As mentioned in task #87, while checking for coolant leaks I discovered that the heater core was leaking. I fixed the bigger leak at the upper radiator hose and left this heater leak for later. It seemed just a minor nuisance where I could keep topping off the coolant level, plus my impression was that replacing the heater core was a big job.
Since then I’ve come to realize that replacement isn’t terribly difficult or time-consuming. More importantly, I learned the hard way that a leaking heater core can be a real safety issue because it will fog up the windows.
This is a very common problem on 850s– probably on every car from the factory because of a poor design in the heater core itself where the plastic frame is not properly bonded to the heat exchanger. After ten years or so (and all Volvo 850s are now at least 16 years old) the heater core will leak a little coolant. Not too much, but enough to be a situation that needs attention.
Even if you are willing to just keep topping off the coolant, it’s a real problem because if you ever use your heater a leaky core creates a fine aerosol of coolant (water and antifreeze) which deposits a moist, oily film on the windows.
This then attracts dust and such in the cabin which just obscures the windows even more. Eventually it’s hard to see out the windows, particularly at night and when the sun is low. Poor visibility is a real safety issue and is what convinced me I needed to fix the leak. It took several passes with cleaner and rags to get the windows really clean and there is a lot of glass on a wagon. This film is also being deposited on the passengers and everything else in the cabin.
You will probably first detect a heater core coolant leak with your nose. When any heat is turned on some air flows around the heater core and the vapor from the anti-freeze chemical moves into the cabin. Most of us recognize the unique sweet odor of ethylene glycol in the coolant. If you’re not sure, just remove the coolant reservoir cap and give it a sniff. If you smell the same odor inside the car with heat on, you’ve got a leak.
Confirmation of a leak is coupled with an observable drop in coolant level in the reservoir. The heater core is located forward of the transmission/gear shift lever under the dash just above the floor. Access requires removal of the dash board lower covers and then pulling back carpet and removal of plastic covers under that.
When I opened up the panels on this car, I found a wet spot on the inside of one of the panels and shiny wet spots around the base of the heater box.
IPD has a great video showing the replacement procedure: https://www.youtube.com/watch?v=cD7L9hzHtNo It is for a 1998-2000 V70 so isn’t exact but only a few details are different.
MVS has a really good step-by-step procedure written up: http://www.matthewsvolvosite.com/forums/viewtopic.php?f=1&t=40547
General procedure to replace the heater core:
1. Remove lower dash covers on both driver and passenger sides.
2. Peel carpet against center console back on both sides.
3. Remove black plastic panel behind carpet to access heater box.
4. Unscrew core frame from heater box, two screws on each side.
Access to these screws is somewhat awkward. It helps to push the seats back, put a cushion on the floor, lay on your back working up and have a good work light.
5. Pinch off both heater hoses on engine side of fire wall to prevent draining engine coolant into the car.
6. Pull evaporator drain fitting out of hole to provide space for removing the heater box.
7. Unscrew tubes from heater box.
8. Pull heater core and tube block apart. Be prepared with rags or towels for quite a bit of coolant leakage (residual in the hoses and tubes plus inside core). It may take some prying and effort to separate.
9. Rotate heater box back and out the RH side (US passenger).
Note access to the heater box may be slightly different on earlier model 850s but the general idea is the same. On models with manual transmission there is also a gear shift cable passing under the heater core to work around.
10. Remove screws holding the heater core in the frame and pull the old core out.
Now ready to install new heater core. While many people insist on using factory heater core replacements, the design is still flawed and will happen again. They are also a bit expensive. I chose to go with an after-market part that claimed to have an improved design. It also comes with two new O-rings for sealing the tubes and is at a very good price on eBay. This is likely a knock-off of a better part but they claim that their design fixes the expansion problem by crimping the tubes onto the plastic tanks. If it lasts five years or more I’ll be happy; not sure how much life the car has left in it.
This new core came with foam strips installed. If yours doesn’t, you would have install new foam strips to match old core.
11. Clean up the frame to remove oil and dirt. Disintegrating foam strip leaves a lot of debris behind. It also would not be sealing well, meaning some air flow through the heater box will bypass the heater core, reducing heat efficiency. New foam will seal better and give better heating.
12. Put new heater core in frame and screw in place.
13. Remove O-rings from ends of tubes from firewall and replace with new ones. If your tubes are corroded they should be cleaned up before installing O-rings. If your pipes are steel (older models) consider replacing with newer aluminum tubes. Always use new O-rings because the old ones will be dry, hard and not pliable.
14. Maneuver heater box back in place and push onto tubes to mate properly. Screw core onto tube block to secure.
15. Screw core frame back into heater box on both sides, making sure the rubber seal is in place all around the seam.
16. Remove clamps pinching off heater hoses.
17. Start the car and check for fresh leaks. Shouldn’t be any with new O-rings.
18. Top off the coolant level as needed, depending on how much was lost in this process.
19. Reconnect evaporator drain.
20. Install plastic panels on both sides of heater box. Note the white plastic tab on heater core frame mates with slot in black plastic panel.
21. Push carpet back into place on both sides. It’s probably a good idea to let the car air out for several hours (windows open) to evaporate much of the spilled coolant from inside the cabin.
22. Install lower dash panels.
I’m going to wait a week or two before cleaning the inside windows again. There is likely some residual coolant in the ducts and air vents which has yet to be pushed into the cabin. Once it’s dried out we can clean the windows for good.
Now I get to do this again on my 850 sedan which also has a heater core leak, just not as bad (no coolant loss and no foggy windows but antifreeze odor with heater on).
$39 for low-cost “improved” design heater core
The heater core is essentially a small radiator, much like the big one in front of the engine. Hot coolant from the running engine flows through the core and air flows through the fins where the heat from the coolant is transferred to the air.
On Volvo 850s coolant flows constantly through the heater core; there is no shut off valve or regulator. Heat into the cabin is determined by temperature settings on the climate control. This regulates air damper position to direct more air through the heater core as you increase temperature. So while hot coolant is always circulating through the heater core, it does not exchange heat to the cabin until the operator selects some heat.