'78 Thermal Vacuum Valve -- "Weak vacuum"

Hi all,

I've FINALLY launched into troubleshooting my emissions equipment. Everything in the EGR system checks OK, except for one item I'm not sure about -- the thermal vacuum valve. It's the device with engine coolant running through it that supplies vacuum to the EGR system after the engine warms up.

In the EGR INSPECTION section of my FSM (p. EC-13), it says:

*Disconnect one end (BPT valve side) of vacuum hose connecting thermal vacuum valve to BPT valve.

*increase engine speed from idling to 3000 - 3500 RPM

*Make sure that thermal vacuum valve is close and that throttle chamber vacuum does not exist at end of vacuum hose. If a vacuum is present, replace thermal vacuum valve.

Then warm up engine above 122 deg, and repeat test. This time, "if vacuum is weak or is not present at all, replace thermal vacuum valve."

I read absolutely no vacuum up to about 1/3 scale on my temp gauge, which is as it should be.

Above 1/3 scale, I do read a vacuum, but it is only about 5-6" Hg -- well short of the manifold vacuum. But then I consider that the vacuum takeoff is somewhere in the throttle chamber, so it could be almost anything, depending on where the tube opens in the chamber.

So what is a "weak" vacuum? I wish they had provided a number. Is 6 in Hg "weak?" Is my thermal vacuum valve operating within normal limits?

Thanks!

Sarah