Cruise control works for a while then starts to lose speed

most of the issues with not holding speed that I've had have been vacuum related. if you can't set the speed or you lose the speed setting when you touch the stalk, my issues have been switch related. the servo and module under the dash have never given me issues, yet (fingers crossed)

Goldie Driver said:

Probably has ac and a cloth seat - fancy city slicker truck.

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…carpet, headliner and an FM radio too.

Here is a 1984 Cruise III document that is quite detailed ... it is before the Electronic CC were installed. If yours is all vacuum this may help in the diagnosis.

Mine is an 85 ... still don't have mine working, but I have not worked on it for a while ... getting painted at the time .. when I get it back I will go through the components.

One other thing .. mine is a Diesel so the Vacuum Pump drives my CC and A/C only.... I do not have reserve canister.

Frankenchevy said:

All y’all with your fancy pants cruise control…

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Goldie Driver said:

Probably has ac and a cloth seat - fancy city slicker truck.

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Frankenchevy said:

…carpet, headliner and an FM radio too.

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Y'all are starting to hurt my feelings.





I am following this thread because my Cruise Control won't hold unless I am above 50mph and then it won't last very long.

And my truck isn't exactly a "city slicker" truck. It does get used but it is a lot nicer truck inside than I expected to find.

Nice looking rig!
To be honest with the market we have in Europe I got what I could, even though Suburban was on my wish list for years it would be nicer to have a Blazer/Jimmy or a short bed pickup, possibly even RWD. Burb looks nice, but the carpets, etc. are far from being practical, however it is really high on attention grabbing scale, mostly because it's size

In a meantime I've inspected my vacuum canister, broke the vacuum nipple off, epoxied it back in place, replaced some more vacuum hose (not, all, I run out of hose ) aaand... It worked for like 10 km cruise to work, no issues. Let's hope that the issue is gone.

Yeap, I bet there's a vacuum leak somewhere. If you broke that nipple - it was probably there. Reckon it's fixed now.

Vacuum leaks suck. I have one and only one sucky line on my truck.
Dakota Digital, Vintage Air, EFI. Hydroboost =no sucky

@Rusty Nail .....lol....Rusty said "nipple"

SquareRoot said:

Vacuum leaks suck. I have one and only sucky line on my truck.
Dakota Digital, Vintage Air, EFI.

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Yep, when the new engine/trans goes in I'll only have three vacuum lines on the whole truck. The big one to the brake booster, a small one from a dedicated nipple on the back of the manifold to the adjustable FPR, and a "T" off to the AC system. The cruise control will be a GMT-400 box mounted on the firewall and a rostra replacement turn signal stalk. No more big vacuum diaphragms mounted on the intake manifold or vacuum brake switches or unobtanium cruise control transducers mounted on the fender. When I first got the truck and was working on getting the speedometer and cruise control to work, I got everything going and then the truck would just go full throttle when the cruise was activated. I quickly disconnected everything and decided it wasn't worth spending any more time on it. I'm looking forward to having cruise control back once I have the overdrive trans and engine power to make long distance drives more reasonable.

A follow-up: Cruise seems to be fixed. Where was the leak - I don't know, I bet that the best guess is from Rusty Nail with the vacuum pod leak around the nipple.
Now it seems to have more power, I can feel how it accelerates and slows down a bit in cycles and even has enough force to downshift from overdrive when I hold the accel slider long enough.

It did it again, so I jerry rigged vacuum gauge on a cowl measuring vacuum past the check valve. I can also see that the system holds vacuum for couple of hours after parking before it finally reaches zero.

I can clearly see the pulses from cruise control solenoid especially when it is at 19 in Hg or so. With each pulse the stored vacuum drops a bit, it is around 14-15, at one moment it went down to 11 or 12 at a slight incline. I didn’t catch the moment when I lose the ability to hold speed but my guess is that after the while the system may be running out of vacuum and possibly the engine at cruise is not producing enough. The moment I switch it off the needed drops and vacuum is replenished.

