Heat comes and goes

[90MJ]

While driving I noticed the heat seems to come and go with different RPMs. The cooling system is full with no air pockets so I am assuming it may be somehow vacuum related maybe? It will blow warm air and then occasionally blows ice cold air then back to warm air again. The controls are also a bit tempermental, sometimes I have to slide it to a different setting then back to where I want it for it to blow from the locations I want. Any thoughts? I haven't owned a MJ in over 20 years until this one and haven't had this truck very long, maybe 5 weeks now, and have driven it maybe 3 of those weeks because I had that leaking fuel sender issue (what a nightmare!) so it is still extremely new again to me. Any common issues you guys know that cause this?

[Eagle]

When the heat seems to drop off -- hold your hand over the top of the dashboard and see if heat is coming out of the defroster vents. If so, you have a vacuum leak. The heater controls are operated by vacuum, and the default position when vacuum fails is defrost.

[90MJ]

I still feel it blowing from the dash vents, its just hot cold hot cold. Very intermittent. I will check the defrost vents on my drive to work soon. Isn't there a vacuum controlled valve under the hood for the heater or is it all undee the dash? Thanks

 

[cruiser54]

Still have the troublesome heater control valve installed? It's vacuum operated and prone to failure. 

Take it out. Plug the vacuum line, and splice the hoses together that were connected to it. 

[90MJ]

Sounds like a plan, I will check it tomorrow. This looks like it is all original under the hood 

[Mbryson]

Is the vacuum thing in just the Renix rigs or on the HO generation also?

[Minuit]

8 minutes ago, Mbryson said:

Is the vacuum thing in just the Renix rigs or on the HO generation also?

HO too. It was eventually eliminated in 1997. In fact, a set of 97+ heater hoses are a common way to delete the valve.