I have no heat in the 99 XJ8. I have checked the auxilary pump it is working. Checked input to pump and very very little fluid no flow. so assumed the bypass valve was stuck closed. after getting coolant everywhere got the valve out. it passed water just fine when i put under faucet. so put back in. does the system have to be under pressure for there to input to the aux pump? car is running but still not anything there. if i squeeze the hose on the other side of the bypass valve water will squirt through


at this point my only thoughts are system must be under pressure and i have a clogged heater core. would like opinions on this

 

Sounds like a clogged heater core.
I would flush it.

 

Disconnect both heater core hoses and with a rubber tipped air blower. Seal it tightly against one heater core fitting and blow to unclog. Start slowly if you know how to use finger control on the blower or lower air pressure. Then reconnect and flush cooling system. If the core is not unclogged first it will never be able to be flushed...a flush doesnt work like draino

 

I am having the same problem as hillsj2 with heater not working in my 1999 XJ8. A/C is fine. I can get some heat if I drive the car long enough, but it is not adequate. Also, the heat increases if I down-shift and increase rpm's. Hillsj2, please post results if you unclog heater core per Brutal's suggestion. Thanks.

 

Rmill, if it gets hot as you rev engine, thats a sign the aux. Pump isnt working. More than clogged. Can be both but check power and ground at the pump, and listen or feel it for operation. Thats its function is to boost coolant flow through the heater core at idle or low rpm. Withoutit you get cold air quickly since the engines water pump wont flow enough at the core

 

Brtual, that makes sense. I will check the pump. Thank you.

 

One of those radiator flush solutions that dissolve scale may help to unclog the heater matrix.

 

it was a clogged heater core. blew it out with air then water. then flush system. then blow out again. the flush process is slow with this car because dont have a good way to drain the whole system at one.

 

Glad you could solve the problem!
And thanks for the feedback!

 

Brutal, I found the heat pump motor. I could feel no vibration. It didn't seem like it was operating. However, there was fluid moving in the front hose--I think it was moving away from the pump making it the outlet from the pump/inlet to the heater core. I couldn't get my hand on the rear hose. I was able to pinch down on the hose and could feel the coolant being choked off. I disconnected the electrical connection to the pump. The coolant flow stopped. There was no noticable change in the pump, itself. Could this still mean the pump is not working--just not pwerfully enough? You mentioned in your previous post that the water pump pushes some coolant through the core, but not enough for adequate heat at low rpm's. Shouldn't the water pump have continued pushing some coolant through the heat pump when I cut the power to the pump? Thanks.

 

Did it pass water in both positions or just the closed position - meaning without the heat on (valve closed) the water will pass through the top tubes (in line) when you have the heat on (valve open) the water will pass through lower (offset) tubes. You would have to put voltage to the input of the valve to determine if it is opening or not.

If you flush the matrix and bypass the valve, connecting the heater lines directly, you will get extremely hot air coming in (though you won't be able to modulate the temp) and you will know that it is the valve going bad.