So here's a weird issue I've been trying to troubleshoot. Ive been noticing that if I'm at a stop, and I let off the brake and slowly get on the gas, my truck has a bit of hesitation. It didn't appear to be fuel or ignition related, but I've been looking at everything. Problem is it isn't consistent, some times it is fine, sometimes it isn't. Even on the same trip, I'm start driving and it is fine, then 20 minutes in it's doing it.

I finally noticed last night what it is doing. At idle I get around 12-15" of vacuum. Well in first as I get going I don't make any boost at part throttle, vacuum will end up going to 0, it shifts to 2nd and boost starts and all is good. Well when my issues happen, I'll start hitting the throttle and it will hesitate, and at 1600 rpm in first it will jump from vacuum to 4lbs of boost, then once rpm climbs further the boost disappears, back to 0 vacuum, and hesitation gone.

I'm perplexed as I didn't think these could make boost in first, plus being intermittent it doesn't always happen. Truck has a 3" pulley, maxes out at about 11lbs. Could this be a bypass valve issue?

 

Hold the bypass closed (the position it's in when it has vacuum) with safety wire and test drive the truck.