Chevrolet Repair: 2000 chevy van lukewarm heat, chevy venture van, coolant level

Question
I have a 2000 chevy venture van that has a 3.4L engine, the issue is low heat or lukewarm heat, the dealer told me it was a external coolant leak, I think he crazy since i have seen nothing on the floor and the coolant level in right in the same spot and hasn't ever moved. So I was thinking the t-stat was stuck open and I change with a new one and the car now at idle warms up to about 3/4 on the temp gauge and then the fans kick on t-stat opens and cools down to 1/4 on the gauge and back and fourth in goes... But the heat got slightly better but still not were it should be i think, not cooking you out of the van.  I have checked for flow of water pump which is good, the temps at the hoses and heater core lines are at 225-250.  so the last thing i was thinking was the heater control switch itself maybe bad though it does change cool to warmer now and lastly maybe a damper door not allowing enough flow of air???? well any advice would be great thank you so much. Adam

Answer
The first thing to check is your coolant level and you said's full so that is ok. I know intake manifold leaks are common with the 3.4L. I'd like you to first inspect the oil. Does the oil look normal or is it have a milky composition to it? If it's milky or chocolate milk looking it may have a leaking intake manifold. Also pull out the PCV valve in the left valve cover and inspect it's underside. If it's gooey and milky that's another sign of a leaking intake.

Ok, assuming that was all ok the only way to test accurately test the thermostat is to drive the vehicle in the cold while monitoring a scanner on coolant temperature. I doubt you have a scanner. You use your temp gauge on the dash but it will not be as accurate. if the coolant temp stays close to 195 degrees we know the engine is hot enough to supply good heat.

Assuming your at a good engine temperature...let the vehicle cool down or do this before you drive it. Remove the radiator cap and inspect it. If it is corroded or rusted you may have contaminated coolant. I have seen this many times.
You can try disconnecting the heater core hose and backflush with a garden hose. If it's real bad you can try Prestone rust and scale remover. It's an acid base cleaner that will clear out any clogs or low flow through the heater core. Follow the instruction to the tee. Refill with fresh new coolant. Buy the premixed coolant, it's better.

Last it is possible that the cold/hot mix door is not closing fully but I haven't seen that too much. I have seen many heater cores plug up using the DExcool. I prefer the new Prestone 50/50 extended life coolant. It works in all makes and models of any vehicle.

Hope this helps.