Question about 1999 GMC Sonoma

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99' GMC Sonoma. The heater quit heating completely about a week ago. I flushed the system and added fresh coolant and even disconnected the lines at the firewall and applied air pressure to blow out any clogs that might be in the core. The coolant runs through perfectly free and the heater now gets warm. Can't find any leaks, but it still doesn't get as hot as it used to. The only other thing I see, is that it takes a few seconds now for the air flow to change from the upper vents, to the floor, to the defogger, etc. Is there some kind of flap under the dash thats not opening all the way or something? Thanks

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  • 48 Answers

Sounds like more than one issue, not getting as hot as before, and slow to change from vent to defrost, The first I suspect you have a thermostat failing, not a big job or expensive either, remove the thermostat putt it in an old pan full of water bring it up to a boil, with a thermometer (I use a meat thermometer from the dollar store as you won't want to use it for food again) watch at what temp the thermostat pops if it doesn't pop at around 180 then replace it.

second is the delay in switching from vent to defrost, again is not to expensive but is a bit troublesome as it sounds like you have a weak vacuum the louvers are moved by little dash pots the dash pot that moves the louver behind the dash could have a bad diaphragm or a crack vacuum line. not expensive just a pain to get at. a can of wd40 with the straw in place can help diagnose the problem if it changes properly when sprayed at time of activation (the line first then the dash pot) you will have found the bad boy Good luck

Posted on Apr 14, 2017

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Posted on Jan 02, 2017

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SOURCE: 1998 GMC Jimmy SLT warm air only from heating ducts

if you claim that your heater core is flushed properly then my bet is the temp regulator is bad.

Posted on Jan 13, 2009

  • 1071 Answers

SOURCE: GMC Sonoma SLE 4.3L 1991- Coolant Temp Gauge Not reading

there are 2 coolant sensors one for gauge and one for ecm

Posted on Jul 23, 2009

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SOURCE: At idle, Heater Core Return Line is flowing only

two things--first replace your cabin air filter.second reverse flush your heater core with a garden hose and that should remove the build up of material in your heater core and restore the flow,hope this helps

Posted on Oct 21, 2009

Testimonial: "Hello; I backflushed heater core, cooling system flush. Heater floor vent temp. went from 96 degrees to 140; Thanks for your excellent advice. Robert"

  • 3 Answers

SOURCE: Heater doesnt work blows cold air, replaced

This is what I did to solve the exact same problem.I got a pump that you can hook to a drill that has garden hose connections on it.I made 2 hoses to pump into my heater core and hook to the pump. I bought some radiator cleaner at Auto Zone and put it in a 5 gallon pail with about 2 gallons of hot water.I pumped the cleaner from the pail into the OUTLET side of the heater core.(By going in the outlet side you are back flushing the core.) I put an old cloth over the top of the bucket and let the outcoming cleaner run onto this cloth acting as a filter for the dirt coming out of the heater core and let the pump run about 10 minutes. Believe me,A LOT!!!!!!!!!!! of dirt,rust etc. came out of my heater core. The heat now works excellent! Good luck.

Posted on Jan 29, 2010

loud_pipes69
  • 122 Answers

SOURCE: OK I HAVE A 1994

sounds like a bad heater core usually located under the dash and stereo

Posted on Jan 12, 2011

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1 Answer

Heat loss in my truck


SOUNDS LIKE HEATER CORE IS PLUGGED. SOMETIMES YOU CAN CLEAN BY FLUSHING COOLANT SYSTEM

Mar 04, 2014 | 1999 GMC Sonoma

1 Answer

My 99 GMC sonoma with a 2.2L is not warming up properly


Possibly plugged or inerporable heater core. Possibly heater core valve (this diverts hot air through your heater system) When you flushed the block, was there a lot of contaminant? 95% of the time, cold heat means thermostat. Also there are a lot of really cheap after market thermostats, you can buy one as low as $6.00. That being said, just because a thermo is new doesn't necessarily mean it opens/shuts when it should. You can look on You Tube on how to test one in your kitchen with boiling water,.

