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Thread: Heat not working - 2003 GMC Sonoma 4x4 Crew Cab

  1. #1
    Join Date
    Nov 2006
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    Rochester Hills, MI
    Posts
    940

    Question Heat not working - 2003 GMC Sonoma 4x4 Crew Cab

    Hi folks,
    It's Sunday morning so I can't do much today but I have a pretty good idea as to why the heater in my truck isn't working. I'm pretty sure that there's a blockage in the heater core. Here's the story.

    Back on Memorial Day weekend 2012, I came out to get in the truck and go somewhere. I noticed a puddle of coolant under the truck. I found that there was a pinhole leak in the radiator. I picked up a new radiator and installed it. Being May, I knew there wasn't any chance of freeze up so instead of filling the new radiator with antifreeze, I just topped it off with water. I planned on running it that way for a week or so to make sure that there were no other leaks. Everything was fine and no leaks.

    I think it was probably August I took it in and had a complete coolant flush and fill. I noticed in November that the heater didn't seem to be putting out as much as normal. Then I noticed that whenever I was stopped at idle, the thing blew fairly cold air. I figured that it was low on coolant but I checked and it was a little low but not too much so I topped it off with some 50/50 I bought.

    It seems to be getting worse and about a week ago, I pulled the radiator cap off when it was cold and the radiator is full. The reservoir is full too. I started it up and ran it up to operating temperature and let it run for at least an hour thinking that maybe there was an air bubble trapped in the system. I would think that would have allowed it to work its way out.

    Yesterday driving down the road, I noticed that the heater really wasn't putting out much at all even while driving at speed. I pulled over and popped the hood. I reached in and felt the heater hoses going into the firewall. One was warm and the other was pretty cold. This tells me that there is probably a blockage in the heater core.

    I really don't want to spend $130 for another flush and fill. I'm going to talk to the shop that did it in the morning and see if there's anything they can do. I might be nuts, but here's my thought. Is there anyway that they may be able to hook their flushing equipment to just the heater core and flush only that portion of the system? It seems like they should be able to because there's just an inlet and outlet hose on the firewall.

    I'm not completely familiar with how they do the flush and fill when it's done, but I thought that they tapped into the heater hoses to do the procedure.

    Does this sound like something that could be done? I would think it would be a lot cheaper than doing a whole system flush/fill.

    Thanks for any input!
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten"

  2. #2
    Join Date
    Jan 2010
    Location
    Victoria BC
    Posts
    87
    Sounds to me like a thermostat problem. It's the easiest and cheapest to deal with and could be what's wrong. If it is stuck open the coolant isn't getting a chance to warm properly, if stuck closed you over heat. Get a new one and stick it in. Take your old one with you so that you get one with the proper heat range for your climate.

    Some info here but a google search should tell you where it is on your particular vehicle if you don't know.

    http://www.justanswer.com/gm/2znwr-w...onoma-zr2.html

  3. #3
    Join Date
    May 2007
    Location
    Kansas City, Missouri
    Posts
    13,448
    I'd probably pull the two heater core hoses and flush with a garden hose reverse of the normal flow, so push water in the side that was cold. Not sure if they make a chemical you can pour in and let it work, may increase the likelihood flushing will work.

    After watching this I can see why one would not just replace it...I feel for you.
    http://www.youtube.com/watch?v=WUa7-d_NeZA
    Darren

    To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault

  4. #4
    Join Date
    Nov 2006
    Location
    Rochester Hills, MI
    Posts
    940
    I'm pretty sure that the stat is fine Brian. The temp gauge shows that the engine is coming up to normal operating temp (just under 210) fairly quickly. If the stat were stuck closed, the truck would overheat. If it were stuck open, it would work but would take a lot longer to get up to normal operating temp. A friend just suggested reversing the hoses on the heater core. This would be a quick and easy check. It may dislodge the blockage and get it working. I might give it a try. But if it does, think another flush and fill would be in order to clean the system again.

