1998 gmc 4x4 runs poorly and sounds like a vacume,air leak .

Have an assistant blow cigarette smoke in the end of the line and look for the leak under the dash,
Thinking that goes either to the brake light switch or the heater control

there is a motor located on the transfer case check to make sure this is working

Hello

I know this is going to sound way to simple, but I would look for a air leak on the intake side. The reason I say this is that your engine runs because the computer feels it knows EVERYTHING about EVERYTHING.. Kind of like *********** N law. Well, when you get a vaccum leak, the computer doesnt know its stupid and raising the idle or POOR idle is one of the effects.

Now I am not at all saying this couldnt me something much more difficult, however VAC leaks are also pretty easy to check. Get yourself a can of SPRAY CARB CLEANER from your local parts store, some times gas stations have it too, wal-marts and places like that have it. Start your truck and SPRAY the CARB CLEANER around places that could have a VAC leak. Like hoses, the intake manifold gaskets and those kinds of things. If your engine suddenly runs REALLY ROUGH or even DIES when you spray a certian spot you know thats where your leak is.

I hope this helps you as much as it helped me

Here is the firing order diagram for that engine, and let me know if this does not solve your problem or if you require any further asistance.

Usually due to vacuum leak. GMC uses poor quality plastic piping going form the left side front of engine by ac accumulator and runs to the driver side of engine

Get a fuel regulator test done. I have same truck. Had same problem. 1 place told me my trans was bad. Another told me cataltic convertor was bad and that the fuel was coming ok when truck was started but when the gas peddle was down the amount of gas per second went down. Fuel pump is your answer. I changed the fuel pump,fuel filter and nothing else and she drives fine now.i hope this helps. My girl friends saturn was doing the same thing. Fuel pump and fuel filter, runs a heck of allot better now.

sounds like a typical pneumatic door problem ...perhaps a vac. leak to the drivers side a/c door (most likely) listen for any vacumn hissing sounds on that side....or a hose duct loose/leaking which i doubt.

Any time your dealing with a 4x4 system the best start is at you front axle. Its primarily a vacuum leak so check to see if the 2 vac lines are hooked up (its good to check the electric connections.). When your 4 wheel drive is engaged and the motors running you should have vacuum at one or both of thee lines depending on if your in 4 hi or low .
If no vacuum at axle check the Vacum switchs(the plug with 4 vac lines) on top of the transfer case it will have 2 feed lines(grey & green) they conect in to the into the vacum harness on back of motor (passanger side). A vacum schematic is normally located on the raditor core support (right under the hood next to hood release)
Hope this helps

I would check for vacume leaks in the engine area listen for air sucking like a vacume ...You can spray a small amount of throttle body cleaner on vac lines and the motor will rev up or bogg down that is a leak and need to fix all vac leaks ........If check engine light is on have your codes checked ....hope this helps

had this problem on my neighbors 98 chevy truck with the same engine !!
it turnd out too be the fuel pressure regulator,you have too pull the upper intake too replace it!!