Heater blows cold air at driving speeds....
 

I checked coolant level in radiator last night after driving home with cold air blowing, coolant seems full close to top of radiator. Normally you would have to wait for coolant to cool down before opening radiator cap. radiator felt warm, so i eased the cap off waiting for some to gush out from pressure, and nothing.....coolant is just sitting there few inches from the top of radiator cap. When sitting still at idle heat blows fairly warm, but at driving speeds merges to cold air. Recently changed water pump, and thermostat, heater core was put in bout 2 years ago, no coolant on passenger side floor, Im thinking thermostat issue, but top hose from engine to radiator is warm when I check it after driving. Will flush system today when I get home and refill.

Re: Heater blows cold air at driving speeds....
 

are you sure that the cables and doors to the air box are working correctly?

Re: Heater blows cold air at driving speeds....
 

Get a cheap infrared thermometer and take the temp of the radiator nipple where the upper hose attaches. Should be low until the thermostat opens, then go immediately to the thermostat temp.

Sounds like you are never getting up to temp. I need to put a cover over the grill in the winter to get up to temp so I have heat.

Here's what I mean for a temp meter:
http://www.harborfreight.com/catalog...er+thermometer

Re: Heater blows cold air at driving speeds....
 

As long as the thermostat is working, there should be no problem getting heat. I have a big block 4 core radiator and I get heat within about 10 minutes in 20 degree weather. With the thermostat closed, water should still flow through the heater core even while NOT flowing through the radiator.

Some thermostats have about 1/8th inch bypass holes and these can cause heating issues. So can low temp thermostats.

Pictures can help too if aftermarket parts are present.

Re: Heater blows cold air at driving speeds....
 

The thermostats flow a little bit even when nominally closed. That 20-30-40 degree water coming into the engine at the bottom, even at reduced flow rate, really sucks the heat. I have a 195 thermostat in mine, and even so I block off the airflow to the radiator core in the winter.

I was surprised that the cover doesn't have to extend across the whole grille. I thought the air would just flow around the corner, but it doesn't. When the electric fan shuts off, the temp comes up even when I'm moving.

So, electric fan helps, because it turns off rather than run continuously, and then I just used a fender pad from Classic Parts to block the direct airflow.

http://users.rcn.com/weyand/smalls/imag0435small.jpg

Re: Heater blows cold air at driving speeds....
 

Check your mixture what ratio water to antifreeze are you running.
I too use a cover in the winter,sure works well for me.

Re: Heater blows cold air at driving speeds....
 

Verify the heat/cold blend gate is working. You can view/adjust it by pulling the glove box.

Make sure you have a 195 thermostat. A lot of folks put a 160 in trying to fix an overheat problem and forget to change back once the real problem is fixed. The PO on my truck did that. The heater never did get very warm until I put in a 195.

Quality control on thermostats today seems poor. I have had more than one fail in a few weeks. Often the real culprit for an overheat issue.

Even hot, my radiator rarely has much pressure on it.

Re: Heater blows cold air at driving speeds....
 

Came out to be stuck open thermostat which I thought but doubted at the same time since I just recently changed it out with the water pump, it was a basic brand from orielly auto parts, 195F, heat blows normal now...

Thanx for helpful tips and advice anyway gm truck comrades..