Hi,
I've owned our 92 940 sedan for about 3.5 years. I can't remember for sure about the first winter but I don't think it has ever heated well since we have owned it. Much lower performance than our 240s. There aren't any issues with the cooling system- engine temp looks good, never overheats, etc. At highway speeds the cabin will warm up fairly well (with recirculating) but if I am idling, it doesn't really get warm inside, even though engine temp is good. Defrost never gets close to warm because I can't recirculate- struggles to melt off snow and ice. We're in central Illinois- recent temps have been about 5-25 degrees most days.

Here's what I've done:
1) During the first winter I smelled some coolant so I replaced the heater core with the FCP Groton core in the spring.
2) I replaced the control valve with one from FCP Groton. I currently have it rigged so it stays at max flow to make sure it isn't a problem with the control system.
3) I changed the water pump. I thought this was the problem because the one I removed appeared to be machined badly at the return line from the heater so that the opening was only about half of what it should have been. This helped but it still doesn't seem like it heats well. I think replacement was a Hepu.

Two questions:
1) Is this performance typical of other 940s? As my wife says, they wouldn't settle for this in Sweden.
2) The fact that it heats ok at highway speeds but barely does in stop and go traffic (with a warm engine) makes me think there is some circulatory problem going on. Either a problem with the new water pump or a restriction somewhere that the higher pump volume/pressure gets through. Does this seem likely? Any ideas what to check?

Thanks!
Lyle

Thought I would post an update to leave a trail for anyone else with similar problems.

To review, I have a 92 940 which wasn't heating well, especially idling in stop and go traffic. I was concerned about it during snowy, icy winter weather because I don't think it was heating enough to melt off the accumulation. It would heat some but it just seemed like it was undersized. I'd been fighting this for at least a couple of years- never had been good since I owned it.

I went through everything in the system and came to the conclusion that all parts were functioning correctly. Engine temp was good, it heated some, and the AC functioned well which implied that all the flaps and recirculation system was good. I had the system flushed to make sure there wasn't some kind of blockage.

The only thing left was the Nissens heater core I had replaced shortly after buying the car. I remember it fitting fine but looking different than the original- more plastic, aluminum instead of brass, etc. I decided that I needed to try changing it out or get rid of the car. I took the old leaking Volvo core to a local radiator shop to see about repairing. They said it wasn't repairable but they sold me a World brand core that looked like the factory core and I made plans to change it.

In the meantime, I took the car on a 4 hour road trip. Going through traffic in a mid-sized town, I heard a pop and steam started pouring out of the vents. Heater core had failed catastrophically. Rapidly couldn't see anything- we rolled the windows down and looked out the side until we could find a place to pull over while the majority of the antifreeze was pumped into the floor pans. Fortunately, I was able to re-route the hoses to the core and we grabbed some antifreeze at the convenience store and were soon back on our way.

I spent 7.5 hours last weekend changing it out and it heats now! We've had some freezing mornings and I sweated on my 15 minute commute to work yesterday even without recirculating.

Looking at the Nissens core, the failure was a split in the plastic end caps that was big enough to drain the system quickly. This core had a very different design than the Volvo or World cores. In those cores, pretty much every fin is a flat passage that the coolant runs through so the surface area should be much higher compared to the volume. The Nissens had 9 pairs of approximately quarter inch tubes with a bunch of fins attached to them. I can see why this might have trouble transferring the heat. The core was definitely the problem- I just can't be certain whether it was the design or if I got one that had some internal restriction that wasn't apparent.

Given the time involved in changing it, in the future I will stick to a factory or "factory-like" heater core.

There's a lot of talk so far and I like it.

First, my 740T and 240T get the car plenty hot with the heaters within a few miles of startup. My specific 740 has a much weaker blower and much less airflow. The 240 has one hell of a blower, even if it's one hell of a job to replace it.

The 740t has issues with its AC. When driving on the highway, the AC would slowly stop flowing. I think the old grease in the blower motor thickens to where the motor can hardly spin. Turning the heat on for a moment, then back off would temporarily solve the problem.

