Low power up hills and has trouble accelerating over 45 miles per hour

I own a 1986 Chevy Silverado C10. I have replaced much of the truck in the seven years i have owned it, including the engine about 35,000 miles ago.
The modulator valve on the transmission became unbolted from the transmission housing a while back (the “tab” of the transmission housing actually cracked off). The truck lost power when this happened and most of the transmission fluid leaked out before i realized something was wrong , stopped and called a tow. I replaced the modulator and secured it with a “Z” shaped clip made of roughly 10 gauge steel that bolts to the modulator and then to the frame that suspends the transmission. Then i noticed a loss of power up hill and the truck has trouble accelerating over 45-50 mph., possiblly has trouble shifting into over drive “?”. I replaced the detent cable next, and changed the transmission fluid, filter, and gasket while i was at it. I have also replaced plugs and wires, and done a general tune up since then, but i still have trouble on hills and accelerating on the highway. I drive up a hill every day coming home from work and the truck slows to roughly 25mph, even if i start the hill at 45mph., and it climbs slow and seems very bogged down. Any ideas? Something to check before i assume the transmission needs replaced.

Sounds like a restricted exhaust or a faulty ignition module.

What’s the best way to check the exhaust? And, can the ignition module be tested? and how? Thanks.

The HEI module will sometimes fail to extend the dwell as RPMs increase, causing the spark to weaken. The problem is quickly diagnosed with a dwell meter or oscilloscope. A vacuum meter will indicate a very direct relation between throttle opening and decreasing vacuum on an exhaust leak and disconnecting the exhaust pipe from the manifold and rechecking the vacuum will quickly confirm if that is the cause.

The truck lost power when this happened and most of the transmission fluid leaked out before i realized something was wrong , stopped and called a tow.

I think that you really have to consider the possibility that the transmission got completely smoked by the vacuum modulator/leak problem. Once all of that fluid leaked out, that’s when you got something that felt like a power problem. The problem was a badly slipping transmission and you smoked it trying to get it to go. Because of that, the problem you now have is the transmission continuing to slip now that its been trashed.

I don’t wish that on you and for your sake I hope its a simple thing like weak ignition or a plugged cat. But I’d have a transmission specialist look at it before I did anything else.

Isn’t there a big vacuum line going to that modulator. Haven’t you created a huge vacuum leak with a gerry-rigged mount? I could be wrong, I am not a GM guy and the only modulator I have ever replaced was on a road trip on someone elses car.

This rig uses TBI, right? have you ever changed the fuel filter and checked the fuel pressure?? Is this a transmission slipping problem or an engine performance problem?