1986 XJ-S that I have owned since 1991. I have no heat.
Hose to inlet side of heater core is hot, but outlet hose is
much cooler. The heater water control valve is opening
correctly. Disconnecting the heater hoses so that I can
check that water flows through the heater core shows that I
can move water in either direction by connecting a water
source. The water pressure required to move significant
flow in either direction is very low.
With both hoses to the heater core disconnected I can blow
by mouth water from the inlet side of the heater core around
the system back to the outlet hose. This is with the engine
cold so the thermostats should both be closed. When I try
blowing water though the outlet hose, the header tank fills
to the top after which I can no longer blow by mouth any
water or air through the outlet hose.
Page 334 of the Haynes 12 Cylinder Owners Workshop Manual
shows for HE engines that the heater outlet hose goes to its
own connector to the bottom of the radiator. On my car
there is no such outlet on the radiator. My hose connects
into the bottom radiator hose that goes from the bottom of
the radiator to the water pump. The Haynes manual also
shows a direct connection from the heater outlet hose to the
header tank. According to the Haynes manual there are three
inputs to the header tank, one from the heater outlet hose,
one from the block coolant fill (just below the cap) and one
from the coolant cross pipe. My header tank has always had
only two sources. The coolant cross pipe and one from the
block coolant fill (just below the cap). I disconnected the
hose from the block coolant fill to see where the water was
entering the header tank. It is coming from the the coolant
cross pipe. I wonder if the left (driver’s side) thermostat
has failed closed and if this would cause the problem.
However I would think a closed left thermostat would cause
the car to run very hot and it does not. Any ideas anyone
can share will be appreciated. One thought I have is that
when I had the radiator core replaced a few years ago they
installed a non HE radiator, but looking a the layout of the
1980 non HE cooling system, the heater should still work. I
am pretty sure the heating system worked fine after the
radiator installation, but I cannot remember for sure.–
Gordon Mortin
In reply to a message from Gordon Mortin sent Sun 13 Dec 2015:
Gordon,
Nor the engine thermostats nor the radiator have any effect
on the heater function, unless someone really messed up with
you plumbing…
Since your heater outlet hose is cool this means that you
have non or very limited water flow.
You say your heater valve is opening correctly… did you
test if water flows through ?
Also did you check that vacuum is turned on and off by your
climate control servo ?
If all above are working as they should you must have a
blockage somewhere in your lines.
To test that you could by-pass heater and valve, connect a
clear piece of hose and see if you have flow.
Best,
Aristides–
The original message included these comments:
1986 XJ-S that I have owned since 1991. I have no heat.
Hose to inlet side of heater core is hot, but outlet hose is
much cooler. The heater water control valve is opening
correctly. Disconnecting the heater hoses so that I can
check that water flows through the heater core shows that I
can move water in either direction by connecting a water
source. The water pressure required to move significant
flow in either direction is very low.
With both hoses to the heater core disconnected I can blow
by mouth water from the inlet side of the heater core around
the system back to the outlet hose. This is with the engine
In reply to a message from Aristides sent Sun 13 Dec 2015:
Aristides,
Thank you for your timely reply. I have been out of town
all week so today is my first chance to respond.
I have disconnected the heater core exit hose from the tube
that runs down the body under the left exhaust connecting to
a hose that goes into the bottom radiator hose. Blowing
through this hose causes water to go back through the heater
core, the heater control valve and then either through the
passages in the right head or the outside the head and block
tube and hoses that return water to the Coolant Crosspipe if
the right thermostat is closed. I believe that with a cold
engine, the water is going to the Crosspipe, then from the
Crosspipe through the hose that connects the Crosspipe to
the header tank. When the header tank is full and now
subject to the 15 lb pressure cap, the water then flows
through the pump lower radiator hose connection to the hose
and tube components back to the top and out of the tube to
which the exit hose from the heater core normally connects.
This tells me that the pathways to and through the heater
core and the control valve are clear. The control valve is
open without vacuum to the control valve and closed with
vacuum unless the heater switch is on in which case vacuum
is closed and the heater control valve is closed.
When I try blowing through the top of the tube to which the
heater core exit hose would be connected, the header tank
fills, but as soon as it is filled, I can no longer blow
through the tube. For some reason water will go backwards
through the system but not forward through the system. It
is like there is a one way check valve in the system that
has been installed in the wrong direction. However I can
find no cooling system one way check valves.
Also my Haynes Manual shows for a 1980 non HE engine the
heater core water exit line connects to the lower radiator
hose. For HE models (I presume 1980 but the manual does not
say) there is a separate connection point at the bottom of
the radiator to which the heater core water exit line
connects. Also the header tank has two hoses, one to the
coolant pipe and one directly to the tube to which the
heater core exit hose connects. My car is a 1986 HE. The
header tank has three hoses. One to the Crosspipe, one to
the engine block filler tube just below the cap and one to
an atmospheric catchment tank located behind the left wheel.
