'87 GMC heating problem
 

My heater is blowing lukewarm air most of the time. Not even warm enough to defrost the windshield. It will blow hot sometimes, then maybe 10sec. later will be lukewarm again. I put a new thermostat in and that didn't help so I got another one and installed it thinking the first one may have been faulty. Still no luck. Anyone have any advice???????
:gmc: :gmc: :gmc: :gmc: :gmc:

low on antifreeze?

do you hear the hiss of a vacuum leak on an AC truck? if its not an AC truck you may want to check theantifreeze or the thermostat.

Antifreeze level is good. I've listened for a vaccum leak, but didn't hear anything. All my vents are opening and closing ok. I'll try to get a piece of hose and listen more carefully around all the connections.

Two things that come to mind. 1, Make sure that rad pressure cap is good and not leaking, if it is you won't get the proper coolant temp to open the thermostat at the right time. and 2, you may have a blocked or plugged heater core. Make sure that the temp of the two hoses from the heater core are both hot to the touch, this means that the heater core is not clogged up. I have always run the engine, turned the heater on full when topping off the coolant to make sure there is no air blockage in the system after changing thermostats or hoses. This is not gospel only what I have found on my trucks when I had a heat problem. Good luck.:canada: :gmc:

Blend flap is not opening is one possible deal. Pull the glove box door off. The top of the heater box is right there THe cable right upo there at the op front is the blend door . Check it. sometimes it gets screwed up if the heater coor hase even been changed. There is a little push clip that retains the end of the cable. Near impossible to get it off without breaking it LOL.


Heater hoses hooked up properly?

The hoses will always route from the intake To the the bottom line on the core with a 5/8 line. The top line is the return and it either goes to the radiator OR it goes to the top of the water pump. Both those locations are return. 3/4 line


Some how a lot of people managed to hook both lines to the returns and it just doesn't moove any coolant through it.

Got my heater finally figured out. I took the advice and disconnected the heater hoses and flushed water through the core. Gets hot enough to burn a guy out now!! Thanks for all the advice!!

Westcasey, glad to hear that you have solved your problem. Sometimes the weirdest things can cause a problem. Happy heat.:canada: :gmc: