Cruise control works for a while then starts to lose speed

[gmbellew]

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)

 

[Frankenchevy]

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

…carpet, headliner and an FM radio too.

 

[rwjtexas]

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.

 

[Redfish]

All y’all with your fancy pants cruise control…

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

…carpet, headliner and an FM radio too.

Y'all are starting to hurt my feelings.

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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.

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[mibars]

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.

 

[Rusty Nail]

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

 

[SquareRoot]

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

 

[idahovette]

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

 

[Bextreme04]

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

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.

 

[mibars]

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.

 

[mibars]

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?

 

[gmbellew]

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