I checked the mono valve operation: Looks OK, default to full heat.
Secondary water valve also OK. 1.1 amp. while running, no problem there.
Vaccum: At idle vaccum is at 14.9 BUT falls to 5-6 for a second or two, then back to 14+. This cycles 2 or 3 times than reverts back to full vaccum. That might be the cause.
What can cause vacuum to fall for such a short period and come back to normal? I checked all vacuum hoses, including those behind the glove box, everything seems normal. Engine idle is very stable (700RPM) and smooth even when vaccum drops.
Also, could a low alternator output at idle cause this problem?