1995 sub rear heat problem

[ellevold]

I have a 1995 chev sub. The rear heat never worked right since the day I bought it. OK I will tell you eveything I did to try to get it to work. The radiator leaked so I bought a new one. Intake gasket leaked so I replaced it. I put in a new thermostat and the heater core in the back is replaced. Still no heat. I took the water line off in the back and the water just trickled out. So I thought it was plugged. So I flushed it and the water flowed through the back just fine so I know thats not plugged.HELP me anyone I just cant get pressure to the back heater core. Also I replaced the water pump just because that was the last thing I could think of. Any Ideas thanks

[Larry Derouin]

There's a 3 way vacuum switch/motor (water manifold) in the heater hoses, between the engine and the firewall. You might want to check the operation of that. Also blow thru your heater core in the back.

[ellevold]

I dont think I have that 3 way switch. Anyway one other weird thing I noticed. When I filled the radiator (cold engine) and waited for the thermostat to open so I could fill it more instead of sucking the level down the water shot out of the radiator about 6 in. Its almost like the water pump is pumping in reverse ha ha. I just dont get this puzzle

[Larry Derouin]

Normal expansion of fluid and air when heated. It has to go somewhere. The cap opening is the path of least resistance. When you fill and run leave a couple of inches between the fluid and the top of the radiator to account for "burping". Add slowly until it gets hot and full. If you take your time it will burp slightly and not "power puke".

Double check for the valving. It should be about 1/2 way between the engine and the firewall.

Larry

[GMMerlin]

Larry is right..on the right wheel well you will find a black plastic manifold with heater hoses going in and out of it. There is also a vacum line that controls it.
I have had to replace a few for rear heat inop......if you have rear heat, you have the manifold.

As far as coolant shooting back out of the radiator, it is quite possible that you have a air pocket in the cooling system. You could let the engine run for a bit, then let it cool down two or three times to burp the air pocket

[ellevold]

I really appriciate your time helping me out with this problem but now I am more confused. Today I decided to put a different thermostat in it. Now It heats like crazy in the rear but the front isnt that hot. The funny thing is its a 195 degree thermostat the engine will run up to about 205 when the thermostat opens it drops down to about 135 to 140. Back and forth it will go. In the mean while the rear heater is blowing really hot. But the front heater gets cold then hot cold then hot and so on. The new radiator that I put in is twice as thick as the original does that have anything to do with it. I just cant seem to get this thing to run a stable temp.

[skiburban]

I have a 92 Suburban, I don't have rear heat, but I do have the same problem with the temperature swing you mentioned in the last post. I too put in a new radiator (same size as the old one - 4 row, I think.) I also have no heat in the front. I'm going to flush the heater core this weekend to see if it's clogged or not. After that I'm not sure what too look for next......

[GMMerlin]

Originally Posted by ellevold

I really appriciate your time helping me out with this problem but now I am more confused. Today I decided to put a different thermostat in it. Now It heats like crazy in the rear but the front isnt that hot. The funny thing is its a 195 degree thermostat the engine will run up to about 205 when the thermostat opens it drops down to about 135 to 140. Back and forth it will go. In the mean while the rear heater is blowing really hot. But the front heater gets cold then hot cold then hot and so on. The new radiator that I put in is twice as thick as the original does that have anything to do with it. I just cant seem to get this thing to run a stable temp.

You should not have that much temp swing with the new thermostat...maybe 190-205 once its open and regulating.
You may still have an air pocket

[ellevold]

I'm pretty sure I have air in the water lines somewhere. Both heaters work great diving down the road but when I come to a idle the front heater turns cold. Thats why I think my temp gauge is going up and down. We can eliminate the thermostat because I put another new one in and it still does it. How do I get the air out?

[utahawk]

Are you sure there is not some piece of debris blocking the airflow of the heater blower? My brand new Burb had no rear heat or AC and when they took it apart they found a part reorder form had fallen into the duct at the factory and it was lodged in the air flow path. They pulled it out and fired it up and the hot air just poured out.

[Fireplug]

First make sure the overflow bottle is full. Then run the engine(take it for a ride) and get it good and hot. open the hood and feel the upper rad hose is it hot and hard like there is pressure inside the hose it not Then let it sit for about 3 hours and go and remove the rad cap and see if the rad is full ,if not fill it and install a new rad cap that can hold 12-16 psi.

[ellevold]

I'm running out of options. What do you guys think if I run the return line from the heater core to the radiator instead on top of the intake manifold? Will this give me more presure to the back heater core? Right now I take the line of the back heater core even with the engine running it barely runs out. I know the hoses are not plugged thanks

[Spencer 4-hire]

The hot to the heater cores should come out of the engine at the intake manifold. From there it splits at a vacuum controlled servo mounted on the right front inner wheel well which controls water to the rear core. The vacuumed control for this servo is mounted on the firewall and is an electric switch that controls a vacuum line running from the manifold to the servo.

Form there, hot water goes into the cores and returns the point near where the servo is and goes back into a single hose via a "Y" fitting. Then it dumps into the radiator just below the cap. If you are getting hot air at one core you should have it at both, providing your vacuum switch is working correctly to allow hot water flow to the rear core.

If one core is hot and one is not, it is a problem related to either the hot water delivery to that core or the air supply through the core.

The hot/cold control temp switch on the dash activates a proportional servo that is located behind the ashtray that can be examined by removing the glove box. Pinch the glove box stoppers in and let it swing out and down to remove. This servo moves a flapper inside the duct to route air through the heater core or the AC core. Check for travel on this servo with the key turned on it should swing back and forth as you rotate the temp control knob from hot to cold. If this servo is not swinging the flap to direct the air through the heater core you may get the results you are describing.

Hope this helps.

Spencer

[K5er]

I have a 94 suburban and have just solved the same problem.
First, I do not have a vacuum switch on the wheel well (same as 95).
What I found was that the "Y" which has the input from the back of the motor and then two ouputs (one to the front heater core and one to the rear heater core) was plugged. The output tube to the back has a flow restricter, much like in a shower head, about 5/32 in diameter (pretty small).
I found the problem by using the the Prestone flush kit Tee, that is installed on the hose from the back of the moter. I put the garden hose to the Tee, then pinched close the portion of the heater hose that leads back to the motor so that the flow from the hose would only go toward the "Y". I then disconnected the "quick disconnect" fitting from the "Y" (the one that goes to the rear) and blocked (pinched close) all other heater lines, including those on the return side).
With this done I turned on the water and found no flow from the output of the "Y". I stuck a small tube cleaner in the end and a small amount of flow started. That is when I found the restricter plate up inside the output tube.
At any rate, I disconnected all the hoses from the "Y" and removed it from the wheel well. I then cleaned it out and now have heat in both front and rear.

Hope this helps.

[DDotson]

You say you replaced the Radiator!!!! If you replaced it because it was bad and someone had put STOP LEAK in it before. You probably have partially blocked holes and hoses. Unfortunatly the STOP LEAK not only plugs holes in the radiator but it also stops up inlets and outlets. And by the way you should always replace parts by manufatures recommendations (original) unless you made other modifications requiring larger (radiator) or lower rated (thermostat) than original.