My 98 GC Limited heater quit putting out heat. It was working yesterday and not now.
The blower works and so does the thermostat. There is plenty of coolant in the system.
I did the system reset thing but not for sure that I got it correctly done.


Does anyone have a solution for this issue ?

 

There is a way to test and reset the HVAC components using the AC/RECIR buttons and rotary temperature control as per the factory service manual. Is that what you did?
It could be a vacuum line that moves the vent door. I had my heater stuck on one summer day. Turning on the cruise control changed the pressure and it switched over to AC. That only worked once. Closer inspection under the hood revealed a loose vacuum line.

 

My 94 Laredo is doing the same. What is the system test you did?

 

Does your 94 have an ac control like this?

 

While there is a plethora of problems that may be associated with your heater not working, one problem may be that your heater core has gone bad. It's somewhere in the dash, so the dash has to come apart to replace if it that's indeed the issue.

I'm unfortunately unsure of any tests you can perform.

It wouldn't be a blend door problem because then nothing would be blowing out of the main vents, and all air would default to the defroster vent in the top center of the dash.

 

I agree with the heater core suggestion. the same thing happened to me. my AC works just fine, but no heat. easy way to tell if it is the heater core is to check the front passenger floorboard, if it is wet, it is antifreeze meaning that your heater core is bad. Hopefully it isn't your heater core, because they are a true hassle.

 

If the heater core fails, it leaks, usually into the passenger compartment. Then you smell coolant & see it on the floor & the inside windshield gets this curious "greasy" coating. But it only fails to heat that way when it's dumped enough coolant that the system is low. And that you can spot because the radiator and/or coolant overflow tank is low. But this original poster said he checked & the coolant isn't low. And he didn't say anything about a smell inside or stray fluid inside. So it doesn't sound like a bad heater core to me.

However, I'm curious to see where this thread goes. I have a '98 Limited with automatic temp control also. You can set it on 80º and the performance is just lackluster. it barely gets to comfortable and definitely doesn't overheat much. And that's when the truck is fully warmed up & it's only in the 50's outside. But my low coolant warning light came on a couple of nights ago. When I checked the radiator, it didn't look low either. I'm running Prestone super radiator cleaner through it right now. I spotted little metallic flakes in the radiator fluid. I'll drain & refill it after the Prestone has had a little time to do it's magic, but I'm not optimistic. My coolant temp is hanging steady at 210, but I'm wondering if the thermostat is failing? Or is the front water pump failing? (I'm at 180k miles.) I'm just guessing now…

Thomas

 

After much head scratching here was my problem....
The coolant reservoir was not going down, making me think the system was full when in fact it was not. I discovered a small sometimes leak at the water pump. This would not show up on a regular basis. Because of this, the level in the heater core was low with the expected results.
After replacing the water pump and correct (195F) thermostat, all is well.
One note however, on my 5.2, the heater return line is a tube that just pushes into the pump. On that end, there is an o ring and I looked around in every parts house in town to no good. It is a dealer item only.

 

Bull!! Do you have any industrial supply houses near you? Take it to them and they'll match you up for about 50 cents.

 

You must live in some city.
No such thing around here.

 

How about a small airport with a maintenance shop? Or even a GOOD hardware store. Tractor hydraulic shop? O-rings are cheap, but not from the dealer.

Again, I don't know where HERE is!

 

The water pump should have came with a new O ring in the box.

 

Yes, that's what my climate controls look like. Someone gave me instructions on how to perform the diagnostic test to pull the codes. Unfortunately, I'm showing no codes present.

So what else could be the problem? I've already replaced the water pump, thermostat, & upper radiator hose. And I very aggressively flushed out the cooling system through every port of entry I could find, including both heater core openings. All came out clean eventually.

It's really not up to handling 10º outside.

 

Get it up to temp and feel how hot the coolant is going into the heater core and is it hot coming back out the heater core. If it's not than you have a blockage or air pocket. If it is hot both ways the blend door isn't moving all the way could be actuator or door in the box is broken. Not alowing the door to move to full hot. Also could be years of crap stuck to the little fins not alowing the air to transfer heat.