Okay I thought I had a vacuum leak on my 2003 F150 5.4, but when I did the smoke test I got nothing. Did the starter fluid spray also thinking maybe it was a intake manifold gasket but had no change in the engine at all. Throwing that P0171 Code leads me to believe that it is only on the bank 1 side of the engine but if there is a vacuum leak I sure has hell can't find it. I am leaning towards the intake manifold but sure would appreciate any input that you all may have.

 

If you suspect an intake manifold gasket leak from the inside of the manifold mounting, sometimes a shot of carb and choke cleaner sprayed into the oil fill hole will allow that to get sucked into the intake and change engine RPM. PCV valve / hose has to be blocked so it can't get in that way. Sneaky, but it works.

 

So I just give it a quick spray into the oil fill and if it changes the RPM then that verifies that it is the intake manifold? Just pulling the PCV out would be ok then blocking off the opening with a piece of tape or something?