'87 GMC heating problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

westcasey · 01-28-2004, 09:17 PM

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???????

shavedchevy · 01-28-2004, 09:20 PM

low on antifreeze?

rolson1039 · 01-28-2004, 09:23 PM

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.

westcasey · 01-28-2004, 09:26 PM

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.

Free88 · 01-28-2004, 10:36 PM

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.

Grim Reaper · 01-29-2004, 01:05 AM

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.

westcasey · 01-29-2004, 08:31 PM

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!!

Free88 · 01-29-2004, 09:25 PM

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

01-28-2004, 09:17 PM	#1
westcasey Registered User Join Date: Jan 2004 Location: kansas Posts: 49	'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??????? __________________ 1987 GMC 1500 5.7L 1985 Chevy 3/4 4X4 2001 Chevy 2500 HD 4X4 2007 Chevy 3500 Duramax 4X4

01-28-2004, 09:20 PM	#2
shavedchevy Registered User Join Date: Nov 2003 Location: Edmonton, Alberta, Canada Posts: 315	low on antifreeze? __________________ Dennis

01-28-2004, 09:26 PM	#4
westcasey Registered User Join Date: Jan 2004 Location: kansas Posts: 49	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. __________________ 1987 GMC 1500 5.7L 1985 Chevy 3/4 4X4 2001 Chevy 2500 HD 4X4 2007 Chevy 3500 Duramax 4X4

01-29-2004, 01:05 AM	#6
Grim Reaper Registered User Join Date: Jul 2001 Location: Atlanta GA Posts: 1,704	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. __________________ Grim-Reaper 70 Pontiac LeMans Sport Convertible, worlds longest resto in progress Looking for 71-72 2wd Blazer or Jimmy Project

01-29-2004, 08:31 PM	#7
westcasey Registered User Join Date: Jan 2004 Location: kansas Posts: 49	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!! __________________ 1987 GMC 1500 5.7L 1985 Chevy 3/4 4X4 2001 Chevy 2500 HD 4X4 2007 Chevy 3500 Duramax 4X4

01-28-2004, 09:23 PM	#3
rolson1039 Registered User Join Date: Oct 2002 Location: NJ Posts: 404	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.

01-28-2004, 10:36 PM	#5
Free88 Registered User Join Date: Jan 2004 Location: Northwestern Ontario Canada Posts: 359	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.

01-29-2004, 09:25 PM	#8
Free88 Registered User Join Date: Jan 2004 Location: Northwestern Ontario Canada Posts: 359	Westcasey, glad to hear that you have solved your problem. Sometimes the weirdest things can cause a problem. Happy heat.



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.