Heater matrix, 740 Volvo

If I put the heater control to the cold setting and start the engine then
put the heater control to hot I get a gurgle through the heater.

If I put the heater control back to cold the gurgle stops, put it back open
again and it gurgles and so on.

I have flushed the heater several times and reversed flushed it but cant
understand why it only gurgles on the hot position.

I was on a run the other day and the heater went cold, I thought Oh the
radiator must be running low on water but as I was on the motorway kept
going.

As I was driving I put the car in to neutral ( shift gears ) and gave it a
few quick revs up to 7000 rpm and the heater started to work again.

Are the two problems linked I would think so.

 

Sounds like air in the cooling system. The "cold" indicates an "air lock" in
the heater core. This has happened to me in the past when i did not totally
"burp" the system when changing the coolant/hoses. Some vehicles are a real
"tough nut to crack". They have "easy ways" - like in the Chrysler 2.2 engine
where you elevate the front end or else use the bleed plug at the top of the
thermostat housing. Not sure what to do on your 740, but yes, there is air in
there.

Perhaps the revving has forced the air out (and into the expansion tank where
it belongs).

Ken

 

In addition, it does it only in the "hot" position because the heater flow
is controlled by a valve under the hood near the firewall.

You may find it helpful to burp the system by turning on the heater and
climbing the steepest hill you can find in low gear. That puts the heater
relatively lower, and the high engine revs should purge it. If you don't
have a steep hill, level ground will have to do.

Ken, my nomination for "tough nut to crack" was my '69 Lotus Europa.
Unfortunately, it was the hoses to the radiator at the other end of the car
that needed to burp, and I never found a better way than to drive it for a
week or so with nearly constant overheating until suddenly it would suck
down the entire reservoir. So why do I still miss that car?

Mike

 

I am already on a steep drive with the car facing up the drive.

 

I vaguely remember changing the coolant in my 240 years ago. I think
the cooling and heater system is virtually the same (except access to
the heater core... grin).

Seems to me the system burped itself in about 15 minutes of warmup
after I finished filling it the first time. After about a week it
had gradually burped itself again and needed about another pint in
the expansion tank to get above "min."

As you guys say, driving the car up and down hills (maybe some
turns?) ought to do the trick. This is really just normal driving.

My S70 is pretty similar. After running the car, re topping the
coolant, and putting my tools away, I still need to add more coolant
after the first drive.

 

How 'bout this? Loosen off the coolant filler cap a 1/4 turn, and leave it
like that on your next drive around town. The system should burp itself well
enough after that. Then, check coolant levels. Top up if needed, and all
should be well.

RS

 

Loosening the filler can will just stop the cooling system from pressurizing and
coming up to operating temp properly without boiling over. Pressurization has no
effect on flow in the system. any air bubbles in the heater core/matrix or
system are at the same pressure as the pressure at the top of the coolant at the
filler cap. Pressurization or not, the air bubbles have to get out of the system
somehow - like folks have said: try driving uphill, reving with the rad higher
that the heater matrix, varying the engine speed once operating temp is reached
(and the thermostat is open).

Yes, that water flow valve HAS to be open - as in choose the HOT selection.
Otherwise no coolant flows through the core.

Ken

 

My point is that sometimes, the filler cap doesn't let off pressure properly
as it's designed to do. They can fail. I've seen it happen. And loosening
the cap a little (not so it is off, just loosen it a little) lets enough
pressure and trapped air (again, I've seen Volvo mechanics do this) out of
the system. It has been done on my car a couple of times, and it works. It's
a closed system, and if the air has no where to go, how's it going to get
out?

RS

 

While the system is closed, the air can still rise to the top of the coolant in
the "overflow" tank. When that happends the coolant level in the tank will fall
by a volume equal to that of the air bubbles. In the closed system everything
has the same pressure - even the air that is above the coolant and any bubbles
within the flowing coolant in the system, and the air bubbles trapped in the
heater matrix. SAME pressure - and when the bubbles surface they don't have to
go anywhere because they have escaped the coolant and now lie above the coolant
in the expansion tank - which is where they should be.

Ken
Canada

 

I get the gurgle as soon as I start the engine which would indicate to me
that the air lock was in the heater, how long does it take for the water to
circulate the engine that is based on idle as if the revs are higher the
water will circulate quicker

 

I see. Thanks for the explanation.

RS

 

It should not take long. I tooled around for less than five minutes,
after installing the heater core. When I went to back out of the garage,
I heard a gurgling as I turned. Haven't heart one since.

The beauty of the Volvo cooling system is that it's self bleeding. If you
have a cheap thermostat or have the thermostat installed incorrectly.. it
won't bleed. I've never once had a problem getting the air out of a red
block cooling system. Altho you will probably have to top it off a few
times.

Poor heat output could be many things (aside from air in the system):

- broken heater control valve
- dying water pump
- vacuum hose to the heater control valve is kinked
- clogged evaporator