In short, my heater doesn't work great and this polar vortex weather is making the drive pretty miserable. I'm a decent DIYer, so I like to think I know what I'm doing. I've done basically everything I've read online short of replacing the engine. So this is what I've done in direct relation to the heater issues:

• Changed upper radiator hose and other leaky cooling hoses
• Backflushed the heater core, but the water came out perfectly clean
• Sealed a leaky water pump gasket
• Changed the thermostat to a 91c thermostat from an 87c
• Fixed the airbox thermostat and replaced pre-heat hose
• Bled cooling system and changed coolant. Cooling system doesn't lose coolant, and coolant is a bright green in colour, no contamination.
• Made sure coolant is always topped up.

In the past year, I've done the following to the cooling system:

• Changed the radiator
• Changed the heater core
• Plugged yellow vaccuum line to prevent floor vents opening up on acceleration.
• Changed the water pump (done in the last 2 years)

I'm honestly posting this as a last resort. Any suggestions? My last thought is there's something wrong with the internal vent system, but the last time I was under the dash, the only vaccuum servo that didn't work was the floor/defrost servo, which still doesn't work. The fan works fine too.

Both heather core hoses are warm/hot to the touch, same as the upper radiator hose. Lower radiator hose is cold, but I believe that is normal. After running for a long while (1 hour), the intake manifold is cold, which I don't believe is normal, even for winter.

I've run out of my own solutions so I'm open to any suggestion really. Thanks for your help!

 

Lower radiator hose should not be cold but the same temperature as the top! You have a circulation problem for some reason. Since you have replaced everything I'd check for air pockets... Take the tstat out if you have to and burp the system then replace the tstat.

 

In this -20c weather, I would think that the radiator would turn the coolant pretty cold by the time it reaches the lower radiator hose, wouldn't it?

 

Well, OK, -20C is not normal environment! May be the t stat is not even open yet. I'd wait before making further investigations.

 

That's the thing...Its been a couple weeks already, and this cold weather doesn't make it any shorter. When the weather is warm, around 5-10c, the heater works very well.

 

You got no problem then, I mean at --20, you can't expect things to function normally. you may want to block the radiator to let it warm better, some type of shield in front...

 

You mentioned in your post that you

•Plugged yellow vaccuum line to prevent floor vents opening up on acceleration.

If you vents are opening/closing on acceleration, then I suspect that your vacuum check valve that come off of the intake manifold has failed and is not doing its job. It is a black/white plastic cylindrical thing that has a larger hose attached to the intake manifold and a small hose that goes through the firewall. Take it off and try blowing through it on both sides. If air passes both ways then it has failed.

Also, having an 87C thermostat will give you less heat in this cold weather. I live in the Chicago area and I always run a 92C thermostat (which is what came from the factory). Its a small difference but when it is this cold, it might help. My car ('90 240) had excellent heat through this cold snap.

 

act1292,
I changed that valve about 5 years ago to initially solve that problem, but I remember making the change, and not noticing any difference, hence proceeding to plug the yellow vaccuum line.

If that valve didn't work, what would that do? My understanding is it simply provides vaccuum to allow the vent doors to move and vaccuum to the heater control valve (which was also replaced!), but the heater control valve should be on full heat all the time, as it operates on full heat with no vaccuum.

The really funny thing is, when I was driving around town, the heat was only lukewarm, but as soon as I entered a tunnel, where the air is slightly warmer, the heat immediately felt like normal, without me touching any controls. Exiting the tunnel, the air became lukewarm again. I believe it would be just around 32f in the tunnel.

 

There is a vacuum reservoir in the dash and the check valve makes sure that the reservoir holds a vacuum at all the time - even when the engine isn't generating a vacuum - like when you are accelerating. When accelerating, vacuum drops. The reservoir drives all of the flappers in the car that open/close vents. This is what led to my reasoning the valve might be bad.

One item I don't see on your list of items checked/fixed is the heater control valve. This valve controls the amount of coolant that flows through the heater core. If it isn't opening all the way, then you will not get full heat. On my 240, I have replaced the original type valve with a simple universal replacement (original is no longer available). Since I don't have a 940, I can't guide you as to what has to be done.

 

Do you remember if the replacement thermostat had the "jiggle pin" and if so did you mount it with the hole up? If not did you drill a bleed hole in it and mount it with the hole up. This is to eliminate the air lock issue that happens with some of these close system cooling systems.

 

act1292,
I replaced my heater control valve about 5 years ago. I have no reason to believe its not functioning properly as when the weather warms up, the heat works great.

jagtoes,
I drilled a 5/8" weep hole in the themostat just for that reason. I also made sure to make it pointing up. I thought for a long while it might be an issue with air in the system, going so far as to burp the system after every long drive, but I'm beginning to think its not.

Here's a slightly obvious question though. What effect does a non-functioning floor vent have on the heat? Apart from the fact that hot air rises, are there any benefits to having this work? Are there other valves this would control?

 

on the 740/940's, the heater valve is 'backwards' relative to what I expected. no vacuum == heater on. vacuum == heater off.

there are flaps for directing the heat 'down' to the foot wells, for the defroster vents, and for the closed air recirculation vs outside air intake.

 

When the heater control valve is not opening all the way, you still will get better heat when it is warmer out simply because you don't need as much flow through the heater core to warm the air up to a comfortable temperature. Last fall I had an issue where my heat control lever (on a 240) wouldn't slide open all the way. Heat worked ok until the temps got below 32F. Then it took forever to warm up.

 

Thanks for all the ideas. The heater still isn't working as I would like, but I picked up a floor/defrost vacuum servo from the local mechanic for cheap, and I'm going to install it. While I'm under the dash, I'm hoping to find some obvious heater issues. Anything I should look out for?

If this doesn't fix it, I'm going to check out the heater control valve. I guess its possible it won't work properly after 5 years.

 

biggest thing is old vacuum hoses cracking and leaking.

 

Update:
I changed the vacuum servo, and didn't find any issues with the vacuum lines. I sealed around the heater core to ensure that no air was getting through the edges and thus not passing through the heater core and warming up. I also used weatherstripping to reseal some edges in the 'vacuum box' where all the flaps are located.

It was -5c yesterday and the heat was better than before. I'm optimistic that I might have finally cracked it, but I also can sit easy knowing that the only possibility left if this didn't fix it is to replace the heater control valve, which is not a huge expense. Thanks for all the help.

 

there's two versions of the 7/9 heater valve for sale. one is actually GM/chevy valve, and it does work on the 740s but I found it was not very progressive at the low end, most of the heat was on quite soon above 'cool'. that was on my car when I got it, and a month or two ago it started to leak, so I replaced it with the VDO valve, which was Volvo's OEM replacement, and now the heat is much more progressive, with the heat **** at 50%, its actually about 50% heat instead of 90%

this is the original NLA,


this is the 'chevy' valve, that I replaced


and this is the VDO OE style replacement, which I found works better than the chevy valve.

 

I currently have the 'chevy' valve in my car, and probably would switch it with a Siemens VDO style.

A question for you Northerner's: in really cold weather, do you find that using the recirculate button affects the heat output from your vents?

 

one caveat on the VDO. be sure to orient it so other hoses and cables nearby don't interfere with its valve lever. the chevy valve is all internal.

I was amused to learn, these valves default to ON. the vacuum line turns them OFF.

 

I never use the recirculate button for heat as it causes your windows to fog up. I live in the Chicago area so our low temps are running around zero for the past month. Heat isn't a problem in my 240. It gets nice and toasty without the recirc.

Recirc is best for running your air conditioning but since mine got removed in the last accident, that button is no longer used in my car.