Heater problems, NOT MUCH HEAT!!

I'm new to this forum but not new to vehicles. I live in Minnesnowda and today it is -15F. Well the heater in my 2004 venture does not work very good. I have owned this van since this past August and I can assume that this has always been a problem but went unknown until this deep cold. So far, I have replaced the waterpump, t-stat, radiator cap, and even the heater core (front). The van runs at a very consistant temp 180(using a IR temp guage on the hoses). If I use vent heat with recirculate air, it will heat okey, better the longer it's on, but to keep the windows clear I have to use defrost then it is little to no heat. I had the lower heater core panel off with a new heater core and van up to temp and running and found that the core will be very hot on the inlet half but very cold by the outlet area of the core (with the blower running). If I was to rev the engine during this test it would improve. I suspect that I need more coolant flow to heat the cold air better. But I don't understand why? Can anyone shed some light on this for me, It's just too cold to handle anymore.

Does a high flow of air come out different vents when you change the direction (with the heat and fan on high)?

If not, the blend door that diverts the heat and A/c may be at fault.

I had to replace the electric blend door motor in my 2000 Silo due to a similar problem.

Yes, all the blend doors are working correctly and the blower blows strong air just cold.

Yes, all the blend doors are working correctly and the blower blows strong air just cold.

Hmmm, are the fresh air vents clean and have the cabin filters been checked?

I mean the air vents under the plastic cowling below the windshield.

The cabin filters (of which there are two) are accessed via the glove box.
The first is easily seen and removed and the second is behind a small door on the left side.

The heating system won't circulate fresh air if it can't suck it in from outside.

The A/C works on the same principle.

Forgot to mention this: I changed out the 180 deg. t-stat and replaced it with a 195 deg one and found quite a difference.

I have replaced the cabin air filters, the t-stat is a 195, and the cap is a 16 lb. This past weekend I tested the coolant for CO deposits and that was negative. It almost seems that I do not have enough coolant flow in the heater cores (front and rear)?

When the engine is up to the operating temp and the heater is on, are BOTH hoses hot? They're supposed to be.

...found that the core will be very hot on the inlet half but very cold by the outlet area of the core (with the blower running). .... It almost seems that I do not have enough coolant flow in the heater cores (front and rear)?
It takes so little water (coolant) compared to air for a heat exchange that normally, one would barely see the difference in coolant temperature.
An air flow of 200 cfm heated from -5°C to near 40°C (104°F) would lower a 3gpm coolant flow by only 5°C.. (from 95 to 90); obviously, you do not have a normal coolant flow...better said, you barely have a flow at all.

SO...either a blockage of some sort exists in the heater circuit, bent hose, or another factor is acting; you have obviously considered the option of blown head gasket (since you tested CO) as loss of heating would be a normal and known consequence (exhaust in the coolant is a very poor heat carrier and impairs the ability of the pump to circulate fluid)...but other signs would normally show though. DO you have to add coolant from time to time ?
There has been a reported case of blockage of a fixture near the base of the air intake...I think I have a picture... www.avigex.ca/xport/heaterinletpipe.jpg
it was a 3.1, but basically same block as 3.4 ..maybe the setup is alike... was the part labelled as #1 I think.... too bad there is such little place for disconnecting hoses and look for real .