Cruise Control Surging

[izzy_Britton]

I just purchased an 86 single cab SWB 305/700r4 cream puff, 81,000 original miles.

She has been sitting for at least 5 years i am thinking closer to 7 years. I just got it smogged, passed with flying colors. All of the vacuum lines were checked by the smog tech and everything was plugged in as it should be. all the lines look good and there is no dry rotting or visible cracking. I have tested MOST of the vacuum lines to check for leaks using starter fluid or brake parts cleaner, and everything seems 100%.

To the issue:

While driving down the freeway at 70ish MPH, when the cruise control is engaged the speed seem to remain pretty constant at the set speed. However the issue is two fold. While at speed and holding, the truck surges or lurches enough that it can be felt and is quite annoying. the second part is that after several miles. say 3-4 miles, the speed slowly starts to decrease. stepping on the throttle pedal slightly will kick the trans down a gear and the truck will take off again until it reaches the set speed and the whole cycle begins again.

for giggles this past weekend, I set cruise at 70, and didnt touch the throttle pedal for about 30 miles. the speed decreased to an unsafe speed and i exited the freeway around 40 MPH. Once on the off ramp i stabbed the throttle pedal quickly once and it took off to gain speed again but i had to stop at the sign at the top of the ramp.

There is a vacuum line going to the vacuum pot on the left inner fender well, it is intact. what else controls the cruise control function?

this is the 38th c/k truck i have owned in the past 20+ years. this is the only one that has/had a functioning cruise control.

Any words of wisdom would be greatly appreciated. Thanks in Advance

Izzy