Heat not working - 2003 GMC Sonoma 4x4 Crew Cab

John Pollman

Member
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1,332
Location
Rochester Hills, MI
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!
 
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-want-replace-thermostat-2003-gmc-sonoma-zr2.html
 
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
 
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!
 
OMG Darrin!

I watched part one and abour 1/3 of part two. I couldn't bear to watch anymore! :rofl:
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.
 
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).
 
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.
 
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!
 
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.
 
Before you do to much make sure there there is no way to shut off flow. Cable or vacuum servo valve. Install a section of clear hose in line with your heater hose to check flow or just open outlet into a bucket..
I have known several Chevy owners who replaced cores only to not fix the problem... The air diverter flap is another good candidate if you have actual flow of coolant thru core...

And I think the power flush and fill are in general real bad ideas.. Great money makers for the shops though which is why so many of the 10 minute oil change places have jumped on the band wagon. Power flush just tends to push the junk into smaller places where it gets trapped or blow out a weakened radiator or core. Garden hose flush ought to be sufficient or something more than a flush is needed..

You mentioned the inlet hose was warm. The inlet hose should be pretty dang hot near 180-200 degrees. Real good trouble shooting aid is these 29.00 infra red thermometers you have to learn to think within their cone of sensitivity. But you can check temperature gradients along the coolant flow path.
 
Heck yeah, it's been COLD and I hate driving a truck w/o heat. Not to mention the defroster doesn't work very well either when it's blowing air just a few degrees about the ambient air temp in the cab. :)
 
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