Given that I can blow backwards through the heater system
and get water at the top of the tube to which the heater
core exit hose connects, it seems reasonable that I should
be able to blow in the direction that water is designed to
flow to and through the heater core and get water at the
heater core exit hose. I cannot and I suspect that is the
reason I have no heat. For some reason water moving in the
‘‘to the heater core’’ direction is somehow inhibited. I am
baffled. Anyone with an idea, please respond.–
The original message included these comments:
Hose to inlet side of heater core is hot, but outlet hose is
much cooler. The heater water control valve is opening
correctly. Disconnecting the heater hoses so that I can
check that water flows through the heater core shows that I
can move water in either direction by connecting a water
source. The water pressure required to move significant
No check valves. Coolant should flow from the A-bank rail
though the control valve, then the matrix, and finally to
the radiator outlet (through a 90 degree elbow connection on
the later cars), where the pump will recirculate it back in
the system.
The control valve is OK when you test, but how is the vacuum
to it?
Steve–
The original message included these comments:
Also my Haynes Manual shows for a 1980 non HE engine the
heater core water exit line connects to the lower radiator
hose. For HE models (I presume 1980 but the manual does not
say) there is a separate connection point at the bottom of
the radiator to which the heater core water exit line
connects. Also the header tank has two hoses, one to the
coolant pipe and one directly to the tube to which the
heater core exit hose connects. My car is a 1986 HE. The
header tank has three hoses. One to the Crosspipe, one to
the engine block filler tube just below the cap and one to
an atmospheric catchment tank located behind the left wheel.
In reply to a message from sbobev sent Sat 19 Dec 2015:
Thank you for taking time to read the long post and for the
link to the heating system parts diagram. That diagram is
the way my system is set up.
With the engine running but cold, I have vacuum to the
control valve. Once the engine water temperature reaches
operating temperature the vacuum goes away and the control
valve opens. I believe this is the way it is supposed to
work. The control valve defaults to open if there is no
vacuum. Before I changed the control valve, the diaphragm
apparently had a leak because the valve would not hold
vacuum. However in this case, the valve was open but I
still did not have heat and water apparently not
circulating. I changed the heater control valve thinking it
was the problem to no heat but it was not.
What none of this explains is why I can blow through the
heater core exit line back through the heater core and get
water out of the pipe to which the heater core exit line
connects. When I blow through the tube as if water were
moving in the proper direction around the cooling system, I
can only blow until the header tank is full after which I
cannot blow any more and no water comes out the heater core
exit hose. This seems to mirror the no water circulating
condition that I am experiencing but I cannot figure out why
I can move water through the heater system backwards but non
forwards. I think if I could understand this phenomena I
would be able to fix the problem.–
The original message included these comments:
The parts diagram can be found here: http://tinyurl.com/o7tgvk7
No check valves. Coolant should flow from the A-bank rail
The control valve is OK when you test, but how is the vacuum
In reply to a message from Gordon Mortin sent Sun 20 Dec 2015:
Problem closer to being solved. I removed the coolant
filler tube cap and blew through a tube connected to the
point where the heater core water exit line connects to the
tube/line going to the lower radiator hose. Water came out
of filler tube (with the header tank cap in place). I
replaced the cap and removed the hose connected to the input
side of the cater control valve. Blowing through the same
tube connected to the the tube/line going to the lower
radiator hose, water comes out the hose that would connect
to the input side of the water control valve. I then
connected a hose to the input side of the water control
valve and was unable to move any air through the hose. I
inserted a screw driver into the input side of the water
control valve. Based on the distance the screw driver went
into the water control valve, the control valve is open and
clear given that the screw driver went in far enough to have
exited the control valve exit tube and entered the hose
connecting the control valve to the heater core. This would
lead me to believe the heater core is plugged however, if I
blow from the heater core exit tube back through the heater
core, water comes out the input tube on the water control
valve implying that the heater core is not plugged. It
appears to me that there must be something in the line from
the water control valve to the heater core that is acting
like a one way check valve. Whatever it is, it moves out of
the way when water moves backwards through the heater core
but plugs the input side of the heater core when water
attempts to move forward through the heater core. More to
come as tomorrow or Wednesday I will remove the input line
from the heater core and see what I find. Thanks to those
of you who have responded with suggestions.–
The original message included these comments:
Thank you for taking time to read the long post and for the
link to the heating system parts diagram. That diagram is
connects. When I blow through the tube as if water were
moving in the proper direction around the cooling system, I
can only blow until the header tank is full after which I
cannot blow any more and no water comes out the heater core
exit hose. This seems to mirror the no water circulating
In reply to a message from Gordon Mortin sent Sun 20 Dec 2015:
Gordon,
The fact that the control valve opens only when the water
reaches operating temperature sounds a bit strange to me…
As I have a different car though I might be wrong, but the
philosophy behind the heating system is to heat the cabin
the fastest possible and not wait until the engine warms up…
At least on my 86 XJ V12 the application of vacuum at the
control valve has no relation to engine temp and the control
valve sees vacuum or non vacuum regardless. There is only a
thermo-switch connected to the heater core that disables the
blowers until the heater reaches a certain temperature so
they won’t blow freezing cold air in your face…–
The original message included these comments:
With the engine running but cold, I have vacuum to the
control valve. Once the engine water temperature reaches
operating temperature the vacuum goes away and the control
valve opens. I believe this is the way it is supposed to
work. The control valve defaults to open if there is no
vacuum. Before I changed the control valve, the diaphragm
In reply to a message from Gordon Mortin sent Mon 21 Dec 2015:
I would suggest you test each component separately to help
you with troubleshooting…
Your theory that there is something in your heater core
blocking the water flow in only one direction is very plausible.
