'99 XJ8 VDP: No heat from heater
 

Except on a rare occasion, I'm not getting heat through my vents. I have some theories, but wanted to get feedback or any additional thoughts.

Here's some information:

- I am leaking coolant at a fairly slow rate from the thermostat housing cap, but I'm keeping the expansion tank filled and check it on a regular basis, until I'm able to replace the thermostat housing.

- I am running the wrong coolant. This was an amateur mistake when I first got the car. I had completely flushed the system and have been running green coolant for quite some time. My plan is to re-flush the system sometime soon (after I fix the leak) and refill with the correct coolant.

- With defrost button, defroster blows ice cold AC air on the window and never heats up. With climate control set to auto and a warm temperature setting, it never heats up...except on a rare occasion where some semi-warm air blows for awhile.

- The car itself does not give any indications that it is overheating.

- I think this might be a coolant temperature sensor problem (perhaps it never detects that the coolant/water has heated up, so it doesn't send any to the heater matrix?) The self-test from the climate controls does show a code 14 (temp sensor problem), however, the guide seems to say that if it doesn't beep when showing the code (it doesn't) then it is an old code that just hasn't ever been cleared.

- Another theory is the thermostat, though if it were stuck closed, I imagine the engine would have overheated when I drove for 5 1/2 hours the other day (and if it were stuck open, that's a problem, but not one that should affect cabin heat...I think)

- And lastly, is it possible that I've clogged up a hose or pump by running the green coolant?

One of the first things I plan to do is measure the resistance across the coolant temp sensor as the car warms up, but I'm open to any other suggestions. We're getting into the cold/rainy season, and heat sounds really good. :-)

The green coolant would be fine as long as it is maintained.

If you have a obd-ii reader and can see the car go into closed loop, or read the temperature as being over 180*F, the sensor is fine.

The trouble spots are normally clogged heater cores or a dead auxillary heater pump.

Thanks. My obd-II reader was able to receive a coolant temp reading, and it was over 180, however, it only got a reading every several minutes. I'm not sure how often it should.

My thermostat housing cap is leaking. But, maybe more importantly, the bottom of my transmission pan has a considerable amount of coolant dripping off of it. Initially I thought that the leak from the thermostat housing was working its way back to the transmission as I was driving, but given the very small amount coming from the housing, and the greater amount dripping off the transmission, I don't think that could be the case. If the heater matrix is blocked, is it reasonable to think that the coolant would somehow leak onto the top of the transmission from the area of the heater core? And if the auxiliary pump were not functioning, could that cause a similar leak?

I would also have a look at the heater hoses that run down the valley, had a pin hole in one of mine that would run down the valley and drip off the bell housing .

Had same problem. Turned out to be clogged heater core. While I have been using the correct coolant since I owned, have been told that possible use of wrong coolant in the past has caused the core to clog. The problem comes back every winter though. I have to flush/backflush the core and heat comes back. Worth checking the auxiliary pump for malfunction too. I had replaced it originally thinking it was my problem.

Cheers

I have a 99 XJ8. Also, almost no heat during cold weather. Not a mechanic, but here seems to be a myriad of problems which may cause this including a clogged heater core. On my car, my mechanic told me there is a blend door beteen the heater box and the A/C evaporator which is binding due to it's age. They were not able to fix it and said there is no available part. They diagnosed it by running a fiber optic camera through the duct.