'88 5KSQ Vacuum Leak Question

[Christopher Ritchie]

I've got climate control problems and a high idle on my '88 5KSQ 
(non-turbo).  These 2 problems may or may not be related.  The engine code 
tells me I need a new ECU, but I don't believe it.  I suspect a vacuum leak 
is the cause of the problem(s), at least for the HVAC system.  I'd like to 
eliminate some areas of where to look so that I can concentrate on others.  
Questions:

1. I pulled the codes on the climate control unit/head.  All was OK.  
According to Bentley, that means it must be in the vacuum part of the system 
and not the electrical.  Is this completely accurate?  Does this mean that 
if the codes are OK, then the problem cannot be in the electrics?

2. The heater control valve will open and close depending on the mode 
selected on the climate control head.  Whether it opens and closes properly 
is another question.  When is the heater valve open?  When closed?  Is there 
always a little hot water running through the heater core?  One of my 
problems is that I get heat in all positions except OFF.  I thought maybe 
the heater valve was bad.  But it opens and closes with vacuum as you run 
through the modes on the climate control head.  The fresh air/recirculation 
flap also works depending on the mode selected on the climate control head.

3. Is there a way to work from the engine compartment to under the dash for 
vacuum lines?  That is, I'd like to temporarily block off vacuum lines to 
help in diagnosing what's wrong.  Maybe I can rule out vacuum lines and 
connections.  If so, then I could concentrate on the solenoids and 
motors/bellows.
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