Intake manifold gasket?

[Tdoa]

I just got a 99 suburban 5.7 with 175k miles. Was told the truck had a coolant leak that couldn't be found. I looked it over and found the radiated cap Completly shot I replaced and found a leak on a block off nipple on the radiator. Simple fix but now the truck is running like crap ever since the cooling system is pressurized from the new rad cap. I tried blocking off the breather tube and PVC valve on the valve cover and checked for negative pressure on the dipstick and found none. Any ideas? Truck ran great before i messed with it

 

[ccapehartusarmyINF.(ret)]

my first question is are you sure the coolant leak is fixed because i had a mysterious coolant leak and found out the water pump had a leak on top of the pump around the gasket and since it was so small off a leak any coolant was absorbed by dust and dirt in that area
and can you be more spacific on how your vehicle is running rough idle ? blowing smoke ? hesitation ?
it is possible that when you replaced the pressure cap youve uncovered another leak somewhere else in your cooling system wich is causing poor running

 

[Tdoa]

I know the leak that I found I fixed but I don't think it was the one that was causing the disappearing coolant. The truck idles fine doesn't smoke but is hesitating pretty bad until you get on the throttle pretty good and then it clears up. Oil is not milky. Ever since I replaced the cap the truck has ran this way. I know the water pump was replaced maybe 3-4 years ago because I helped do it. Truck has been in the family for 10 years

 

[ccapehartusarmyINF.(ret)]

okay disapearing coolant oh lord im assuming you havent found any disapearing onto the ground wich means that its either disapearing somewhere in the engine wich is bad
or you have a small head gasket leak maybe im speculating here but its small enough to wre the missing coolant its just being boiled away causing poor running conditions but not actually blowing smoke out your exhaust

 

[Tdoa]

I'm thinking the intake manifold because it seems like every truck needs it done sooner or later. And I know this truck has the original

 

[ccapehartusarmyINF.(ret)]

yeah i can go with yah on that

 

[99Vortec]

Is the overall coolant level dropping still? (On the "Hot fill" mark?)