BarLeaks or calc deposits could very well do that.
Fill your heater core with white vinegar and leave it
overnight. It does miracles with calc.
Then blow water through it with a water hose and see what
comes out… it might solve your problem.
Hope that helps,
Aristides–
The original message included these comments:
Problem closer to being solved. I removed the coolant
filler tube cap and blew through a tube connected to the
point where the heater core water exit line connects to the
tube/line going to the lower radiator hose. Water came out
of filler tube (with the header tank cap in place). I
replaced the cap and removed the hose connected to the input
tube connected to the the tube/line going to the lower
radiator hose, water comes out the hose that would connect
to the input side of the water control valve. I then
connected a hose to the input side of the water control
valve and was unable to move any air through the hose. I
In reply to a message from Aristides sent Wed 23 Dec 2015:
You are exactly correct and I was wrong. I have vacuum at
the water control valve when the engine is running and the
blower control switch in the off position. With the blower
control switch out of the off position, the vacuum disappears
and the heater control valve goes to full open. The water
temperature sensor above the driver’s side foot well (LH
drive car) works properly, but there is no warm water to the
copper tube to which the temperature sensor is fastened.
This is because there is no water flowing through the heater
core. I will blow the water out of the heater core and fill
the heater core with white vinegar this afternoon and let it
sit over night. Thank you for the suggestion.–
The original message included these comments:
Your theory that there is something in your heater core
blocking the water flow in only one direction is very plausible.
BarLeaks or calc deposits could very well do that.
Fill your heater core with white vinegar and leave it
overnight. It does miracles with calc.
In reply to a message from Gordon Mortin sent Wed 23 Dec 2015:
Problem narrowed to either the hose between the heater
control valve and the heater core or inside the heater core.
I removed the heater control valve and disconnected the
heater core exit hose from the tube that leads to the point
just in front of the bottom heater hose where the heater
core exit water joins the engine cooling system. I tried
blowing through the heater valve to heater core hose. I was
unable to blow any air through the heater core. I tried
blowing air through the exit hose back through the heater
core and exiting the heater control valve to heater core
hose. I was able to easily blow air. I connected a water
hose to the heater core exit hose and was easily able to run
water back through the heater core exiting at the heater
control valve to heater core hose. The exiting water was
collected in a five gallon bucket. While the water was a
light rust color, nothing exited that would have blocked the
heater core. I ran a total of 15 gallons of water in this
manner. I then tried blowing air from the inlet side hose
to the exit side hose of the heater core and was able to
blow air. I then ran water from the inlet hose through the
heater core exiting at the exit hose. Water ran easy. I
tried blowing from the entrance hose and water easily exited
the exit hose. I added a water hose connection point in the
inlet hose to the water control valve.
I drained the cooling system and then flushed it until the
water ran clear. During this process I had heat in the car
(lots of it). I let the car sit overnight. Yesterday I
drained the water and refilled with a 50/50 mixture of
antifreeze. I ran the engine to normal temperature. I had
less heat in the car than the day before and the heater core
exit hose seemed a little cooler than the entrance hose.
Today I drove the car about 25 miles at 50 mph or greater.
I had no heat and when I stopped for gas I checked the
heater core exit hose. It was not near as warm as the
entrance hose.
Apparently I still have the same problem, but have now
narrowed it to either the hose from the heater control valve
to the heater core or something in the heater core. The
only thing I can think of with the hose is that it has come
apart inside the hose but attached at the top or bottom so
that water flowing in the proper direction through the hose
causes the fallen material to plug the hose, but water
movement in the opposite direction opens the hose. If it is
not the hose, then something must be stuck in the heater
core that blocks the heater core with flow in one direction
put pushes out of the way with flow in the reverse
direction.
