Alright I just replaced all my gaskets on my intake manifold a week or so ago and the other night I was running someone and after we ran my car started leaking coolant again around the manifold, well after awhile it quit and I drove it about 25 mins home and it wasn't leaking anymore.

Well now my car still leaks sometimes but it only starts to leak when I get my RPMs up to around 4,000. What could be causing my gaskets to go out so soon and easily, also why does it only leak when it gets into the higher RPMs?

 

did you visually inspect the intake itself for damage? the plastic intakes are known to crack and leak. where are you seeing coolant at? back near heater hose or front near thermostat? Those are the most common crack spots. try pressure testing the system and visually looking for the seeping coolant. asking us wont get you far since we cant see the car

 

The coolant is leaking under the heater hose. Yes I have looked at it for cracks, its leaking the same place it was last time when I replaced the gaskets. I've even taking it to a local ford dealership where I know I guy that is a mechanic there, he even looked at it for me. He said it leaking around my gaskets, he took a mirror and look where it was coming out at. I'm just trying to figure out why my gaskets aren't holding up like they are suppose to, and what could be causing it.

 

under the intake where the gaskets rest, is all composite, over time it deteriorates and cracks. brand new gasket or old gasket they fail to seal. i've never done just intake gaskets on a 4.6 if it had a coolant leak....always is the intake itself

 

When I was doing a coolant flush I had the thermostat replaced and I'm guessing the stress of having a new gasket (thicker when new) where the coolant inlet meets the thermostat was enough to fracture the inlet and I started leaking coolant. Ford made a mistake of not making intake manifolds with metal inlets and instead used plastic. Over time with heat and cold it breaks down the plastic and will lead to cracking.

There was a recall some years later. The new intakes have a metal inlet.