    Thanks for the input!
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten"

  5. #5
    Join Date
    Dec 2006
    Location
    Oliver Springs, TN
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    1,726
    I'm with Brian, I'd start with the thermostat first. If you replace it just remember to put the new one in before you put the cover back on!

  6. #6
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    Nov 2006
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    Rochester Hills, MI
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    OMG Darrin!

    I watched part one and abour 1/3 of part two. I couldn't bear to watch anymore!
    I'm PRAYING that a back flush works! Actually it's NOT leaking (thank God), it just appears to have a blockage.

    Keeping fingers crossed.
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten"

  7. #7
    Join Date
    May 2007
    Location
    Kansas City, Missouri
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    13,448
    Quote Originally Posted by John Pollman View Post
    I'm pretty sure that the stat is fine Brian. The temp gauge shows that the engine is coming up to normal operating temp (just under 210) fairly quickly. If the stat were stuck closed, the truck would overheat. If it were stuck open, it would work but would take a lot longer to get up to normal operating temp. A friend just suggested reversing the hoses on the heater core. This would be a quick and easy check. It may dislodge the blockage and get it working. I might give it a try. But if it does, think another flush and fill would be in order to clean the system again.

    Thanks for the input!
    I'm thinking the thermostat should be working if one side is hot. Reversing the hoses sounds promising since you'll have heated fluid pushing the crud out, but would hate to see all that go back in to the radiator. Just doing some googling, it looked like there are some chems for using to flush a core (same stuff as the flush kits you buy).
    Darren

    To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault

  8. #8
    Join Date
    Dec 2007
    Location
    Tellico Plains, Tennessee
    Posts
    4,352
    I don't know if the newer trucks and cars still work this way, but back in 1963 I had a '59 impala. On a trip home for Christmas I ran into a snow storm in Williams, AZ, stopped for the night, next morning my radiator was slushy... not enough antifreeze and I overheated about 5 miles out east of Williams.... cooled the car down, went back to town and had a service station look at it... they added more antifreeze.... at same time I noticed no heat in the car and asked them to look it over... they messed with it for over an hour and couldn't find a thing wrong.. so I put on another pair of levi's and a second pair of socks and headed for Texas.... when I got home, mentioned to my brother in law about no heat. Being a mechanic, he grabbed a screw driver, lay across the front seat and stuck he head under the dash... he tightened a screw clamp on the heater cable under the dash. It cost me $100 in Arizona and no heat.... he fixed it in 15 minutes.

    If your heater works on a cable slide, check to make sure it's tight and moving the cable. May not be the way they work now... more likely the doors open and close on servo motors this day and age.
    Chuck
    Tellico Plains, TN
    https://www.etsy.com/shop/TellicoTurnings
    My parents taught me to respect my elders, but it's getting harder and harder to find any.
    If you go looking for trouble, it will usually find you.

  9. #9
    Heater core replacement took me about 4 hours on my 1999 GMC Jimmy with 4.3 engine. The dash tilts like it on hinges you just need to unbolt the steering column and let it sit on the seat to let the dash tilt all the way.

    On mine there was an access port right where his heater resistors are located. Nothing about that in the service manual as they had you evacuating the a/c and removing the fender to remove that whole assembly just to get to that one stinkin' bolt. Little voice in my head said to cut out the access port and there was that bolt!

  10. #10
    Join Date
    Nov 2006
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    Quote Originally Posted by Darren Wright View Post
    I'm thinking the thermostat should be working if one side is hot. Reversing the hoses sounds promising since you'll have heated fluid pushing the crud out, but would hate to see all that go back in to the radiator. Just doing some googling, it looked like there are some chems for using to flush a core (same stuff as the flush kits you buy).
    That's what I'm thinking Darren. If I try it and it works, I'll probably take it back in and have it flushed again to try to clear out the crud.
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten"

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