90 GM LS: Heater Hose and return hose are cool and have no pressure

[Eric_S]

Hi, I hope you guys can help me with this.

I have a 1990 Grand Marquis LS and the heater is not working. The fan still blows, but it will never get hot. So far I have done the following troubleshooting.

- Checked coolant levels. It is full and good.

- Checked all the doors and motors underneath the dash to make sure they are working. I can see the motor move on the ATC when I switch between Defrost and Max AC. Also, I can hear a difference when I switch between Heat and AC

- Unhooked the white hose with a clear end going to the motor underneath the glove box. I plugged the hose, and I still got no heat.

- Replaced the thermostat with a 195 degree model.

- Checked the fuses for HVAC and climate control. They were still good.

- Flushed the heater core with water. A bunch of brown muck came out for the first few seconds, then it ran clear.

The car has these symptoms:

- Losing radiator fluid somewhere.

- Has had stop leak put in it.

- The hoses going into and coming out of the heater core are cool and don't feel like they have any fluid in them.

- I replaced the bypass hose going (the little one behind the radiator with an S curve) a couple of months ago.

- No dampness underneath the dash.

Is there anything else I can try?

The slack and cool hoses going into the heater core seem to be the most suspicious, but I'm not sure where to go from there.

[cld783]

is your rad full of coolant? i'm sorry if this seems like a stupid question but i've seen it a million times. If everything checks out im guessing something is plugged up from the stop leak. you say you flushed the heater core, have you also flushed it from the radiator side?

[Eric_S]

Yep, it is full of coolant. Made sure of that not too long ago. I'll put it into the things I've checked.

I have not flushed the radiator since I put the stop leak in. I will probably do that tomorrow morning.

Thanks for the reply.

[gadget73]

Does the upper radiator hose get hot and build any pressure?

it sounds like either the new thermostat is bad and allowing coolant to bypass it, or you have an air bubble. Try pulling one of those little coolant hoses off the side of the throttle body. Thats the highest point in the cooling system. If therre is any air in the system, most of it should purge out through that spot.

[Eric_S]

Nope, the upper radiator hose does not get hot or build pressure. I will add that to my original post, along with that I have replaced the radiator cap.

It would really suck if the new thermostat is bad. I bought it from O'reilly's, and it did look a little different than original. BUT, my car was doing this before I replaced the thermostat.

[marquisman]

you dident buy a fail safe, did you? had very bad luck with them sticking open.

[91waggin]

Thermostat is easy to check with a pan of boiling water and some pliers to dunk it with

[marquisman]

if its a fail safe, it will be open when you take it out

[86VickyLX]

I bet you do have some air in there. Coolant likes to disappear when there's air in the system. If there isn't any coolant on the ground, I want you to open your radiator cap, and start your car. Put your heat on full on high and let it run. Any air in the system will come out. When it's all gone, the water will start to over flow, at that time, close the cap and add coolant to the resevoir. By then, you should have heat. And everything should be operating as it should.

[Eric_S]

I took the car out to a mechanic my Dad recommended. When we got there we checked the upper radiator hose and it seemed very hot and had pressure. The heater hose was still slack and cool though. He recommended replacing the coolant temperature sensor.

I went and bought one and installed it and then took the car for a test drive. When I pulled back into the driveway I checked the heater hose. It seemed to be getting hot, so I got back into the car and turned on the heater. The air still wouldn't get hot. I got back out and checked the hose again and this time it seemed cooler and still slack.

EDIT: I just tried belching the radiator again, and this time the coolant wouldn't even reach to the top of the radiator. I think all I need to do is wait for it to cool down, fill it up, and try again.

[Eric_S]

Just tried filling up the radiator again, and after a test drive it still doesn't seem like it can build any pressure in the upper radiator hose or the heater hose.

[gadget73]

Pull one of the lines off the side of the throttle body. That should get the air out. Its the highest point in the system.

[Eric_S]

Well, I've pulled all the lines off and let the air out, and belched the radiator again. Still no heat.

I've been overlooking one thing. The radiator never ever gets hot. I can drive for 20 minutes and stop and take off the radiator cap with no spray, and the water never boils. Until that coolant actually gets hot I will never have heat.

I think the thermostat I installed was bad out of the box. I thought it would be stuck in the open position when I took it out, but it was closed. Anyway, It was a 192 degree Murray Plus+ model. I am replacing it with a Superstat 195 degree one tomorrow morning.

[marquisman]

i still dont trust that t-stat. pull the upper rad hose off the t-stat housing and you should be able to see if its stuck open with out removing the whole thing.

[gadget73]

Put it in a pan of water and heat it to make sure it opens properly.