So I have two more questions:
-What’s the typical vacuum at cruise at 55 mph for the 350 with 700R4 in OD, 3.7x gears, 33 in tires? Engine compression is fine and even enough (tested cold)
-What’s the vacuum needed to actuate the throttle to hold the speed?

highway cruising vacuum may be 10-12 inches at 75mph or so. I'd guess at 55mph it would be higher.

check your lines and reservoir well. with the system off and holding vacuum, wiggle and jiggle the lines and see if it drops.

if all is good there, you may have to take the solenoid apart and clean up the disintegrated foam as it may be plugging up something. somebody recently posted a great video on how to repair the solenoid

A new issue surfaced: I had a sudden acceleration from cruise control, so I decided to finally investigate the servo.

Based on this Youtube video slowing down like I had before is usually a case of a dirty release valve. I suspected that acceleration could be caused by dirt in a vacuum valve.

I documented the process for anyone troubleshooting their system.

Servo removal is straightforward, 3x 7mm screw on back hold the servo to the bracket on 350 TBI engine.
Remove the 2x Torx to reveal valve body. Red coil with metal plunger is a release valve, the green one with plastic top is a vacuum valve. The middle one is a position sensor.



At this point plunger and a rod from release valve can be removed and I advise to do so as it’s easy to loose. To disassemble solenoids you need to remove three further torx screws, however you can keep the top one in palce if you want to remove valve assembly as a whole. If you do remove it then watch the coil wires as they are fragile, there are also couple of small parts in between that will fall off.



Thee valve body is friction fit via o-ring to the back of diaphragm. With two (or three) screws removed you can pry it off.

Here are the solenoids removed and disassembled from the top, and a description of parts:
Release valve (normally open)
Base -> spring -> sealing cup -> metal plate -> piston (?) -> wavy spring washer -> coil -> thin rod -> plunger

Vacuum valve (normally closed)
(From top) Base with a rod that sticks out -> o-ring – metal plate – o-ring – coil.
(from the bottom) White orifice tube -> spring -> rod with a rubber seal -> base.

Once disassembled you can see the dreaded foam filter. Mine looked intact, however it turned into dust upon touching. After cleaning the remains I replaced it with a strip cut form dish sponge as it is resistant to high temperatures.



The bigger issue was the vacuum valve. I tried to remove the white orifice, but it cracked into pieces and a spring from valve popped through the now-enlarged hole. The plastic was like a very hard chalk. I had to remove it with a tiny milling bit and bunch of sharp tools to scrape the remains.




I found that a brass stovetop gas nozzle is a good replacement for that white orifice. They come in various nozzle sizes and the standard M6 x 1 mm thread can be filed down a bit to make it press fit into opening (tip: mount it into drill to file it down)



I tested the cleaned, reassembled valve by sucking on a piece of hose and suppling 12V to the coil to actuate it. I still got intermittent leak upon closing that I couldn’t get rid of with any further cleaning. I replaced the spring with the stronger one I had (the bigger one above) adjusted by shortening to actuate at about 10 V applied to the coil. This solved a valve leakage. Another source of leak I found was from two flattened o-rings, sandwiched on both sides of metal plate in a valve assembly. I didn’t have replacement so I cleaned them as much as I could and applied some Vaseline with hopes of softening the rubber, it did the trick.

The above was enough to fix my cruise control, it holds the speed nicely, roughly +/- 1 mph of setpoint, acceleration via the "ACCEL/Resume" slider works correctly and I also no longer can feel kind of cycling that I occasionally felt before, where every second or two it slightly accelerated and slowed down. Vacuum valve is also holding vacuum correctly, I can change HVAC controls hours after shutting off the engine.


FYI: Connector has 5 spades, from left to right:

Vacuum valve + | sensor coil | common ground | sensor coil | release valve +

Applying 12 V at any polarity actuates valves, they actually start to operate at about 6 to 7 V. Solenoid coils draw about 0.25 A at 12V, so they are roughly 50 ohm each. As they share common ground and work at any polarity you can conveniently actuate both by applying 24 V (or less) to Vacuum + and release +.

I hope it helps anyone struggling with a malfunctioning cruise control servo.

Great post!