Hit-or-Miss Heating

maybe a sticking heater/exchange box unit

Was it like this before you had the thermostat replaced?

Hello, and thank you and others for the quick response. I had drafted my OP up last week and while it was true I've since stopped getting heat going home as well. A couple observations:

1. I'm not sure if the '90 Sable has a heater core flow control valve. Even if it does and it's intermittent would that explain:
2. There have been several isolated instances where the temperature indicator (engine has been running for a while) has gone to its maximum value and just as quickly dropped down below below the midpoint before returning to the midpoint. My conclusion here would be low coolant but the coolant appears to be full. I'm speculating that something is restricting coolant flow such as a partially-blocked radiator or somewhere else other than the heater core but before the thermostat. Any further insights are most welcome. Sincerely,

You could have air in the system. That could have happened when the thermostat was changed. I didn't see where you gave an answer to the question in the post you are responding to:

"Was it like this before you had the thermostat replaced?"

-jim

Your shiny new thermostat is a piece of s***?

Just a stab in the dark; whenever I replace something and things get worse it always seems to be one of three things: I broke something else during the 'fix', I put in a faulty part, or what I fixed overstressed some part of the system that was on the hairy edge, and it broke.

Some thermostats can be put in upside down, I seem to remember.

Also some GM variants had special side flow thermostats or some such, and even though they might fit similar cars, they did not work interchangeably. I dont think Ford went this route, but if the problem came on after he changed the thermostat, air in the system, or a thermostat malfunction is certainly to be looked at.

I have worked on older Fords that I had to take a hose and blow the crap out of the heater core and make sure the heater valve was working. Havent worked on a Sable or Taurus however.

I don't think there's any problem with coolant flow through the heater core. I'll bet your problem is the blend door in the heater box. I don't know how you would check/repair something like that.

You should verify that your coolant level is OK and there's no leaks. If that's the case, change your thermostat. I think that your heater problem is unrelated to your coolant problem.

I dont know about Sable. On some cars you can get to them well enough to manually operate the doors. If they are vacuum operated, you have another burden to bear.

If the doors are stuck, you can sometimes free up the mechanical pivots of the blend doors with a little shot of aerosol lubricant.

A Sable mechanic would likely know all the systems.

I dont understand why this system would work in the afternoons, but not in the mornings,and now wont work at all. I am sure it will make sense once he gets into it

Really helps to have a good manual when you are trying to DIY something like this.

First thing to do is make sure the engine temp really is up where it needs to be. I don't think this is your problem but it is an easy place to start. Also check your heater hoses and make sure both of them are hot. Two hot hoses tells you there is coolant circulating through the heater core.

You said you set the temp to 85 which tells me the car has an automatic climate control system and I think this is probably where your problem actually is. Most common is a blend door that is sticking. When that door doesn't move the air isn't directed through the heater core so... no heat. It could also be a problem with the temperature sensors not knowing the temp of the car or just a general fault with the automatic system.

If you aren't used to working under the dash and you don't have arms with five elbows I would probably find someone that does a lot of this type work

Steve B.

I don't know anything about these systems because in the past, if it went bad, I'd just run without the heater. The idea of working under the dash is pretty unappealing - thank God I live in the tropics. My guess is that the Ford uses vac servos to operate the doors and one could do an initial check with a hand held vacuum pump. I'll bet the system tends to leak when the rubber hoses are cold.

There are different systems and you really have to know what you are doing.

Some GM systems, for example, us a vacuum modulator module that is a little difficult to diagnose without the proper tools,and is DAMNED expensive to replace. This system operates blend doors, etc.

Sables and Tearasses are just something I dont know anything about, and really dont want to learn.

I wish it were as simple as it used to be, with just a manual linkage to the blend doors, a simple valve, etc. But it isnt always that way.

mvh

It certainly sounds like an air bubble. Just to be sure, are you checking the coolant level in the cooling system itself rather than in the overflow tank? Wait until the engine is dead cold and then remove the remove the pressure cap. Top it up. After the initial fill you need to "burp" it--run it through a full temperature cycle with the heater on-- then top it up again. After that you can leave the cooling system sealed and simply watch the fill lines on the overflow tank.

then "burp" it. h

It's surprisingly tricky to fill radiators these days because they tend to be lower than the top of the engine. On my Taurus, I can tell if it's full by squeezing the top radiator hose. The first time I changed the coolant in the car, I loosened the clap on the hose that went to the top of the engine. Now days I don't bother - I'll just run it through several heating a cooling cycles and purge air bubbles by squeezing the hose.

I just read a couple of TSB's on the blend door having issues with the cable etc.. It also mentioned something to the effect if you are having a problem with the blend door turn the key off and back on and see if it works then.

I had a heat problem in a Taursu casued by a clogged heater core. There is a bypass hose right across the input to output of the core located on the firewall so just becasue you have flow through the heater core hoses doesn't mean the flow is actually going through the core, if the core is clogged, the flow will go through the bypass hose.

I carfully pinched the bypass hose with vise grips to force flow through the core and back flused the core. that fixed it for me.... YMMV.

Mark

There is an in car temp sensor behind the glove box and a sun temp sensor under the left side of the dash. If there is no air in the system, I'd take a look at those. HTH, Ben