First diagnose the problem: is the heater core the prime suspect?
Some symptoms of a heater core leak are:
* you are losing coolant when the heater is used.
* you are NOT losing coolant when the heater is NOT used.
* you can smell antifreeze when the heating is on.
* you can see condensed vapour exiting vents when the heating is on.
* your car's footwell carpet is wet.
* windows steam up with soapy-looking film.
Heater core failure can be a major problem because of access difficulties. If your car is a Volvo 850 then the job is not too hard, but I wouldn't recommend trying this instructable unless your car has almost all of the above symptoms (heater core might not be the cause), are a reasonably competent DIY mechanic, and enjoy the challenge of working in inaccessible and unpleasant places to save yourself a large garage bill.
The job might take you approx 4 hours of solid work, largely depending on how much antifreeze leaks out and how much you care about cleaning it off the carpet), and it will be messy.
This Instructable illustrates the repair in a right-hand-drive Volvo 850 1994 GLT 2.5 AUTO. If your Volvo 850 is left hand drive then procedure is similar but you'll be removing the heater core radiator from right hand side instead of left. In either case: you'll be removing heater core from passenger side footwell, which gives you better access than driver side footwell.
All loose removable carpets should be taken out at first sign of coolant leaking onto them, and replaced by absorbent rags or newspaper.
* torx screwdrivers (several sizes) and philips screwdriver.
* dry newspapers
* plastic sheeting/bags
* carpet cleaning kit e.g. wet vacuum cleaner.
If the heater core does need replacing you will also need a new heater core:
* Volvo 9144221 Heater Core/matrix/radiator. An OEM (non-Volvo) part costs circa US$150 or £100).
* two o-rings (3545586S) for installing matrix - you can probably re-use the ones that are already there, but maybe be a good idea to replace whilst you have access.
I replaced my heater core after seeing info at volvospeed.com - you might want to view that page too.
There's more info about Volvo 850 A/c & heating system at volvoclub.org.uk, but I wouldn't use hose clamps on the water pipes to core, because in my car they appear to be solid! So try to stop the pipes at outlet rather than by clamping.
Step 1: Passenger-side footwell access to heater core case.
* Disconnect battery.
* remove panel under passenger dash. Keep those screws in centre console tray.
* Peel down carpet carefully.
* Unscrew and gently remove black plastic panel to get access to white heater ducting.
Step 2: Driver-side footwell access to heater core case.
Now that you can see the white plastic heater ducting and two water pipes you should think before disconnecting the pipes: might the leak NOT be caused by the core being faulty? For example, is the leak coming from the connection between the two pipes and the core? Test for this visually and with dry tissue paper. If the connection is the problem, try tightening the central screw (mentioned in next step).
But assuming that the connection itself isn't the problem, this instructable will proceed with REMOVING THE HEATER CORE.
Step 3: Disconnect heater casing and core.
* Ease the white casing apart at the black seal joint. Don't force it too far.
* After the seal is opened you need to detach water pipes (secured by 1 screw) to completely remove the lower part of the white casing, which has heater core attached to it.
Be ready for the coolant (fairly low pressure because engine is off). You might be able to create a waterproof reservoir under casing, using plastic sheeting and rags.
I found that the right hand pipe gushed a cupful of coolant then stopped. I quickly temporarily lashed the left hand pipe with bicycle inner tube, which completely stopped leakage even when left overnight.
Now you can carefully ease the heater core back towards gear lever, twist and shuffle it out via passenger footwell.
When you have removed the heater core and ducting, separate them by undoing four torx screws. It is impossible to replace new heater core the wrong way around because of arrangement of screws, so don't worry about noting the orientation.
You are now holding your heater core - let's hope it's faulty!
Step 4: Confirmation of diagnosis
Assuming you have found a faulty core, get yourself a new one:
Volvo 9144221 Heater Core/matrix/radiator (an OEM part costs circa US$150 or £100). Order new o-rings while you are at it.
Ft the new core into the white plastic using those four torx screws, and the o-rings on the pipe stubs.
I didn't check my o-rings and all was OK, but if for some reason they went missing or were damaged you would find that your system could leak at this joint. Don't take this risk.
Step 5: Reconnect the heater core.
Once plumbed in screw the white ducting case back together - if you haven't already. Now the job should be functionally complete, so before replacing panels and carpets and TEST YOUR WORK:
Reconnect the battery
Top up your coolant under bonnet/hood, to mid-position (I found that this needed just 1 litre of coolant).
Replace cap securely on coolant reservoir.
Turn heating OFF. The heater core only gets pressurised when the heating controls are on AND the engine has warmed up after maybe 5 minutes.
Run the engine with bonnet/hood open (DANGER!) and if coolant level drops significantly switch off engine and check for leaks where you connected new core to two pipes. I found that coolant level DID drop but that there were no leaks from the system - I just topped up and assume that the coolant level drop was just due to flushing bubbles from the cooling system.
Run engine with heating ON (and wait for engine to warm up so hot air flows into car) to check that everything seems OK.
Hopefully you'll get the same result as I got: after one or two minor coolant tops up everything settled down nicely, there were no leaks from the core, no more noxious coolant vapors and condensates, no more wet carpets. If you can run the engine with the heater blowing hot air into your car and without any leak from the pipe-core joint, you're probably in luck!
Step 6: Assuming you've fixed the leak ...
I also used silica gel bags because the weather was cold and damp. It's best to do this with the carpets exposed to help ventilation of the carpet. Also you will still have access to the heater core housing to double-check that that pipe joint isn't dripping even after the heater has been used.
When carpet is dry replace black plastic side panels, and carpets. Then fit the under-dash panels into position, taking care to connect the panels correctly with the guides before screwing securely. If the panel is not in its guide you'll be able to bend the edges unreasonably.
If you try this instructable, you are very welcome to 'Add Comment' about how you got on and how to improve these instructions. Thanks. Dom.
Spurious bonus material: This stunt was a part of a Swedish Volvo commercial demonstrating the 850's crash survivability.