Hello. 1994 Volvo 940 non turbo. A few days ago temp needle started climbing. I stopped. Checked coolant level. Was below minimum. So added coolant. ~ 25 miles later same thing. I have looked really carefully at all hoses & fittings and can see no dripping or leak. Done with car running.

I only checked engine compartment area/under hood. Could leak be from heater within the car ? Thinking maybe as coolant passes thru there. No wetness on either driver or passenger side.

Not sure but I think what may be heater valve which is on one of the two coolant hoses that pass thru firewall into car is not working. Valve does not move when turning from cold to hot air. Shouldn't it ? Could this need replacement ?

Thanks in advance for any info & advice with this problem.

So this weekend I finally resolved this matter. Turns out leak was from heater control valve. When AC is on it closes and then guess due to pressure leaked. Didn't notice as only have AC on while driving. Leak would drip atop tranny which is hot and evaporate. While I was at it I replaced water pump as it was moist and weeping around top seal. All is good now. Thanks everyone.

I REALLLLY hope you used new Volvo brand heater hoses. If not, check this periodically because I have never heard of a brand of aftermarket hoses for this location that lasted more than a couple of years.

If your expansion tank cap is old , replace it, a wonky pressure cap will do strange things with your coolant.
Now for tracing leaks, test dont guess.
Go to your parts store and get a little bottle of UV dye for cooling systems and a set of UV glasses and a light ( like the ones used for checking ac systems for leaks, you can get a cheap set for about $15) pour the dye in the expansion tank and run the thing up to temp and drive it., then preferably in a dark place . run it up on ramps and get under there with the uv light with the glasses on and go over the thing slowly. if there are any external leaks, there will be a bright yellow/green snail trail leading you directly to the point of the leak. No more guessing!
If you dont find any external leak, well, it's time to run a compression test, because you may have a head gasket problem., but before jumping to any conclusions, replace that cap.
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-------Robert,:'00 XC70. '95 960, '93 940t, '90 240 wagon, '94 940SE, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '92 Ford F350 diesel dually

Check the transmission level and see if it's filling up with coolant via a radiator/trans cooler internal failure.

If not that, your heater core theory may be real. Or also check for a heater control valve failure (common). But I would think you'd see a puddle under the car if that was the case. Look around the water pump real well.

OP back. Thanx for reply Chris. I checked tranny. Both level & color are fine fortunately. Forgot to mention I put in new thermostat. Didn't help. Just got in from checking heater core area. It is fine. Dry. No drips or leaks there.

How does one test heater control valve ? Is this valve the one I described in my original post ? I honestly am not sure where it is. If valve is problem can I bypass it until I get part ? How do I bypass it ?

I checked water pump. It is a bit moist but I find that common as rubber donut usually does weep a little. Doesn't look any more or less moist then it always has. Sounds fine. No dripping from it while watching it while running. However
maybe it has gotten worse and drip is from bottom not top where I am looking from. Maybe heat of engine is burning up & evaporating coolant drip/leak. Perhaps I should change pump.

Any moisture on or around the water pump is not a good thing. Often times they will leak worse as the engine cools down and then tighten up and stop leaking when the engine is warmed up. That may be your problem.

The hat seal between the top of the pump and the engine head is a common point of failure. If the pump isn't leaking around the shaft you can get a gasket and seal kit for about $5 and re-mount the pump. However a lot of people feel if they're going to the trouble of removing the pump to re-seal it, they might as well go back on with a new pump. It's a matter of $'s vs time.

Dear Hoola,

Hope you're well. How many miles on the car? Factory-installed water pumps are pretty durable, especially if the drive belt is properly tensioner. I replaced water pump that had served for 138K miles, because the top gasket started to fail, and I had to do the timing belt/tensioner. Others have reported even longer service lives for water pumps.

Are the heater hoses and heater valve the original, factory-installed items? If so, these items should be replaced pronto, as they're well past the end of their service life (no more than 15 years, if the car lives where ambient tempertures do not much or for long exceed 90°F (32°C).

I'd use Volvo brand heater hoses. Installing heater hoses is not easy and they are not easily inspected. Further, if a heater hose or a heater control valve fails, all/most coolant is lost within a minute. That usually means a headgasket replacement. Thus, the additional cost for Volvo brand hoses = versus "aftermarket" - is worth bearing. It is not possible to know much about rubber items by looking at them. A hose's quality is determined by how the rubber is formulated and processed.

The heater valve need not be Volvo Brand. The heater core can be bypassed by connecting the heater hose nipple (on the side of the head) with the coolant pipe at the back of the engine. A short length of generic heater hose - of the correct diameter - can be used for this bypass.

Hope this helps.

Yours faithfully,

Spook

The heater control valve is mounted near the fire wall in the engine compartment and is vacuum controled. It is fully closed when the AC is on Max and open the rest of the time. If the vacuum line going to that valve is broken or cracked it will not close. I had a similar loss of coolant on my 940, it ended up being from water leaking out of the back of the head and evaporating. I could not find it until it got bad enough to leave a puddle on the ground.

Bill