I have a 92 F150 with a 5.8. My air conditioner won't stay blowing out of the vents it switches to the defrost vents. I can make it come back to the vents by downshifting or raising the engine rpm. I found the air solenoid that controls the air flow to the vents and the line that hooks to it . It seems to have very low vacuum pressure. I traced that line out under the hood and then to a block in the intake manifold. After that I am lost as to where to look for the problem.Any help would be appreciated...

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1992 F150 SC 4WD,5.8L,E40d, 180k miles

 

Check for proper engine vacuum, then see if it is the same at the hose ending at vacuum diaphragm (if not, bad hose), if the same vacuum both places, check vacuum diaphragm for leaks. If you don't have a vacuum pump to properly check this, then attach a vacuum hose to the vacuum diaphragm and suck on it to create vacuum and see if it holds in the open position (mmm tasty). If you just have low vacuum at engine, you will have to check all your engine vacuum system, and possibly intake & carb gaskets, and worst case scenario internal engine problems. Hope this helps, good luck.

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2000 F-150 XL,RC,LB,5.4,4R70W,3.55LS,
4-wheel disc/ABS,Chestnut/Parchment,
Ford bedliner & gas/wheel/spare locks,
3" cold air box modification,Superchip,
Dynomax ultra-flow welded 3" cat-back.