Hi Folks,
I have a 03 Venture with about 60k miles.
The past few days the heater has been blowing cold air while at idle. I have read on forums and websites from low coolant to engine rebuild...
I checked the coolant and yes it was low but the reservior was not out and the engine was not and has not been overheating. I filled reservior and did not see a improvement. If I start engine and let it sit at idle at stop light or in morning to warm with defrost on it will blow ice cold, unless I am in vehicle revving motor. If I sit in it and watch the temp gauge and rev engine it will heat up to just over half and the thermostat will open and you will start to feel warm air but if I let off while it is cooling back down it will start to blow cold again.

Thank you for any feed back and/or suggestions and Merry Christmas!

 

-Edit-
Check if your baffles "Cold/Hot" temp control is vaccum operated. If so, you may have a vaccum leak. you'd notice subtle stumbling in the engine too I believe.
If you have a vaccum leak then fixing that should not only make the vehicle run better, but fix the heat as well.
End Edit

I'd also look for coolant leaks.

If there aren't any, then I'd still be looking at the thermostat.
The water temp gauge only measures the temp of the water on the engine side of the thermostat (on most models).
If the thermostat is sticking, then you could have a "False" reading on your temp gauge.

You can actually test the thermostat by taking it out of your car, and "Boiling" it in a pot of water. There is a stamp on the end of it indicating the temperature it should open at.
With a thermometer in the pot of water, boil it. when it opens, note the temp. compare to the "Stamped" rating. if it is within 1-2 degrees, it's good.

If the thermostat is good, THEN I'd start thinking about the water pump as you said it's fine while "Revved" but when you let off the gas, it gets cold again. It might be worn out, and only pumping at higher RPM's

Other experts?
(I love being wrong.. cuz then I learn too.)

Aces.

 

With the engine warmed up feel the heater hoses they should both be warm. If they are its a control problem as stated above. If there is a big difference the core may be plugged.

 

So about 8 months ago the check engine light came on and it started idling rough...i took it to trusty auto zone and they said the light was caused by cylinder #2, the OEM plugs are suppose to be 100k plugs but I bought a few anyhow and pulled #2, it actually looked great, very little carbon or build up. I checked the wire, nothing appeared wrong. I took it in to shop and had them basic service and also replace fuel filter, but still missing, but it was not constant, light would come on and off, so I thought maybe bad gas so i bought some fuel system cleaner and miss was gone and check engine off was off within 15 minutes and did not come back....until about 4 months later, added fuel system cleaner first and again it worked BUT this time it was cylinder # 5...and wouldn't you know it started again about a week ago, a few days before this heat issue.
Related or just very coincidental?

 

Personally I would take the time & flush out & back flush the heater core. When water pump has to lift coolant to upper levels like these SUV's, any restriction in the core, can cause flow problems, that's a lot of the reason that it feels o.k. going down the highway, but cooler at idle. More RPM's = more & stronger flow through heater core. I removed my heater hoses, attached 2 pcs. of garden hose & a drill operated pump to the fittings at firewall, and circulated CLR through the core. Then plugged one end & poured CLR in until full, and let it sit in there overnight. Flushed it out next day, both directions, and hooked everything back up, bled out the air, top up coolant. Difference was very noticeable, and all was well. Cost approx $20. If no small pump available, gravity feed it, but take some time to let it sit in there & work a bit.

 

I grabbed water pump, antifreeze and thermostat from NAPA and started to tear down and found the intake manifold leaking and appears to have been for a while. I have called around for estimates and it is around 2 days and $700 to $1k to repair. I called 4 repair shops and everyone of them said it is very common and they do a few every month, one place said that the factory used a plastic gasket and before I told him my mileage he guessed what it was... ...I can do somethings but I feel with my lack of tools and experience this would be best suited for the pros...anyone know a good mechanic in Salt Lake City, Utah?