So three basic things affect interior heat temps. Thermostat, plugged heater core/.valve, and insufficient coolant or liquid to tranfser heat. If your interior coolant (engine temp) is working normally and your vehicle reaches operating tempature according to the needle, your thermosat is probably not the cause.

My bet is somehow, your heater core is plugged so that their is not enough "flow"

Feb 28, 2013 | 1999 GMC Sonoma

1 Answer

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All kinds of things to look at.
Start with coolant level in radiator. Very often if the radiator is low, the heater will not work, or work well, because if there is air in the system, air will rise to the top, leaving the heater core with an air bubble.
Whenever you replace any hose on the car, you must run the engine with the heater on at a medium high RPM (1500 RPM) for a while (5-10minutes) to "burp" the air from the system.
You'll need to make sure the system is full of coolant after that.
Then there could be a partly plugged radiator, a pinched or crimped heater hose, or the heater control might not be allowing the hot water to flow fast.
Be sure to test the temperature of the radiator, to be certain the thermostat is keeping the radiator coolant at near 200 degrees. A thermometer in the top of the radiator will tell you.
If you have a sealed system, you might VERY carefully feel all the hoses to determine the heat inside the radiator after the motor has been running for 10-15 minutes.
Thermostats can get stuck in the "open" position, requiring replacement before you will get heater heat.

Jan 05, 2013 | 1998 GMC Sonoma

1 Answer

Coolant level ok no leaks new 195 deg. thermostat still very little heat from heater defective heater core?


I am assuming your vehicle has the pink antifreeze called Dexcool. This is the recommended coolant for all late model GM cars and trucks. This coolant tends to form a pasty sludge in the cooling system which can lead to overheating and/or plugged up heater core. Try flushing the cooling system with a good aftermarket product such as prestone radiator flush. First thing to do is drain all the pink coolant, then add the radiator flush and fill with plain water. Run the engine according to directions on the flush container. Drain the radiator again and refill with plain water. Run the engine for a few minutes and drain again. Keep repeating this until all the pink antifreeze has been eliminated. Fill the system with 2 gallons of green antifreeze. Make sure the antifreeze container says it is compatible with aluminum engine parts. Hope this helps. If it does, then you won't have to replace the heater core.

Dec 16, 2010 | 1999 GMC Sonoma

2 Answers

NO HEAT BLOWING COLD AIR


Make sure the coolant level is full. If coolant levels are ok you made need to replace the thermostat .You can disconnect the heater hoses going to the heater core and flushing water through with a garden hose at low pressure. If the vehicle has the 4.3L engine these are quite prone to heater cores plugging up. Would try flushing the heater core first.If that does not work most likely the thermostat is causing your issue. Be sure to fill coolant back up after servicing and allow air to bleed from the cooling system.

Dec 01, 2010 | 2003 GMC Sonoma Extended Cab

1 Answer

I have a 2001 sonoma which is running high on temperature. I just changed thermostat and it still running too hot. The heater does not blow heat and when I took out the thermostat it seemes like it needed...


The radiator might be clogged. Take it and have the system flushed out,and back flushed, that will tell if there is a clog somewhere. If the radiator is too clogged, it will need to be replaced.

Mar 28, 2010 | 2001 GMC Sonoma Extended Cab

1 Answer

REAR HEATER PROBLEM


your lines going to the back may be plugged up and the antifreeze is not able to flow through the rear heating system. disconnect your cooling lines at the back and see if there is coolant in both sending and return line hook a garden hose to the lines see if you can flush out the whole heater system in the back

Jan 07, 2010 | 1999 GMC Suburban

4 Answers

The heater still won't work after I changed the thermostat on my


Get a female garden hose adapter from the local hardware store. Disconnect the heater hose from behind the fan pulley and attach the hose connection. Disconnect the other heater hose behind the alternator, aim it towards the ground and flush water through the heater core. This worked on my truck.

Feb 12, 2009 | 1999 GMC Sonoma

3 Answers

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Considering the age of the car, your heater core could be partially stopped up. Check the coolant in your radiator and see if it is a rusty color. Flushing the coolant system might help get your heat flowing again.

Jan 01, 2009 | 1995 GMC Sonoma

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