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1991 Chevy Suburban (Weird Heat Problem)

Guys/Gals, I have a really strange problem with the heating in my 91’ Chevy Suburban. For some reason, the heat will not work when I close the hood. I’ve burped and flushed the radiator, heating core, checked the blend doors/actuators, you name it. The only other thing I can think of that it might be, is the thermostat.

Any ideas as to what to do and why the heat works when the hood is open, but not closed? Thank you!

Make sure a hose between the engine/heater core isn’t being pinched off by a hood hinge when the hood is closed.


I don’t see any hoses that look like they’re getting pinched when the hood is closed…

A mystery to be sure, is there an under hood light that goes on when you open the hood?

Not when I open the hood. There is a light that I can turn on though when the engine is running. I even disconnected the sensor that gets engaged when the hood is lifted/closed to see if that would somehow do something but no luck.

Just thinking there is a backfeed that supplies power to the blend doors to provide heat when the hood is open.

If there is, I cannot find it… I’ve looked everywhere, with the hood in different positions, to try and locate some kind of hose or electrical switch that is being engaged when the hood is down with no success. The only button that I can locate is one on the driver’s side. However, it makes no difference if that is connected or not. The heat still only runs when the hood is up. I’m out of ideas.

Hood open Indicator? Have you checked the fuses and relays?

I’ve checked the fuses… Not sure where the relays would be though.

What is the heater temperature output with the hood open and closed?

Not sure of exact numbers. However, when the hood is closed, it is blowing cold air, no heat at all. When the hood is open, there is typical heat.

If it is a dual heater model 6.5 diesel, I had a similar problem caused by the plastic water flow splitter just behind the motor close to the firewall. There are two of them. The plastic had become soft and flattened at the hose coming from the motor when pressure changed. I made two new splitters out of copper pipe tees and pipe reducers since hose sizes are different, and silfloss brazed them together rather than 50/50 solder. My problem went away after that.

How would the splitter be effected by hood open or closed?

Engine bay temperature rises when the hood is closed apparently just enough to make the plastic splitter soft enough to partially collapse. I also separated the two splitters that were touching from the factory.

I have run into some weird issues on GM, One I put a socket in for a spotlight, if the spotlight was plugged in backfeed from the spotlight with a chargeing circuit caused the engine to keep running when the key was turned off.
My thought was it is an electronic control for the heat, fuse or relay out but for whatever reason if a light was being powered when the hood was raised it provided juice to the heat control via backfeed.