I would IR-temp probe everything with the heater selector turned off and the valve replaced with a piece of copper tube. With no air cooling the core, the in and out temps should be equal, and within a few degrees as the head temps. Then I'd turn on the heater and see how warm the temp is and if it ever reaches the maximum temp at idle. A short twitch and release of the gas pedal might change the vacuum available to the system without pumping tons of additional coolant around.

I replaced a water pump on my 740 and all but half of two fins had corroded away. I think it was a HEPU pump based on photos, but it had been on the car for at least 3-5 years.


Any weird noises from the blower motor?

Does it have automatic climate control?

The vent air only gets hot on recirculate - both on the highway and at idle? If the heater core is indeed hot, maybe the motor is not up to snuff or the vent doors aren't sealing or shutting correctly.
What's the vacuum at idle?

Maybe there's a blockage for air flow over the heater core? I don't think there's a lot of hair floating around in your heater core area, but it's a problem with PCs that their cooling fins get blocked with hair and dust.

Good Luck!
--
1990 740 Turbo, on its way to stock specs, maybe beyond

"I can't remember for sure about the first winter but I don't think it has ever heated well since we have owned it."

The heater hose/valve connections must be wrong, installed wrong by the PO or the PO's garage.

I am replacing the radiator in a 1995 940 now and since I'm up to my elbows in dirt, rad fluid and tranny fluid and if you would like I can dig in there and post a connection diagram.

(The original post yesterday disappointingly disappeared.)

Tom

Hi,

Another reason heating systems can fail to produce heat at idle, is the thermostat is installed backwards and or it does not have a jiggle valve to allow the system to burp the air pockets out.

[img src=http://i883.photobucket.com/albums/ac34/gus123456/thermostat.jpg]

Hi,

Another reason heating systems can fail to produce heat at idle, is the thermostat is installed backwards and or it does not have a jiggle valve to allow the system to burp the air pockets out.

I pulled the thermostat this weekend and it's in the right direction with the jiggle valve in the right position (up). From feeling the radiator hose, it seems to be functioning ok. So I think it is ok.....

I'll take a look at that given that it seems like it is a flow problem. I can't recall if the thermostat I put in has a jiggler.
Thanks@

I spent several hours this morning diagnosing and comparing with our 240. The best test I did was with a thermometer checking temps coming out of the vents with recirculating off. Outside air was about 25 degrees F. Doing this, at highway speeds, I could get around 90 degree temps, in stop and go in town (30 mph with a stop sign every 4-5 blocks) I could maintain 80. When idling (like at a long stop sign or in the drive), after 1-2 minutes it would drop to 70 pretty fast. By comparison, the 240 followed a similar pattern (maybe not dropping off quite as fast) but was 30 degrees warmer ranging from 100-120.

Thermostat is a 92 degree temp and seems to be functioning ok. Temps at the thermostat housing and upper radiator house measured with an infrared thermometer seem comparable to the 240. The cooling system seems to be functioning fine.

Inlet and outlet hoses to the heater core both heat up but there does seem to be some drop off. I had a hard time getting reliable readings with the thermometer on these hoses.

Heater control functions (I reattached the vacuum line) and does affect the heat level. Recirculating controls seem to work ok.

At this point, I am thinking it is some type of partial restriction in the coolant flow. I probably should try replacing the valve with some straight copper just to be 100% sure. If that fails, I will probably tough it out till warmer weather when I can do a thorough disassembly and flush and backflush all the components.

Any other thoughts? Thanks for all the help with this.

Here's a thought. I have read how on some Jeep models when the water pump gets old the fins within the waterpump degrade and the pump is no longer ale to push fluid at sufficient volume to get heat to core properly, but yet be just strong enough to keep engine from everheating....just a crazy thought I spose but you are digging...

It may be crazy but seems to fit with the symptoms so it's probably worth looking into. The pump is relatively new so I doubt it is an age issue. I replaced it maybe a year ago. The pump that came on the car when I bought it had the port for the return pipe off-centered so it seemed like its effective opening was only half of what it should have been. As I recall, replacing it helped a bunch but didn't totally resolve it. It is a Hepu which generally gets good reviews but I can't remember what the fins looked like.

