Heater matrix, 740 Volvo

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

Rock wrote:

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

Rusty wrote:

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

Rusty wrote:

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

Pressurization

core/matrix or

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

-- alex