1998 GMC Sonoma Repair Heater

Tiny
TRAFFICFLOW
  • MEMBER
  • 1998 GMC SONOMA
Heater problem
1998 GMC Sonoma 4 cyl Two Wheel Drive Automatic

My heater is barely working I changed the thermostat. How do I tell if it is the cable or the heater core is plugged?
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Sunday, January 3rd, 2010 AT 8:46 PM

9 Replies

Tiny
KHLOW2008
  • EXPERT
Hi Trafficflow,

Thank you for the donation.

Note the cable position at valve at fully opened and fully closed position.
Disconnect the cable and check the valve operations manually.

Operate the valve manually and compare the fully opened and closed position. If there are any difference, readjust the cable and retest.

If no difference, fullly open the valve and with engine running, compare the heat of both hoses, they should be similar if coolant is flowing through.

Did you recheck the coolant level after the thermostat replacement? Any air trapped in system can cause the heater to work erratically.

Another possible cause would be the air mix door.

To test if the heater core is clogged, disconnect both hoses and use a water hose to flush through. If water flows freely, it should be good.
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Monday, January 4th, 2010 AT 7:20 AM
Tiny
TRAFFICFLOW
  • MEMBER
Good Info. Can you send a picture of where it is?
I have no idea where it is or how to get to it. Thanks
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Monday, January 4th, 2010 AT 7:52 AM
Tiny
KHLOW2008
  • EXPERT
I am unable to find any clear diagrams for the heater valve and other items from our database. The heater valve should be in engine compartment near the heater core hoses.

Just before we proceed further, I would like to know the following.

What is the actual problem that your are facing? No heat with blowers functioning correctly?

Are all the various vents selection working correctly?

The vents and air mix are activated by vacumn and if they are not functioning correctly, it could be a clogged or leaking vacumn line.
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Monday, January 4th, 2010 AT 10:48 AM
Tiny
TRAFFICFLOW
  • MEMBER
I do not think it is a air issuer. The blower works and when I change the vent from floor to dash works. It is just barely getting warm. Thanks
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Monday, January 4th, 2010 AT 12:28 PM
Tiny
KHLOW2008
  • EXPERT
As the vents are working correctly, the issue shoudl not be from the vacumn.

Check the heater hose and valve near the firewall.
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Monday, January 4th, 2010 AT 3:26 PM
Tiny
TRAFFICFLOW
  • MEMBER
I checked the heater core and there is no blockage? Air lock possible Water Pump?
I do not see no valve on either line If it is in the dash why is the hoses not getting warm?
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Monday, January 4th, 2010 AT 6:54 PM
Tiny
TRAFFICFLOW
  • MEMBER
Okay this is what I have
I checked the heater not clogged
Changed thermostat
Heater dioor moves to direct from defrost to floor
Cold to warm switch works it getts real ood when I move
No heater valve in cooling at all I checked
This is what woeks.
When I leave fan on low there is actually some heat coming out the lower vent, I will check for air block but I had the heater core lines of.
What do you reallly think it could be?Thanks
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Tuesday, January 5th, 2010 AT 5:11 AM
Tiny
SERPENTRACER
  • MEMBER
Does the hoses of the heater core get hot?
If they don't you need to flush it out or bleed the system better/more.

If they do then you can start looking into the controls.
Vacuum operates all the doors of these trucks.

A common problem with these trucks is a bad vacuum line. They go into the cab near the brake booster.
Rubber vacuum lines on these trucks seem to last about 4 years.
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Tuesday, January 5th, 2010 AT 11:00 PM
Tiny
KHLOW2008
  • EXPERT
Here is a flow chart for troubleshooting. See if you can come up with any information that might help us get to the bottom of this.


http://www.2carpros.com/forum/automotive_pictures/192750_HeaterDiagnosis98SonomaFig01a_1.jpg



http://www.2carpros.com/forum/automotive_pictures/192750_HeaterDiagnosis98SonomaFig01b_1.jpg



http://www.2carpros.com/forum/automotive_pictures/192750_HeaterDiagnosis98SonomaFig02a_1.jpg



http://www.2carpros.com/forum/automotive_pictures/192750_HeaterDiagnosis98SonomaFig02b_1.jpg

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Wednesday, January 6th, 2010 AT 6:18 AM

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