Or perhaps the O-ring to pre-heat(heater core leg) got fouled up in there to block the flow? Did you use excessive amounts of RTV or something in there?

I have a '92 740. I assume our cars have the same non climate control heat/ac system. On my 2.2 mile downtown commute to work in 18-20 degree weather the heat starts to get warm at about 3/4 mile. Pulling into work the temp gauge is at center and the heat is warm enough that I don't want to get out of the car. I only let the car idle for about 30-40 seconds before driving off and take it slow. Within 10 miles of 60 mph thruway driving I'm turning the blower to low or off and cracking a window open. I'm one who likes really toasty cars.

All that said our cars should have the same heater performance.

Maybe try this:

Remove the heater water control valve and see if its actually opening fully by looking through it. A new one could be defective in some way. The previous owner of my car just bypassed the valve when it broke.

The new heater core could be partially plugged or defective. Are the heater hoses getting hot at operating temp? You could disconnect the heater core outlet ( ENGINE COLD ) attach a small lenght of hose secured into a large pan under the car, start it and see if it flows well. Should almost shoot out when reving the engine. Be wary of possibly hot coolant. This should confirm proper coolant supply pressure to the core as well as a free flowing core. Don't run it too long.

Could it be possible to check that the temperature blend door is moving completely and freely? Given the symptoms it seems unlikely though. I don't know where mine is or else I'd try to help.

Good tip here checking the temp of the heater hoses. On of the things we checked before doing the heater core on buddy's Ford truck was that the hose was hot going to the core, but hardly warm coming out. It did have a good clog in it. Yeah, it was a Ford F150, but it still works on the same principle.

Do you have a ballpark on how much temperature differential there was? I checked mine with an infrared thermometer and was seeing a drop off between them (don't remember how much for sure- maybe 20 degrees F) but figured that was just the heat transfer.

The heat in my 1992 740 is not even close to as strong as it was in my 240. I have to turn the heat all of the way on in the morning and turn it down as the car finally does warm up. I have not replaced the heater core but I have replaced just about everything else.

Mike

This is the big question for me- as you said, your 740 is not nearly as good as your 240- is it a design problem or is something wrong with our car? I wish I knew someone in town with a 940. The output temps were about 30 degrees lower on our 940 when compared to our 240. The heat in it just doesn't seem "good enough".

Thanks!

Here's something that comes to mind. How well did the heater work that 1st winter before the heater core was replaced? Could the replacement heater core not flow as well? Some sort of possible restriction. I just helped a guy replace his & there must be full flow to stay warm. Just a thought.

I checked with my wife and she is sure it wasn't heating well prior to the core change out. I think you are right about the most likely cause being some kind of restriction- I'm trying to figure out where it could be. Not too many places left....

"...I currently have it rigged so it stays at max flow to make sure it isn't a problem with the control system..."

Just a reminder - max flow is achieved with NO vacuum reaching the valve (diaphragm relaxed), and applying vacuum to the valve CLOSES it. Is it possible you rigged it opposite of what you intended?

Sorry if this turns into a double post- I responded earlier but may have done something wrong- it's not showing up.

I have the vacuum line totally disconnected and have manually pushed the piston to verify it is moved all the way so It is open. So I should be good on that front. Good question though- at this point I probably need to question all my assumptions.

This may be your problem. If you disconnect the vacuum source, the valve will naturally default to the OPEN position. (I know, it seems backwards) So if you forced it the opposite direction, you probably forced it SHUT.

I probably didn't say that the best. I made sure it was the open position. It flows well at higher rpms.
Thanks!

Is the coolant level topped-up to the proper level. If it gets a "little bit" low the engine will still circulate OK and the temp gauge will read OK, but the heater core will starve. In these cases sometimes racing the RPMs higher will force hot water up to the core, but when idling the slower (less forceful) circulation will not reach the core.

I hate to be a persistent PITA about the valve, but have you considered removing it from the equation totally and substituting in a short piece of copper pipe - just as a test?

I've thought about replacing the the valve with copper but haven't gotten to it yet. I've verified the valve functions to some degree- I reconnected the vacuum line this morning and changing the valve affects the temp. But there could be some restriction and a 10-20% restriction is probably what I am chasing here.

