It's definitely a lack of vacuum, but there may not be much you can do about
it. The first thing to do is check the heater control itself. Some of
these have a vacuum switch that tends to come apart over time. I've
replaced a couple over the years. The switch in question is what directs
the vacuum to the right places as you move the lever on the control. You'll
know it by the vacuum hose gang plug attached to it.
Next, look for any vacuum leaks under the hood. Look for cracked hoses,
etc. Listen for whistles and noises. Then, you can use carb cleaner and
spray the hoses while the engine is running. If it changes rpm, stumbles,
etc., you know you have a leak at that location.
Having said all that, an engine with 300k miles is going to be pretty worn.
It's going to have a good bit of cylinder, ring, and valve seal wear, and
will not produce as much vacuum due to lower compression.
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