Vacuum issue 98 Mazda

[juice90]

An issue has developed in my 98 B4000 4x4. The heater /ac keeps changing from what ever I have it set on, IE a/c, vent, heat, etc to the default defrost position, so I seem to be losing vacuum and then getting it back again.

Does anybody know where there is a vac diagram available?

It looks like the vac lines run out from under the passenger side, under the blower motor, then connect into where the vac line is that runs the vac for the front hub lock on the pass fender under the coolant reserve and air filter.

Is that correct? or does it route some where else?

I plan to remove the vac hubs and change over to manual, so plan to get rid of that junk anyways. So if I am correct, I should be able to disconnect the hub crap and re route the vac line for the heater/ ac system.

No vac leaks that I can find, just seems that its either getting blocked off, or diverted.

 

[Mark_88]

Not sure of the exact vac line layout on a 4.0, but the symptoms you describe indicate that a check valve is either missing (recently, since the issue developed) or not functioning properly.

On earlier models like the 2.3 setup, there is/was a round ball that attached to the vac line that goes into the firewall to activate the various settings in the cab. That ball contains a check valve that stopped the vacuum from affecting the inner workings of the hvac for the cab and sounds like it is either not functioning or is missing in action.

The vac leak would probably cause an eventual CEL indicating a lean condition and you'd have rough idle as well, so it sounds like the check valve either in the ball or whatever setup was on your system has failed.

Trace the vac line again to see if there is any other object or direction it may be going...the ball is pretty obvious so you should recognize that immediately if present...otherwise, this is where you will need the diagram to see where the check valve is located...although from my experience with early vac diagrams, they are not exactly well documented and usually show representative symbols as opposed to what you would see under the hood.