got a 2004 GMC sierra with a 5.3 vortec. been giving me some heating issues. it will get up to temperature but will not blow heat in the cab unless i give it throttle. today it acted up and went up to 240 degree'* so i turned my heat on and gave it a little throttle so the coolant would flow. its using coolant but there is no noticeable leak. can anyone give me a idea whats going on or if you have a this problem?

A few thoughts. One is a lower intake manifold gasket has failed. You may also have a bad water pump and possibly a semi clogged heater core, you may have all three problems.

A coolant pressure tester is what I would do next. If it doesn't hold pressure, likely a LIM gasket or water pump. The heater core may be semi clogged as a result of a LIM gasket failure, make sure you clean it out if you repalce the LIM Gasket.

ive put a pressure tester on it and it does hold pressure.

You are losing coolant. You do not mention finding it on the ground, so I am guessing the water pump is not weaping coolant. I would take a pinpoint flashlight and check for signs of coolant around the intake manifold at night. I had the same exact symptom (not blowing warm enough heat) you mentioned on 3 vortec 4.3L engines, and each time it was a LIM gasket failure. I also had to clean out the heater core. I found that if I cleaned out the heeater core prior to the LIM gasket replacement, it would temporarirly fix the heat problem, for about a week or so.

What does surprise me is the temperature you are reporting. Normally a LIM failure in a vortec engine results in lack of pressure buildup, and keep the engine from getting above 180 degrees in cold weather.

BTW, cleaning out the heater core is pretty easy on many RWD GM vehicles. I have clean mine at the car wash, as I had the problem while traveling for work.

I simply go to car wash, relieve coolant pressure, and and remove coolant hoses going into and out of firewall. I insert the pressure hose, and force water through the core openings at the firewall at low pressure. I typically see some gunk flowing through, and then see the clear water. I then put it all back together, make sure no air is in the colling system and enjoy the heated drive.

You may also have a leaking heater core? Do you smell coolant in the interior or see any signes of coolant in the interior?

i do not find any leaking on the ground. or see any leaking. just noticed the coolant tank is low everytime i look at it. the vehicle has no problem heating up it just doesnt blow heat. its heating up faster then it has since ive owned it. bout it in august and havnt had no problems with it untill this heating issue came up.

Have you done a coolant flush?

no i have not.

A lot of the trucks have issues with the temp actuator, stick your head under and see if it changes when you move the temp

The 5.3L is a LS1 based engine so it has the dry intake manifold, so coolant wouldn't be leaking there. Sounds like the heater core is getting plugged up. With the truck up to operating temp feel both heater hoses, they should both be hot and the same temperature. If one is hot and one is cold you have a plugged heater core and it needs to be flushed out.

The waterpump gaskets are common to leak on these engines also, a lot of times they don't leak very fast so when the engine is hot the coolant will just evaporate on the hot engine block before it hits the ground and leave leave a stain under the waterpump gaskets.