The first thing you might do is to drain the antifreeze down to a level below the heater core.

Option 1. Unhook the heater hoses from the heater core and connect them together with a piece of pipe the same size as the heater hose connections. 1/2 copper pipe often works. Secure the hose loop in the engine bay.

Option 2. Unhook the heater hoses in the engine bay and remove the hose that has a valve present. Then loop the remaining hose to that hose's connection on the engine. Secure the hose.

Both options by-pass the heater core - you can drive the car without heat.

After you remove the heater core, check it visually.

In my 544 the leak was a crack where one of the connectors attached to the core - I easily repaired it with solder.

Or you can take it to a radiator shop and ask them if the core can be saved.

If you try this, get an auto hose removal tool for about 10 bucks - search the web to see what one looks like - many auto stores have them

rock auto has a 100% aluminum one by spectra for 65 dollars

It doesn't at all surpise me it's your heater core, just you didn't describe enough symptoms in your initial post for me to declare it the most likely source of your problems, plus it's not the easiest of jobs so you're best to eliminate other possibilities before getting all caught up.

Once you get past reading all the effort required to do the replacement as described in my old FAQ article (keyword in that is tedious, not horrendously complicated requiring a lot of expertise) then spend most of your time trying to source a decent replacement heater core. Quality is perhaps more important than price as you won't be wanting to repeat the project anytime soon.

Ideally, you're looking for an all-metal core with brass/copper/even aluminum side tanks, and preferably not plastic tanks as is now the more standard design in both rads and heater cores. All-metal heater cores used to be available from a few sources, but are now increasingly rare unless they're really old stock. Many old posts describe where to still find all-metal cores, but I'd guess most of them are no longer available. Don't trust listings in catalogues that picture metal side tanks as they may be supplying an "updated" design (as in plastic), so do query suppliers directly about it being an all-metal tank before purchasing. Having said all that, and despite the warnings of others, if you are forced to go plastic then I find reputable manufacturers like Nissens tend to be better than average. I've had good service out of plastic sided Nissens rads, still going strong after 10 years. Main thing is to respect your cooling system, keeping them topped up, replace t-stats, heater valves and hoses before they fail, use a quality long-life anti-freeze and do regular flushing. Also, if you buy through some of the better web suppliers of Volvo specialty parts, you get a decent or even lifetime warranty, although your time and effort is probably worth every bit as much as the cost of a heater core.

Good luck with your project!
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

The Volvo 700-900 FAQ has the procedure.

https://www.brickboard.com/FAQ/700-900/HeaterCoreReplacement.htm

How long has the coolant been in the engine? What antifreeze do you use when you mix up 50/50 coolant mix with distilled (never tap) water.

I'd not ever replaced a 700-900 heater core.

See engine cooling system info here:

https://www.brickboard.com/FAQ/700-900/Cooling.htm

Though I'll guess you've read through these Volvo 700-900 FAQ files.

Some more heater core leaks info:

https://www.brickboard.com/FAQ/700-900/HeatingAirConditioning.htm#HeaterCoreLeaking

So I hope others that have can provide you more guidance if the heater core replacement procedure here is incomplete or needs updating.

Also, info about electrolysis and heater cores that leak after replacement. A radiator shop may be able to mend the leak. Though if a leak formed inside it in one place, maybe elsewhere is another fault solder weld or something?

Dunno what heater core brands are best for your RWD Volvo.

Questions? Hope that helps.

MacDuffed.
--
Donate NOW! Give your brickboard.com a big DONATION!!! Find the on brickboard pages!