Dear Irisius,

Hope you're well. You've done everything to maximize flow of heated coolant through the heater core. I hope you changed the heater hoses, when you changed the heater valve. These hoses, if factory-original, are well past the end of their working life. They don't owe you a penny. Heater hose failure = total, rapid coolant loss = engine over-heat = possible head gasket replacement.

This is an air-handling problem. The clue: at highway speeds, airflow brings warmth into cabin from the heater core. When stopped or moving slow, ambient airflow drops or stops: too little warmth arrives.

The blower likely is not working to full capacity. When on "5", the highest setting, you should feel a blast of air and the motor should make a dull roar. If these conditions do not prevail, I think the blower switch, resistor, or motor need replacing.

If motor speed increases, with each movement of the switch from "1" to "5", then the switch and resistor are in good working order. I'd then focus on the blower motor.

When the blower is operating, does air come out of each of the four front center console air vents? Here's a test. Put the motor on "3", with all four front console vents open. Close the passenger's door-side vent. Does that increase flow from the remaining three open vents? There should be some increase. Next, close the passenger's center console vent. Is there an increase in airflow from the two vents nearest the driver? If air output rises, then the internal air-handling system is in good working order.

That narrows the problem to the blower. The motor may simply be at the end of its working life. Even though it may spin faster, as the switch is moved from "1" to "5", it still may not reach full operating speed.

Hope this helps.

Yours faithfully,

Spook

Thanks- that is a good "outside the box" suggestion but I doubt it is airflow related. I will run through your tests, though. It seems to be blowing a good amount of air and the AC works fine during the summer. I probably wasn't clear enough- when I idle, the airflow seems the same but the temp drops. If I drive at say 40 for a little while and then stop, it heats up while driving and then proceeds to fall when sitting- air flow seems constant but just cools off. Also, if I up the rpm's while stationary, it does run warmer. Really seems like the coolant isn't circulating well enough.

Also thanks for the good reminder on the heater hoses. I did change them when I did the heater core. Those things are easy to forget.

Have you checked/replaced your thermostat? There are several available-- you could pick one that lets things heat up more before opening.

That said, my old 240 had much more impressive heating than my current 940.

Good luck!

I found my records and I did change to a 92 degree thermostat the first winter. I did some comparisons between my 240 and the 940 using an infrared thermometer on the thermostat housing and upper radiator hose and it seems like it is in the ballpark.

Hi,
I can't remember for sure but I think I did change out the thermostat and I believe I upped the temp a little.

How much difference is there between your 240 and 940? To us, the 240 delivers a huge amount of heat- almost enough to bake with :). The 940 by contrast stays cold (or at least cool) in stop and go traffic and I'm not sure the defroster will melt snow and ice in stop and go traffic. At least my confidence on the melting isn't very high. The defroster does very little to warm the car. My only way to get the car warm at all is to have the heat vents (not defrost) on recirculate and be driving at a reasonable speed. To us, the heating level in this car isn't acceptable. So I'm trying to figure out whether this is a design problem or something specific to our car.

Thanks!

I second Herb's suggestion. There are two temps available(dont recall what they are) I just changed to the higher temp on my 94 940. Not dramatic but noticeable.

I have had 15 Volvos. I now have 91 740 wagon, good heat, don't need the fan after a few minutes.
92 740 wagon, very good heat, don't need the fan after a couple of minutes
93 940 sedan, 268k mi, decent heat, have to turn the heater knob higher to get similar heat to the wagons, but mostly don't need the fan.
I am near SF so it's not that cold, but we have had nights with ice and frost in the am so it's not always warm.
I think my cars are similar to the 240s I've owned in terms of heat, so it's not the design of the 7 series.
After all that's been done I think the thermostat you're working with is likely an issue. The one in my 92 failed recently and the heat was not there; I replaced and all was well again. Get a good one, no cheapies.

I'm no expert but I'm just wondering how many gallons of fluid leave the heater core per minute and at what temperature. Is there any way to measure that? Perhaps with a flow meter of just count the length of time to fill a gallon jug.