The heaters are just small radiators. Turning on the heaters increases
the total cooling capacity of the system. In many cases it will be
enough to stabilize the engine temps. Most cases of overheating are
transitory such as climbing a long hill on a warm day. Turning on the
heater as a aid to the cooling system has been used as far back as
heaters have existed. It has saved my butt a few times. Just last year
on a camping trip a blown fuse disabled my radiator fan but I was able
to proceed by running the heater at full speed. Of course it was a very
hot day and I was miserable so I fixed the problem as soon as I reached
camp.

Some cooling problems are too fundamental to be helped by the additional
radiator capacity of the heater. If there is no coolant, or if the water
pump impeller is not turning, the heater won't help. If the head gasket
has failed such that combustion gases have replaced the coolant at the
pump then a lack of circulation will prevent the radiators from doing
much.

Mark

Malcolm Stebbins wrote:
>
> > At this point you can shed additional heat by turning both heaters on
> > full blast, but you won't like being in the same vehicle with it. <g>
> >   david
>
> In my experience, this is has a VERY temporary effect, maybe a
> minute or two relief for the engine, then right back to over-heating
> (form whatever cause). M