I have a 91 s-10 P/U 2.8L It has a vaccum problem.

This is for a 3.1 liter engine.

Your whistling is quite possibly a vacuumn hose leak on the passenger's side, top of the engine. Look closely at the hose connections coming from the PCV. Small holes can develop in these vacuum line connections, and you will experience a rough idle, as well as the "whistling" noises.

check your push rod going from the pedal to the booster a quick way is to pull the vaccum hose and see if they release the vaccum not being released can cause sticky problems also check your vaccum hose also

The coil pack should should be the device you plug your spark plug wires into. if it is round, then it is a distributor not a coil pack, if it is square and looks like there are relays coming of it, it is a coil pack. to replace this, you simply mach the wires to the corresponding diodes on the new device, unscrew the old one and screw in the new one. plugging in any connecters you might have forgotten.

actually i would say it is the vaccum actuator on the frontend thats not applying, i dont think its a transfer case issue.. if you look on the punkin on the frontend, you will see it, it will have an electic plugin and a vaccum hose that goes to it...

you should check your hoses. having a vaccum leak will cause the car to roun rough.

purchase a haynes manual-its detailed in all parts of the vehicle--those discount places havum

If you think you have a vaccum problem, the best thing to do is replace all the vaccum hoses. As you replace the tubes, do not take both ends off at the same time. Never take two hoses off at the same time. ONLY ONE END AT A TIME. Then replace that end, run the hose, cut it and stick it on.... You have a nine year old car, and that is not unheard of to have vaccum problems, and hunting is next to impossible. It is just cheaper and faster to replace them all and then you KNOW what you got.. "That Problem Has Been Eliminated" if you chasing another symptom.

the vacuum modulator that the vacuum hose connectes to at the transmission has a ruptured rubber diaphram, replace the vacuum modulator and the vacuum hose.

check the vaccum hoses on the passenger side under hood. I think that there are two hoses but it might be three. they get brittle and leak. but they move a little valve that you can move yourself just to see if heat works then. it is inline with the heater hoses I think. Bear with me though because it has been about 3-4 years ago when I figured this one out. it is definately a vaccum problem. there is also a set of vaccum hoses under passenger side of dash, several colors in it. might want to look there because sometimes kicked by peoples feet but money is on the ones inline with heater core. just move linkage and see if it gets hot air. Hope I helped and didnt confuse.