Hi all,

Well, the time has come to replace the heater core in my '94 940, which I bought for $500.00 because... it had a bad heater core!

I'm not too worried about the job, but did have one question: where the tubes come through the firewall, which is In and which is Out/Return? The heater core was bypassed when I bought the car, so it's not obvious to me. If I missed this in the FAQ, my apologies.

I assume that the hose with the heater control goes on the In side?

Next month: heater core on my '90 240. I'm dreading that one.

As always, thanks!


Monty
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Past: '79 245, '91 240, '88 245. Current: '90 245, '83 245, '95 940

I do not know anything about the 900 series cars so I will get that out of the way as a minor disclaimer. I drive 240’s and my question is. Are 940’s heater cores really that much easier to do that you knowingly bought the car anyway?

In thinking about your question, I will suggest that you hook up the heater core like the radiator on the front of the car. They work the same as they both lose heat.

They use a radiator system to work with natural thermal convection. Hot air and water rise. That said, as hot water cools it sinks to the bottom because it become more dense or heavier. The pump works less because it is going with the flow. The system will continue to flow when the pump is not running. If you notice, the bottom radiator hose connects to the suction side of the water pump and the pump uses a turbine design.

The thermostat (a valve) is on the outlet side of the pump and goes to the top of the radiator. The direction of flow is important because is suppose to sense the hottest heat flow right out of the engine cooling passages.

As a regular valve, that does not need that requirement; it will not make any difference which line it is connected. They can design that valve to fit anywhere, even though it seems they to like putting them in hard to reach places.

A shut off valve can be directional for a few reasons.

Most valves designs want the flow to come up to and under the seal, not on top of it. There is less chance to for a seal to stick downwards and requires less effort to open a valve. There is less chance for any contaminates in the medium to rest around or collect on the seating surface during the lesser flows rate upon closing.

Pressure should be under a seat and not around a moving actuator side of the valve. This is because of packing seals. When a valve is off the static pressure is highest, you want that on the most solid portion of the valve. I consider an inlet to be opposite the serviceable or assembling side.

I leave it with; go with nature and what feels natural.

Phil

"Are 940’s heater cores really that much easier to do that you knowingly bought the car anyway?"

Well, I'm hoping so. From the reading I've done, the 240 seems more involved: the whole dashboard has to come out. Guess I'll find out soon.

Yeah, I bought the car knowing it needed a heater core. It's a fine vehicle: 230K, nice leather interior, runs and drives like butter. I got two quotes from mechanics for doing the core, both were in the $1,000.00 range. The core itself is $100.00. The previous owner knew that he had a $1,500.00-ish car with a $1,000.00 problem, so I got it cheap.
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Past: '79 245, '91 240, '88 245. Current: '90 245, '83 245, '95 940

I've replaced 5 heater cores in 700/900 series cars. You may find it easier to pull the dash, although I haven't been for replacing the heater core. Accessing the top screws holding the heater box together is difficult with the dash in place. I replaced the dashboard in my daughter's 740 last summer, and it took just under three hours. When the dash is out the access for replacing the heater core is easy.

The cheaper plastic and aluminum heater cores are a lousy fit, and I recommend getting a brass and copper heater core. The last one I bought was $135.
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john

John,

The old heater core is out. Took about 5-6 hours. Really not too bad, and I think much easier than working under the dash of a 240. Re those two top screws: obviously the design engineer was a sadist 8-)

Yes, I spent for the brass/copper core. Put it back together tomorrow and see what I've got.

Monty
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Past: '79 245, '91 240, '88 245. Current: '90 245, '83 245, '95 940

I have some sketched diagrams somewhere...

FAQ: Install the new valve and tighten the clamps. [RMagoo] The valve goes between the heater hose connected to the short pipe coming out the cylinder head and the heater hose entering the bottom tube of the firewall connector . The arrow on the valve points toward the firewall. The top heater hose goes to the red pipe behind the head leading to the water pump.

Me: The top is the inlet.

Tom

Someone correct me if wrong, but I'm fairly sure the pipe behind and alongside the engine block goes into the suction side of the water pump, into the same chamber as the lower rad hose. Thus it will draw coolant out of the heater core.

If I'm right, the pipe/hose coming out of the cyl head provides the inlet to the heater. This makes sense from a heating standpoint, as that is where the coolant will get hotter, sooner. That's what you want from the heater on a cold day.

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Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.

Thanks. Totally missed it.
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Past: '79 245, '91 240, '88 245. Current: '90 245, '83 245, '95 940