Here’s an XJS heating question for you all. Apologies if it’s been addressed before but I haven’t found much on this specific presentation of a heating issue.
I turn on my '89 and allow it to properly warm up. Once temp guage is showing its habitual spot (a bit below N and I know my car’s thermostats are goo so the engine really is warmed up) I turn on the heating system in manual mode and set the temp dial all the way to 85. I should add that my blower fans both work on all settings (low, norm, high) and defroster blows like it should when selected as well.
As soon as I turn the system on the vents adjust as they are supposed to and the majority of the airflow is from the footwell vents. The center console vent shuts off like it is supposed to as well. And then nice hot heated air blows out mostly from the footwells. So clearly the system knows I want heat and it delivers that heat without issue.
But then after a minute of good heat, it just disappears. The air keep blowing from the correct vents but it becomes ambient temperature or maybe even a little cold. If I turn the dial to full cool and wait for the vents to switch and then turn it right back, I can get another minute or so of heat until the same thing happens. This cycle can repeated pretty much indefinitely it seems.
I’ve read a lot about failed heater valves, etc. but I’m unsure that that is my issue considering that I can get heat, if only temporarily.
I’m wondering if it could be the ambient temperature thermometer or maybe a different thermometer somewhere in the system? Any thoughts on this?
I should add in case it’s relevant that my A/C system does work but poorly. It’s not charged fully and I haven’t had the time to find any leaks and/or sort out an R12 option or substitute. But seeing as we are moving into colder weather, I’m more interested in getting the heat working reliably before I address the cooling recharge (I
do know that getting it work fully properly is important too though not just for passenger comfort but also because of its role in cooling return fuel).
Any thoughts on what might cause this short term good heat that disappears but can be gotten back by adjusting the temp dial?