thermostat)

 

Actually not kicking out of overdrive, I manually shift out of OD to improve torque going up a little hill. It has me puzzled. And the only vacuum hose that looks odd is the one going to a large black bulb on the hood. It looks too tiny to carry a normal amount of air to that location. The other vacuum hoses have been changed.

No doubt, the 5.0L is really not the engine for this van. It really needs to be swapped for a 350.

 

Oh so it stays in overdrive and lugs up the hill but falls off speed.....Personally I like it when the tranny does not shift too soon. Ford trannys hunt in and out of gear and fail because of it.
I give mine a little gas before the hill to help out, but the first shift down should be the torque converter unlocking, then the downshift to drive.
Do you have the 3.42 rear? If so then you have longer gears than mine too.

I had a 1990 vandura with the 305 and the turbo 350 tranny. It was not too bad. Not nearly as good as my 02 express though. I miss the 16 mpg in the city though.