Overheating, not building pressure.

[dvom]

I just put together a stock 2.7t now 100 miles later it is starting to over heat. I noticed even on the first startup the cooling system had no pressure opening the cap when warm. So I bought a new cap from the dealer. I have only recently correctly bled it from the heater core and engine pipe and had no change, I had the front wheels up in the air and raised the tank to be sure. I flushed the heater core as best I could and what came out was clean.

The fan clutch is good, the electric fans only run with the engine off, is this correct? Radiator is fairly new, I did not replace the waterpump or t stat, but looks like the next step. Possible blockage or headgasket, anything else im missing? Has anyone seen a car not building any pressure?

[christianb5s4]

The fans running with the engine off is not correct, I'm willing to bet something is off with your fan relay.

[axnjaksn]

did you bleed it from the 5 or 6mm hex right next to the power steering pump??

[S4tech2]

I would bet the water pump isn't flowing enough coolant. If the t stat was stuck closed it would be too much pressure

[slowSfaux]

Or let's just address the obvious... If there is no pressure in the overflow tank when warm you have a large leak somewhere. Unless I misunderstood your post, you need to deal with that before you worry about what your fans are doing.

[dvom]

Yes I have to start with the pressure issue, there are no leaks, it has been on the lift and it looks great. I thought I may have a crack in the reservoir or the cap/o ring sealing surface is bad, but I just put a pressure tester on the reservoir and it held 15 psi great for 1 minute.

The pressure in the system is created by the expansion of coolant from heat, not the water pump IIRC. The car has been good on 10-15 mile trips and has only been overheating at idle(which would tell me the water pump is ok), but fans or no fans I need pressure.

[codeopp1990]

Sounds like the fans to me. They should run whenever you get over a certain temperature and should stay on with car on or off until it drops to an acceptable level. Would also explain why it only overheats at idle when no air is moving over radiator.

[christianb5s4]

Sounds like the fans to me. They should run whenever you get over a certain temperature and should stay on with car on or off until it drops to an acceptable level. Would also explain why it only overheats at idle when no air is moving over radiator.

That's what I was thinking as well. Plus, the fans should not turn on when the car is off.

[slowSfaux]

That's what I was thinking as well. Plus, the fans should not turn on when the car is off.

That is the purpose of the afterrun pump. The electric waterpump under the intake manifold and the fans run for a few minutes after shutoff if the temperature sensor (dedicated sensor just for the afterrun system, I believe it's on the back of the motor in the coolant crossover pipepipe on the driver side) reaches a set temp. The oe sensor will only engage the pump at really high temperatures, so the system almost never runs unless you're overheating, or the sensor could have been swapped with the lower temp option like many here have done.

No matter what the cause of the fans not working is, the problem of not building pressure is going to remain, and even with running fans you'll still overheat. The cooling system uses pressure to raise the boiling point of the liquid in the cooling system. Boiling coolant can't carry any additional heat, so fans running or not, if your system can't build pressure you will overheat under "normal" operating conditions.