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I have a '90 Formula 350 that I converted to a 5 speed. Cruise control has worked fine for years. A few months ago when I was trying to use it, it would engage but it would slowly start decelerating. Looked under the hood and could detect no vacuum leak. I have a '91 Formula 350 with 12K miles on it so I swapped the control module/vacuum actuator out of it to my '90. Took it out for a drive and it did the same thing. It's been a few months since I've been in the car so I drove it today for a long drive and it displayed the same symptoms, after initial start up but after I had driven it 15 miles it worked and held my speed at the set point. I drove the car for an hour and stopped for an hour and drove back. Continued to work. I'll check all the vacuum lines under the dash but can't understand why the cruise works when the car gets warm?
You're still losing vacuum someplace. Go back and check the lines using a hand held vacuum pump. Don't forget the vacuum lines (if there are any) running under the dash to the brake and clutch switches. Check the switches as well. Don't forget the check the servo for leaks.
If there's no outward external vacuum leaks, I would suspect the one-way valve is no longer working. Trace the vacuum lines that feed both the HVAC and cruise control. You should find a check valve. The attachment shows the vacuum line routing in my '92 Lumina (disregard the notes) - the F-Body looks different but is basically routed the same way.
Check the check valve using the vacuum pump. Sucking on each valve nipple (weee!) will also confirm if the valve is working.
The check valve holds and isolates the devices that require vacuum to the vacuum tank under conditions of low/no vacuum; such as when the engine is under load. If the check valve is always open, loss of vacuum will cause the HVAC will default to defrost and the cruise control servo will lose vacuum and not maintain set road speed.
sorry, I was out of town and I'll get to the car soon and let you know what's up. FYI, I did swap servo's with my other car. Condition still existed. Can't figure why it works after 15 miles of driving though?
