so i've been trying to hunt down the problem that ive had with my 98 1500 with the 5.2. it runs really rough. at an idle it will start bogging down and chugging. ive i give it a little gas itll come out of it for a bit and start doing it again. It goes away and seems to run ok when its warmed up (just really bad mpgs. About 7 highway and slight lack of power) I've been really busy at the shop and havnt had too much time to work on it but since its been kinda slow i decided to look into it more. Ive looked at all the live data and it all seems to be fine except for the manifold pressure. My scan tool (snap-on solus) shows it staying around 9.6 to 9.8 inches of vacuum. i thought that was low and decided to hook my vacuum gauge to the intake. My gauge read 19 inches steady. and for some reason when i drive it looking at the live data, when i stomp on it the scan tool reads a spike in vacuum to about 28 inches. at WOT the vacuum shouldnt spike, it should drop, so i checked the vacuum tube connecting the map sensor to the TB for leaks but didnt find anything. so i decided that i should replace the sensor. I just put a brand new one it and its giving me the same readings. all the data seems to be ok other than that so im not sure where to look from here. its got good fuel pressure (48-50 psi), i took a fuel sample and looked for water or dirt but it was clean, checked for vacuum leaks and didnt find any, intake pan gasket is in good shape, injectors are are firing properly, its got good spark. any diagnostic thoughts or advise is appreciated.

 

If you snap the throttle it should drop to 0 then rebound a little higher and settle back at where it was..19psi . Keeping at wide open is eventually going to increase vacuum rather quickly.
I rhink 97 and older have an egr valv...could be worth checking that out along with idle air control sensor.

 

Unplug the front O2 sensor, see if it runs any better.

The MAP giving bad numbers on vaccum is kinda strange..... clogged cat?

 

IAC is clean and properly working. i tested it a few weeks ago with the scanner. ive tried the removing the o2 sensor but it doesnt change, the cat was gutted out before i bought it. the vacuum spikes immediately when i open it up, it doesnt drop and rebound like it should (only on the scan tool, my vacuum gauge shows the proper WOT drop). my truck doesnt have egr, its a 98.

one thing i forgot to mention is when i drive without the engine at op. temp., for the first 30 seconds or so, it feels like the engine is loading up and feels like im hitting the brakes slightly. if i let off or stomp on it it will come out of it but if i dont do anything, the engine will load up and knock once or twice and it will come out of it and drive fine after that. ive never seen a problem like this at the shop so im lost.

 

What does the scanner say engine temp is?

 

i have this same issue, everything is the same as you described. if i stomp on it it pulls out of the *******. i changed the iac, pcv, fuel pump, evap cans, and filler neck because i also have the check engine code p0446. there was stuff in the bottom of the tank i vacuumed out. ran perfect directly after the fuel pump change and i put on a flow-master 40 series. but this afternoon and since then it chugs but its more intermittent and my cel light is still on Ive checked for leaks and there is none i can find.

i think my cat is gutted cause its not factory pipes on when i changed the muffler, i tapped on the side and it sounds hollow.cant stick anything in it cause they welded a bend ans long pipe in until the muffler.

 

when the truck is at op temp the scanner reads about 195-200.
when the truck initially starts its around 70-80 (obviously depending on ambient temp) and steadily climbs to op temp.