1998 XJ8 Vanden Plas

If I have the heater blow out only into the main cabin, it's hot. If I have it blow out only to the floor area, it's luke warm. If I have it blow out into BOTH the main cabin and floor area, it will often go cold (or, at least, not blow hot). There is also an odd vibration sensation around the brake pedal when it blows into the floor area.

This seems to be an intermittent problem (when parked, heat it often fine when blown into both areas). I tried reading all the posts in the forum but did not see anything similar, so would really appreciate any advice.

I'm assuming the heating core is fine since the main cab easily gets hot quickly. Is it possible there is a blockage in a pipe going to the heater around the floor area? What's the easiest way to diagnose and fix this?

Thanks!

 

It sounds like the flaps that control the direction of the air flow are not behaving as expected. I will have to leave it to other folks that are more knowledgable on the x308 platform. I have a lot of experience with the old s3. It was done via vacume lines. If you have heat then he heater is fine.

 

Thanks - I assessed the air vents, and no apparent blockage. I can now verify that the air seems to go from hot, to lukewarm, to sometimes a little cool by itself - most prominent when the car is in motion (if I park, the heat will now come in quickly and it will stay hot). I really appreciate any additional insight from people who may have experienced this issue.

 

Have you checked out the cabin temperature sensor ? Apparently the little grille gets clogged up and can cause problems. It's got a little fan in the housing to pull air through it. It's located in the trunk/boot release area. I'm sure there's something in the archives about cleaning these.

 

I believe that my VDP is doing the same or nearly the same thing. Look for the "Yet More Heater Problems" thread. Can you tell if there is a periodicity to the temperature swings? Mine is about 3 mins hot, 3 mins cool, and so on.

No one seems to have a "eureka" insight into this particular issue. Just check out the booster pump, valve, cabin sensor. But in my case will await warmer weather.