4.3L blowing hot and cold

[dubbs]

I have a 2000 half ton with a 4.3L. Over the past few weeks the heater has been blowing colder and colder. Now its just luke warm. The temp outside has been -8 to -25c. I have just put a new thermostat in it and still no heat. The engine temp gauge goes to just below half way and will move up and down when you drive. I can get some heat to come out when I drive it hard and only when the rpm's are up. The truck still has the orginal dex-col in it with 86 000km's on it. It seem like the heater core is pluged, or maybe the water pump is not moving the coolant through the system. Is there any tests I can do or anything you would sugest trying. On a side note, how does the dex-col become bad and what does it do to the engine? Thanks for your time.

[asepgrad98]

have you changed the cabin air filter within the last 15k miles?

[02 Silverado Z71]

dubbs,

 

You may be correct on the water pump. Do this...

 

1. Drain all coolant. Use the engine block plug to completely drain the block.

2. Remove water pump

3. Flush the hell outta everything, and let drain

4. Install a NEW water pump

5. Install fresh mixture of DexCool and clean water

 

Your truck should then cook you to a "well-done" temperature!

 

But seriously, your desription of the problem leads me to believe that your water pump is failing.

[Bpot]

Before you do all that, get your truck up to operating temperature then feel the top heater hose going into the heater core and feel the bottom one. If one is hot and the other is cold you have a restriction. Best bet is to remove hoses and backflush the heater core. If they are both just warm you have a flow problem and most likely the water pump and flush job.

[dubbs]

I have checked the heater hoses. The inlet side is hot and the outlet is cold. This leads me to think that the heater core is pluged or the valve that controls the flow of coolant is broken. Does anyone know where or how the valve works. There is no A/C on this truck so there is no cabin filter. I can't see any valve so i guess its on the inside of the firewall. Thanks for the quick answers.

[dubbs]

Well I fixed my truck last night. Took it in the shop and i flushed the heater core and cooling system. The heater core was pluged. I found small bits of brown crap and scale and also some bigger plastic parts. Put it all back together and it works good as new. The only thing i worry about is where the plastic came from. Does any one know if they are running a plastic impeller on the water pump?