'77 vacuum question

[Rick Dorminy]

How much vacuum is used in the operation of the heating/AC control? If there is a leak there, would that be enough to cause carb problems?
thanks.

[Wayne W.]

Re: '77 vacuum question

It is such of a small hose that it would cause little problems, but there are lots of places elsewhere that in combination might.

[Michael W.]

My experience is that

if the leak is big enough - an open hose somewhere- it can cause the engine to stall at idle.

To see if it's your headlight system at fault, disconnect the tube from the intake manifold and temporarily plug the opening.

[Steve Junkersfeld]

Re: '77 vacuum question

Rick, I just went through a similar situation with my 77 L-48. I had low vacuum and a persistent miss. The problem was a leaking heater valve vacuum control hose. I replaced the hose and the situation improved - but it took replacing all the vacuum hoses to get the engine vacuum back up and the engine running well. I suggest you remove all the vacuum control hoses (headlights, cruise control, vapor cannister, and heater), and get a baseline vacuum reading on the engine. Replace hoses one at a time to locate the culprit. If your hoses are original - replace them all. Good luck, Steve

[Tom R.]

Re: '77 vacuum question

Rick:

It was touched on but your best bet is to begin diagnostics at the manifold vacuum port. Vacuum goes three places and then spreads; engine operations, headlamp operations and heater controls. The heater controls including vacuum operations are in places you may never see. For example and as you've described, the vacuum comes into the heater control through the firewall to the heater controls. From the switch it operates several plenum doors that control the flow of both hot and cold air...one of which runs back out to the passenger fender. All of these have little vacuum hoses but while they are not as subject to the engine compartment elements as those within the compartment...they can cause nuisance problems. The vacuum also operates the hot water shut-off valve (A/C). You might not even notice this vacuum source is disconnected or that it is a vacuum source unless you know that it is! That sounds redundant but I think you get the point.

Dr Rebuild offers some of the best graphics and diagrams and are helpful for tracking this stuff down. Diagrams and a vacuum pump gauge and this will allow you test both the system and the individual devices such a switches, plenum doors and their actuators. good luck and keep us posted.