I was listening to Car Talk and one guy was asking about why his car intermittently overheated. In the course of asking his question, he mentioned that he would turn on his heater to help take heat off of the engine. Would this have and significant effect on the engine temp?
It doesn’t have an overwhelming effect, but it can certainly help. The heater in your car is a heat exchanger, smaller than but similar in principle to the radiator. When you allow engine coolant to flow through it (and especially when you turn the heater fan up) you can transfer a fair amount of heat.
In most cars, your heater operates by circulating engine coolant through a heat exchanger in your dash, and blowing air over the heat exchanger to remove heat. Thus, if you remove heat from the heat exchanger, and your thermostat is already wide open, you will end up cooling the engine down. But the real question is, how much? In my Honda Civic 1200, I could see the temperature needle drop very slightly when I did it - but then, t was a small, aluminum engined car. In most cars, I confess I have a hard time seeing it make a substantial difference.
Had a Buick 350 V-8. When it would overheat and the red light came on, I could turn the heater on full blast and light would go out in a matter of 30 seconds.
Might that be because your thermostat’s working, so the feedback mechanism it represents keeps the engine at the same temp? If that mechinism is not working properly (which it isn’t for whatever reason if the engine is overheating) maybe the results are different? Dunno. I’m curious, too. I used to do it when I had a dodgy old clunker, but I don’t really know if it achieved anything or was just a placebo.
In days gone by in light trucks pulling a heavy load in the summer time, it would take a gallon of kerosene in the tank and all the windows open and the heater on high all day to make the difference between sitting on the side of the road and 'Getin her Done".
Really sucked but that is what it took.
Absolutely right - good thinking. If all is working well, heater use won’t significantly affect engine temperature - you don’t want it to and it’s designed not to. But if there’s a malfunction that keeps the system from removing heat from the engine like it should (which of course means things are not operating as designed), running the heater can make a quite noticeable difference.
How it was explained to me:
When the heater is not on, there is still water in the heater core from the last time you used the heater. This water is not part of the engine cooling system until you turn on the heater. At that point, the cooler water in the heater core mixes with the regular engine coolant, dropping its temp. Also, the heater acts as an additional heat exchanger. On top of that, the heater fan comes on, moving more outside air over the whole system.
Sounds reasonable to me, but my idea of precision auto mechanics is to get a bigger hammer.
I think I’d agree that theb most critical factor is that overheating often occurs when the normal cooling path is compromised. The heating coils isn’t a good substitute for, or a great supplement to, the radiator – but it’s the only one you’ve got! Besides, it pretty mcuh can’t hurt (yes, you might blow an about-to-fail seal or othr part of the heating system, but that’s probably negligible, compared to blowing an engine today, and having the the heating system fail a few months later anyway. )
Forgot to say, this was Summer in Wash DC, 95 outside, and learned quickly that you put the heater controls to *defrost[/] with windows down all the way.
I guess I should have made the obvious observation - that problem diagnosis might make a difference. If the engine cooling system is not working because you’ve had a total loss of coolant, then I can’t imagine turning on the heater (or anything else except stopping immediately) making a difference. Partial loss? Might make a difference. Dunno how these things work. But if there is some other flaw, maybe it will help. As you say, probably can’t hurt much more.
This hasn’t been the case in cars for at least 40 years. Up to the early 60’s, cars had heater control valves, with the heat off coolant would not reach the heater core. With the advent of high pressure cooling systems, the heater control valve became a weak spot so automakers eliminated it and cars now use a sealed heater box under the dash. Coolant is always passing through the heater core, when you need heat, the heater box is opened and the warm air is directed to where you want it. The newest vehicle I ever owned that still had a heater control valve was a 64 Chevy Biscayne.
I had a big ol’ 65 Pontiac Catalina that would overheat. I had to drive home in the dead of summer in stop n go traffic with the heat on. I almost freekin’ died. But I made it home!
If you have a blockage in the cooling system, like a stuck closed theromstat, or a radiator blockage, the heater core could provide a good measure of extra cooling as the circulation of fluid through here does not go through the thermostat nor radiator, and if you have the heat on high, you reduce the temp of the fluid.
But I said this in my post:
which means that if the thermostat is wide open - meaning that you’re already above the closing/regulating temperature. Or are we talking about something else here that’s not clear? 
It damn sure worked on my '65 Ford pickup. No doubt about it.
I’ve done this successfully on a couple of other vehicles (older cars too).
I had put a bigger motor under the hood and hoped the old radiator would do the job …but NOOO!
I installed a bigger radiator and it’s been cool ever since.
(went from a straight six to a high performance 302")
Always remember, radiator caps go bad. That’s usually one of the first things I check on a vehicle that overheats for no “obvious” reason. 
Car Talk had a puzzler once about some AMC car from the 60’s IIRC that had a switch on dash labeled “desert”. It would vent the heat outside the car rather then into the cabin, acting as an additional radiator for hot weather use. I assume cooling systems were less efficient then and it made more of a difference.
This is not accurate. While some manufacturers - Ford and GM come to mind - eschew heater control valves (there are other ways to regulate heat), plenty still use them. Chrysler, Honda, and Toyota are among them.
In terms of the OP, it doesn’t matter whether coolant is always flowing through the heater or not. What matters is whether heat is being removed from the heater. This is accomplished by flowing hot coolant through the heater core AND blowing air across the core to remove the heat.
That’s true as far as it goes, but the thermostat being open is not the defining characteristic. In normal operation, the thermostat cycles between being open and closed to regulate engine heat. If it stays wide open, and everything else is working properly, the engine will be overcooled, not reaching its normal temperature. Now, if it is stuck closed, or partly closed, or if the radiator is restricted, or the radiator fan doesn’t work, there will be overheating to a greater or lesser degree. In these types of situations, operating the heater will typically make a noticeable difference. Whether it’s enough difference to prevent severe overheating depends on the particular situation.
Again, absolutely right. One symptom of significantly low coolant is no heat from the heater, precisely because there’s not enough coolant in the system to fill the heater core.
I have an 87 Corvette with a heater control valve, and I’m pretty sure I’ve seen one on other newer vehicles also. Even with a valve, most of the cooling you would get comes from the heater fan pulling in cool(er) outside air and passing it over the heater core. Without that fan on, the area around the heater will warm up to the temperature of the coolant and stay there.
Had a '90 Camry that would overheat on summer highway drives. The working solution was to turn the heater to max and open all the windows. I eventually replaced the radiator, which solved that problem but then the defroster stopped working. Sigh.