Truck engine is running at higher rpm at 65 than it should, like it is not kicking into 3rd.?

Question

It is a 77 GMC High Sierra with a 350 auto and a rochestor 4 bbl. When I rapidly let off the gas it backfires.

This truck has a 104K original miles on it, it was owned by a elderly man and sat for 10 years without much use. It runs really smooth otherwise, engine is very quiet internally. It has the factory wheels and tires.

Answer 1

Check the vacuum tube to the carburetor from the vacuum advance module on the transmission. If it is leaking, your truck will not shift properly. If it has transmission fluid in it, the diaphragm in the modulator is ruptured.

Answer 2

If you have the wrong size wheels on it this affect will occur.

Assuming you have the stock wheels:

1. It sounds like you might be one quart low on tranny fluid and it sticks in 2nd gear. My '85 did this when it had a small leak.

2. You might need to check and or replace some of the rubber vacuum lines. They will dry rot and crumble and create small vacuum leaks.

The tranny modulator has some affect on shifting but I noticed on the older models of GM and Ford that the tranny will still shift up and down without the vacuum line connected to the tranny modulator. The shift pattern is just different and probably costs you a few MPG's.

The backfiring can come in handy if you want to scare the poop out of someone. Is it running OK otherwise? You might just need a linkage adjustment. As the motor mounts and tranny mounts age they turn into chewing gum sometimes if exposed to certain fluids and heat and the tranny settles and the linkage needs adjusted. Not too hard to do. Just chock the wheels before you climb under it with the motor off and the tranny in Neutral.

Good Luck!