Re: 300sd Heater stops at 45MPH

"My 81 300sd has good heat around town but loses it on the road above
45
mph. I have noticed that there is little heat at 2800 rpm but as I
slow down at precisely 1500 the heat will suddenly burst out of the
vents as if some one had opened a water valve. The heat will keep
coming until you get above about 2400 rpm and then it just drops to a
very little amount of heat. The engine temp is 80c and I have pulled
the wires off of the mono valve so it will stay in the open position
and I still get the same results so I figure that it is not a fault of
any thing electronic. I think the water just stops flowing at high
speeds.
Any one got any ideas?
Thanks,
Bruce Buchanan "


The description of the heat bursting out has me confused. Does the
volume of air remain constant? Is it coming out of the correct vents,
ie footwells for heat, and does that remain constant?

If it's just the heat that disappears, with the monovalve out of the
picture, there isn;t much left. I assume you've checked the coolant
level and made sure it's full? Usually low coolant behaves the
opposite, with heat loss at idle occuring first. The only thing I can
think of is that there is some kind obstruction in the heater core loop
that only gets blocked under higher volume flow, which would be pretty
strange. I'd try flushing the cooling system using one of the
adapters that allow you to connect a garden hose into the heating hose
path.

.

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