vacuum problem

i have a 91 4.0 aw4, 242, regeared to 4.56, 35x12.5 mud tires. here is my problem.... my heater control(the one that selects defrost/heat/bi-level/etc.) will only blow out of the window defroster even if i select something else, and my cruise control acts weird too, when i set the cruise and take my foot off the gas i decelerate for a while before the cruise kicks in. also, i have a vac gauge installed and i noticed that i only have these problems with highway speeds where i am running very low vac(like 5 on the gauge). so if i'm driving into the slightest bit of a wind then i have barely any vac and my heater selecter and cruise control will not work properly. any suggestions?

Sounds like you may have a vacuum leak. Check your vacuum canister and the hard plastic line that leads to it. The line passes under the battery tray and can get destroyed by acid.

i relocated my canister. i searched under the hood with a stethiscope and am almost certain there is no leak there.

also, i replaced the canister with a vac canister off an older f150. its bigger and shaped a bit different but i assume that makes no difference?

Good way to tell is to pull the hose out of the canister after the engine has been running for a while. If you hear vacuum escaping, you're good. If not, you have a leak.

you need to get 14+ hg inches of vacuum pressure at idle (it will have to be warmed up)

look for rotted connectors, loose lines, etc. put the stock canister back on and see if that changes things (it might)

at idle i get about 16-18, its at highway speeds that the problem occurs

Well that's weird. Vacuum will drop a little when throttle is opened (air can flow more easily so there is naturally going to be less negative pressure in the intake) but it shouldn't bottom out. Maybe a leak is developing after the engine enters closed loop? Do you have a Renix with EGR (bad solenoid maybe)?

You could test pressure at 2000 RPM cold and warm in your driveway to confirm.

not a renix. also, when i accelerate(not too hard) i keep a good amount of vacuum in all gears until the torque converter locks up at around 50mph, then i have like 2-4 vac until i reach cruising speed then it returns to about 8-9 and if i-m going into the wind a bit i'm sitting at around 5. and if i accelerate to pass my vac drops again unless it downshifts.

the TC lockup drop is normal (RPMs drop which results in less vacuum created, yet throttle is still open, so air is flowing more easily, resulting in much less vacuum) but it shouldn't go quite that low. try with the factory canister--you may have too much volume to build sufficient negative pressure for reserves