Warm Weather Causes 3rd Gear Shifting Problem Warm Weather Causes 3rd Gear Shifting Problem

Q. I have a 1982 Chevrolet K20 pickup with 350 cid and a TH350 transmission. My problem is that when the weather gets warm and the engine is up to normal operating temperature, it shifts very slowly into 3rd gear when I accelerate and let off the throttle. Otherwise it doesn't want to shift into 3rd. When the weather is cooler, or on shorter trips when engine temps are lower, it shifts normally. This transmission was rebuilt just before I purchased the truck, which has about 15,000 miles on it. All fluids are at the proper levels. Does anyone have an idea as to what the problem might be?
The only time the truck is run a little hard is getting up to speed in traffic. Typically it's driven only 7 miles to and from work, at about 35 mph. I wouldn't think there would be any internal problems, since the transmission was recently rebuilt, but you can never be sure of the quality of the job you are getting done. When it starts to mis-shift, it is very slow and sluggish to shift. Later when the temperature warms up more, you have to run the speed up in second gear, then let off some and it will usually shift up after several tries. When it does shift up, it shifts firmly. Could this be a sticky valve?

A. Your problem could be a number of things, but first I would check the vacuum modulator. It's a cheap thing to check to start with. It could be clogged or have parts blocked. This might not be the problem, but it's worth the quick check to save time and money. It could also be that your clutch pack is worn down, or that your direct clutch area is worn down in 3rd gear. It's improbable that the shift valve is part of your problem. Other possibilities are that you are losing pressure in the direct pressure area in 3rd gear, or that you are loosing pressure when the fluid gets warm or hot.
3rd gear and reverse are the same clutch pack. How does reverse work? If reverse is okay, then it is probably not an internal problem. In that case, it might be a weak spring on the 2-3 shifter valves or the governor is hanging up. If reverse is okay it should be a valve body problem. The best thing you can do is to bring your car into a transmission shop to be looked at and fixed.

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