Heater Problems

I just purchased a 1998 GMC JIMMY with 115000kms on it and when I turn on the windsheild defroster there is barely any heat comming out even after I have driven several kms. The fan motor works fine but the air is barely warm
comming out. Does anyone have any idea what could be the cause. All other cars I have owned the air temperature coming out is hot. This truck's air temperature coming thru the vents is mildly warm. Thermostat seems fine as the temp gage on truck shows normal. Any suggestions?
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Plugged heater core of faulty heater bypass valve.
Doc

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I had cool air coming from my truck when it showed hot once. Low coolant.... 1990 GMC f/s Jimmy 350.
GMC Gremlin

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Thanks guys After looking at this problem under the hood...I noticed the A/C compressor turns on when heater control is in defrost position. When I turn knob to dash vents the heat is fine. When I turn knob to A/C positions nice cold air blows out. So what can cause compressor to turn on when I have the heater control knob set to defrost?

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Thank for the information DOC. I know I did not ask the question,but always wondered about the compressor kicking on while the defroster is on. I noticed this in my last 3 Chevy Trucks. Makes sense to me.
Thanks Doug
Learn something everyday and this group is the place to do just that.I read this group everyday and always learn something .
I had have asked a couple questions and received the correct solutions each time.
Thank You everyone for your time and knowledge.

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That's the way it works. The AC compressor comes on to cool the air AND dehydrate it. If you move the temperature lever to the mid point it will defrost the windshield very quickly.
Al
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Did I mention anything about defrost? Because I sure didn't mean on defrost. Heat & vent it was cold. Add a couple canteens of water and hot air all the way home.
GMC Gremlin
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I had a similar problem with my 98 blazer when we first bought it. Noticed it about 2 weeks after we bought it. Dealership said it was prob a bad heater core and offered to replace it for $500. We told them no way since we just bought it. After they said they would eat half the cost, I ended up taking it to a radiator shop who checked it and hooked a hose up to the firewall (port for the heater core I am assuming) and flushed it out. A big blob of crud came out and then after he hooked all the hoses back up, the heat worked fantastic! And they only charged me like $30 I think. Well worth it in my opinion and I kept their business card in my wallet while I still lived in that town to take any future radiator problems to them.
Dave

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Sam wrote:

Like Doc says, plugged heater core. Very common on the Jimmy/Blazer line, as they have the 4.3 engine that was the worst engine when it comes to the Dexcool coolant. If the coolant level is allowed to run low, the dexcool will sludge up in this engine (cast iron block) and it usually ends up plugging up the heater core. We usually will back flush the heater core and change the Dexcool. In most cases, this restores your heat.
Ian
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Hi thanks for your comments, just wondering if you would be able to tell me how to go about back-flushing the heater core. Also if it is a plugged heater core , why do I get good heat when I turn knob to dash vents?

ends
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Sam wrote:

Well, unfortunately, none of us on here happen to be mind readers. So when your original post says:

then I give you a diagnosis based on what you tell me. Then you change the story....which throws the diagnosis out the window. Which is why it's so important that a customer make sure they relay accurate information as to what the truck is doing. Mind you, once we get it in our bay, we can usually figure out if/what the problem is without the customer input. But that's not possible on the internet.
Ian
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In order to flush the heater core, you need to find the two hoses that run into the firewall. You should see 4 or 5 relays left of it that are mounted on the firewall. Take them both off, just run a garden hose through it, who cares if the shit falls on the engine, you can wash it off.

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