I have problem with heating on my 740 87. Intensity of heating depends on outside temperature and movement of the car. When I stop at traffic lights almost cold air is coming out, but on the freeway it is quite hot. Blower motors are working fine. Last summer I changed radiator and thermostat but it did not help at all. Level of antifreeze is OK. Can somebody help me, please?

Check for proper operation of the vacume controlled heater coolant valve mounted on firewall driver side. This valve opens to allow coolant into the heater core and is operated by a vacume line from the heater controls. Can't remember if it opens or closes when vacume is applied. I had trouble with mine when the vacume line was getting crushed and blocked where it passed near the has pedal. Vacume changes with idle and cruising so you may have a fault with the feeding of vacume to the valve.

Thank you guys for your opinions. Water pump has been changed 4 years ago and heating was normal after that. I don't have heating coolant valve in engine compartment, previous owner "fixed it" by removing it completely and by connecting two hoses with metal tube connector. According to mechanic it is OK because there is another heating coolant valve inside the car behind dashboard.
Maybe Dave is right it could be vacum leak inside. Do you know where and how I can check vacum lines for leak?

Do you have ACC? If not it would news to me that there is a coolant flow valve in the dash. But there are vacume operated servos which open and close to direct the air according to the control settings. The main vacume "switch" is right behind the heater controls, there is a vacume source line which goes to the engine and then other lines branch of to the 3 (I think) servos. When the PO removed that valve did he plug up it's vacume line?

check for vacuum leaks in the climate control system...also if the car is a turbo, it should have a vacuum resavour under the engine in front of the crossmember.

If you notice the heat is weak while idling, but adequate when 1500 rpm or more regardless of vehicle speed, you might suffer from what I believe to be an under-performing coolant pump.

Many on this forum decry Volvo's use of a stamped impeller in their replacement pump kits, but my alternative opinion based on my own experience and a common sense eyeball comparison is this: The "desireable" cast impellers may not deliver the volume of coolant at low rpm because their fin geometry is less than half the depth of the stamped version.

The photo below shows two cast versions. Notice the profile of the fins on the left pump are half the profile of the right pump. The stamped impeller is twice again the water mover that the pump on the right is. It makes the heater work at idle, in my opinion.

Check your heater control valve. It could be that the vacuum hose going to it is cracked? Or if the valve is still original, it could be shot. The heater control valve is located under the hood, between intake #4 and the firewall.

Matt, I was reading this thread with interest because I also have weak heating. How do you troubleshoot the heater control valve, i.e. how do you know it is open when it should be? If it is due to vacuum leak I can see that a leak in the vacuum hose leading to it would cause it not to open. However, if it is a faulty valve/motor, how do you know and how do you check?

It seems to me also that apart from the heat being weak, my blower strength is weak. Any ideas on why the fan isn't blowing as strong as it should?

Regards, KJ.

Thats a tricky one Mark. you've already tried the 2 most obvious things. The only thing I can suggest is you flush through you heater matrix but in the opposite direction of normal flow. you might have a blockage there and this would hopefully remove the crud.

Good luck,

Rich.
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'86 245, B230A. 161k. A few modifications here and there old boy.