Has anyone else had this problem?

When I hit about 50mph and above, my 1976 280 blows very hot air (actually burned my leg during an hour-long drive) from the lower and driver/passenger side heater vents.

The center vents will blow nice cool air while all the other vents blow hot hot hot.

Reduced speed, in-town driving = no more hot air.

I took my 1975 240D out today for it's first run after making all the repairs
and I noticed hot air coming out of my car also. I checked the firewall behind the engine and noticed the two plugs flaps were broken and missing so I pluged them. You might want to check yours. Hope this helps

not sure but worth checking , at speed there is air flow against the flaps that are closed,could you have a weak vacuum or weak linkage holding the flap closed and the air is pushing it open?
I have this isssue in my 84 300

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Ron
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Will be checking the flaps this weekend.

If that's not it, into the shop it goes!

I'll post the outcome as soon as I can.

Thanks for the suggestions!

...that your heater valves are closing?

If you are getting cold air thru the center vent, then the air is either being heated by the cores or you have a hole in the firewall (if it's a hot day).

Jim

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I can't seem to get those vents to close.
I haven't looked closely at anything yet. I'll ask what is probably a real silly question - Are the valves are located on the firewall?

Someone mentioned that I may have a bad heater core.
Any thoughts on that?

Even if the cores are heating the air, I should be able to close the vents and keep the air from coming inside - yes?

I'm sure you can tell I know VERY LITTLE about the mechanics of this (or any) car, so my apologies for all the questions and my lack of understanding!

The hot water valves are located on the firewall right beside the brake booster. The cables can be rusted and prevent them from moving. Slip the coiled ends off the levers and see if the valves themselves will close.

There should be a "crossover" diaphragm just above the accelerator pedal on the console. It's run by vacuum and is controlled by the A/C twist knob. Make sure it is closing the flap (or opening).