Heater doesn't get very hot

[Skunksmash]

Another heater thread I guess. My 87 heater seems to have lost a lot of its functionality. The truck has to have been warmed up all the way for at least 15 minutes before the heater will barely start to work. It always had a good heater up until this winter.

I made sure its full of fluid, and it seems to be. I did just replace the radiator, but I drove it several times and filled it up each time it sucked more fluid down. So there should be plenty of fluid. I think there can be air in there, but I'm not real sure how to tell if that's my problem.

Once the truck has been running for about 20 minutes, the heater will finally start to blow somewhat warm, but it never really blows hot like you'd want it to. There is no leaking going on in the cab floor. If it is the heater core somehow, are they difficult to replace in these trucks?

[buddy_1]

What does your gauge read? Have you checked the thermostat for proper operation? Sounds like it's stuck open.

[craig113]

If you just replaced the radiator and sure that there are no air bubbles in the system, next thing I would do is to check the thermostat and make sure there is not a blockage. With the new rad. you might have better flow and have loosened up some calcium and blocked the core and thermostat. Back flush the core and make sure the stat is no plugged before you over heat and blow a gasket.

[Rubble]

Check both heater hoses coming out of the cab,once the engine is warm they should both be hot to the touch.Heater core is pretty easy on these trucks too..

[rich weyand]

Or you may just be cooling so much better with the new radiator that the engine never gets up to temp. Thermostats have a bleed, so they always flow a little bit. My truck won't warm up to temp in the winter, even with an electric fan that doesn't run unless it is up to temp, unless I block off the grill.

[motornut]

what is your rad mix?

[Skunksmash]

Originally Posted by motornut

what is your rad mix?

Rad mix is a little more water than antifreeze, but not by much. I just bought a couple of those 50/50 jugs from oriellys and used those. It was still a bit low, but I just added water until the radiator didn't show to be low anymore.

As for the temps, they hover around the 177 area. Thermostat was replaced when the radiator was, with a 185 degree thermostat. The gauge is a digital gauge so I can see exactly when the thermostat opens if I'm watching. It will go up to about 191 and then drop really fast all the way back down to 175. If you watch you can see each time it opens, though it varies from the first time when you start up the truck. Once its warm it isn't so predictable.

[rich weyand]

I don't have the book for 1987, but the 1973-1978 parts book specifies 195* thermostat for all engines -- six, V8, and diesel -- from 1973 through 1978.

[Skunksmash]

Yeah I'm not real sure what it was supposed to be so I just replaced it with what came out of there. Can that 10* difference affect the heater?

[rich weyand]

10 degrees more is 10 degrees more everywhere in the water jacket, so yeah, it makes a difference. It will also make a positive difference in power and gas mileage.

[tucsonjwt]

Blend door not opening all the way? Bad vacuum actuator? Just guessing, as usual.

[Just call me Sean]

Being fuel injected it would need a 195.

I just worked on my vans heater. The core was clogged, I had to flush it backwards and forwards until it finally cleared. Blows hot now.

[Skunksmash]

Seems like a whole lot of things could potentially be wrong. Might just take this one to my mechanic. He has a tendency to diagnose things in 10 min that take me hours if not days.

I might can back flush the core, that sounds easy enough. I'm assuming you just plug the water hose into the lower heater hose and unhook the upper one?

[RUN GMC]

Originally Posted by Just call me Sean

Being fuel injected it would need a 195.

I just worked on my vans heater. The core was clogged, I had to flush it backwards and forwards until it finally cleared. Blows hot now.

Same here on mine......shot the hoses with a temp gun the hose from the intake was 184 degrees and the return hose to the radiator was 89 degrees at the core and 128 degrees at the radiator end which tells me I have a blockage in the core I believe......and the reason I get a little heat is due to the supply side getting radiant(maybe it is more like conductive heat) heat where the supply side and hose heats up and heats up the core some upto the blockage.....will perform the back flush tomorrow and finish replacing the last of the vacuum lines......I will disconnect the supply hose from the core raise it up and clamp to a clothes hanger the disconnect the return from the radiator blew a little air (low pressure until I see a little stuff come out) through it then hook up the garden and flush clean both ways..........

[wilkin250r]

The last heater problem I fixed, the guy had both hoses hooked to the low-pressure side of the water pump. He had very little flow, and thus got very little heat.

Once we hooked up the hoses properly, it cooked him out of the cab. It gives you yet another possible solution to investigate.

Also, whenever I back-flushed anything, I spent $12 on some PVC fittings at the local hardware store, and made a fixture to attach both a water hose and a air nozzle. When I flush with water, I give it some shots of air as well to "shock" the water passages and help knock stuff loose.

[RUN GMC]

Originally Posted by RUN GMC

Same here on mine......shot the hoses with a temp gun the hose from the intake was 184 degrees and the return hose to the radiator was 89 degrees at the core and 128 degrees at the radiator end which tells me I have a blockage in the core I believe......and the reason I get a little heat is due to the supply side getting radiant(maybe it is more like conductive heat) heat where the supply side and hose heats up and heats up the core some upto the blockage.....will perform the back flush tomorrow and finish replacing the last of the vacuum lines......I will disconnect the supply hose from the core raise it up and clamp to a clothes hanger the disconnect the return from the radiator blew a little air (low pressure until I see a little stuff come out) through it then hook up the garden and flush clean both ways..........

Worked on mine today...1st disconnected hoses like I said then back flushed both ways and also with water.....I saw nothing come out of the core except green fluid then it lightened upto clear water......I did however lower the from the cap on the radiator to below the return from the heater core to see the flow return into the radiator......I started up the motor to see if flow was good and it was and used temp gun to check the hose temps as it warmed up meanwhile I removed the glove box to watch the controls move back and forth freely which they did......the heat came up and heater started working real good the heat out the vent was upto 123 degrees , the supply to the core was 148 degrees and the return to the radiator was 108......my question is did I over fill my radiator to high and it slowed down circulation through the core back to the radiator by being above the return inlet.......as I test drove it around town this evening the temp out vents drops down to 112 degrees but it still runs me out of the cab so im happy now.....but i wonder if it was overfilled? Thanks for the thread it really helped me out a lot...........