Anyway, more to come. I am ordering a new inlet hose
tomorrow and will replace it sometime this week.–
The original message included these comments:
This is because there is no water flowing through the heater
core. I will blow the water out of the heater core and fill
In reply to a message from Gordon Mortin sent Sun 27 Dec 2015:
Gordon,
Remove the heater valve and bench test it.
Apply vacuum with a syringe or something.
Vacuum = No flow - valve is closed
No Vacuum = full flow - valve is open
The heater valves can also be directional, and usually there
is a flow arrow… Is could be possible it’s installed the
wrong way around, or that its the wrong type of valve.
From what you write I understand that water flows freely
through your water heater, on both directions, so I don’t
think the heater is your problem.
In reply to a message from Aristides sent Mon 28 Dec 2015:
It is the recommended original valve and there is only one
way to mount it.
There is no vacuum to the valve. I checked that and it is
‘‘zero.’’
Before I back flushed the heater core with plain water, I
could not blow (lung capacity) air or water through the
heater core. After I back flushed the core with plain
water, I was able to blow air and water through the heater
core in the direction flow is supposed to go. After I
filled the cooling system with water and ran the engine I
had excellent heat. The next day I drained the water and
filled with 50% antifreeze/water. I have no heat and the
heater core exit hose is much cooler than the entrance hose
indicating the heater core is blocked in the normal water
flow direction.–
The original message included these comments:
Remove the heater valve and bench test it.
Apply vacuum with a syringe or something.
Vacuum = No flow - valve is closed
No Vacuum = full flow - valve is open
is a flow arrow… Is could be possible it’s installed the
In reply to a message from Gordon Mortin sent Mon 28 Dec 2015:
Gordon:
I suspect that if you removed the
vacuum operated heater valve in
front of the bulkhead and spliced the
heater lines together that the heat
would come right on.
The ‘‘jump diagnostic technique’’!!
carl–
The original message included these comments:
There is no vacuum to the valve. I checked that and it is
‘‘zero.’’
In reply to a message from cadjag sent Wed 30 Dec 2015:
Gaining on the problem, but also going backwards a bit. First
I jumped the heater core by connecting the output of the
heater valve directly to the heater system water return line
to the radiator. Hot water running through the hose so heater
control valve not the problem. I then filled the heater core
with a 50/50 water/antifreeze mixture by pouring the mixture
slowly through the input hose untill it came out the output
hose to make sure the core did not have an air lock. I
connected the input hose to the heater control valve and the
output hose to a longer hose that I could loop around and
connect to the heater system return line to the radiator. I
disconnected the vacuum to the heater control valve and also
connected a vacuum gauge to the vacuum line so that I could
read any vacuum on the line. This configuration gave the
following results:
Once the car warmed up the entire heater water line was
hot to the touch. No vacuum on the line.
With the heat temperature control full hot (85 degrees)and
the heater control switch in the ‘‘Auto’’ position, the air
compressor came on and shortly afterwards the hose connected
to the heater core output line cooled down. There was no
vacuum on the vacuum line.
Air at the dash corner vents started out very warm but not
hot but soon turned cool.
When I turned the temperature to full cool, the center
vents put out cold air but there was still no vacuum on the
vacuum line.
Moving the temperature control from full cold to full hot
does not seem to engage the heater servo.
A week ago I could hear the heater servo operating whenever I
moved the temperature selector and when I went to full cold
with the temperature selector I would get vacuum on the vacuum
line to the heater control valve.
Where I think I am is that I now have two problems. First is
that I have lost power to the servo. I fixed this problem
last year by replacing the servo, but apparently I have the
problem back. Even with the servo working and even though the
control rods were adjusted in accordance with Kirby Palm’s
book, I had no heat at the heat vents and cold air at the
tunnel vents. So the second problem is that even with the
rods properly adjusted, for some reason the air conditioned
air is cooling the heater core. This would explain why the
output of the heater core as measured by the output hose
temperature starts out hot until the air compressor comes on
after which the heater core output hose cools significantly.
The heater core output hose should always be hot except when
the temperature selector switch is in full cold. If the
heater core water is not hot, then the temperature sensor
switch does not enable other things to be energized.
So next problem is to find out why the servo is not working.
I suspect it has lost power and that happened last year.
Turns out a relay under left (driver’s side LHD car) was bad
so no power to relay box behind left tunnel panel.
More to come.–
The original message included these comments:
In reply to a message from Gordon Mortin sent Mon 28 Dec 2015:
I suspect that if you removed the
vacuum operated heater valve in
front of the bulkhead and spliced the
heater lines together that the heat
would come right on.