just thought that you all would love to know something cool.
my 96 5.2 was losing power and popping throught the intake when it was crossing from open loop (cold preset curve operation) to closed loop (warm) i've know for a while that my upstream o2 was failing but for a lack of funding and just being a minor nuisance i left it alone.
other symptoms i've seen on customers vehicles have been loading up at idle and ocasional stalling.
if you quikly change throttle position (like 1/4 to 1/5 fast) it clears up for just a sec till it goes back to closed loop. full throttle it goes away because you go into open loop.

to test for the o2 causing this if you are lucky enough to have access to a scanner wait until it would do it (normally in the morning or on the way home for me) and watch your o2 sensor. your voltage should change somewhat rapidly .5 to 1.5 is what myne was suposed to show. but it just goes low and stays low then it's bad if it's in closed loop. i changed myne and it fixed everything. for those of you with the old paper belly pan gaskets i'd recomend doing this and your gasket at the same time if it's leaking because it actually can cause the o2 to fail.


i hope this helps someone out.

 

thanks man! any help like that goes a long way on this forum. happy new year dude!

 

I'm having the same problem, once the truck warms and I assume its changing over to the closed loop. It throws a fit and almost sounds like the timing is way off. I've already changed the 02 sensor with no improvement. Not sure what your talking about the paper gasket, is this the intake plenum gasket that I've seen others post about. Just replaced the crank sensor, no change. Any help would be greatly appreciated.

 

happy new year guys.

the paper gasket would be the plenum gasket. you can look down into the t body and see if it has a pool of oil in there to tell if it's blown. at work we plug off the vacuum supply to the pcv valve and pull the vent hose off. with the oil cap on and dip stick in put your thumb over the vent hose end and see if you build vaccum or pressure. a good healthy engine will build a small amount of pressure (normal from them blow by that any engine has) usually about 5 to ten seconds is long enough to tell. if you get nothing then something either leaks (cap or stick could do this) or the plenum gasket is blown but not bad bad. you can still go a bit on it like this. at this point i would tell a cust to save up for it. if you build vacuum it's deffinetly bad.
this is what i think killed my o2. i didn't have this issue till after i fixed my severely blown plenum gasket. it also started not wanting to idle. the o2 went because of being rich all the time from the oil. my cat will be gone soon most likely.
the iac valve probbly sat full closed all the time and that's where it was stuck. i cleaned it and beat it around and got it working. once in a while it hangs still. but it's improving and i have one for it should i chose to get up and do it. if you do your plenum pan i would suggest atleat getting the steal gasket that you can find anywhere almost. i got myne from the dealer i work at. i had to do it a second time (still sucked a fair bit of oil in and i wanted perfect.) and i used axle rtv (grey) on both side the second time. it's heald up great so far and im about 7k miles into it now. alott of ppl get a aftermarket aluminum plate that replaces the steal pan. personally i think either one is fine. i had considerd the plate then having a buddy who has a tig weld it together for me but that's overkill. im not running boost so i don't really need to worry with it. this was only on the 39/5.2/5.9 magnum engines by the way. by the way born if you have a scanner you can barrow that's the best way to test but if not you can resort to voltmeters.
if it does it when it warms up but not when it's cold you can rule out throttle position sensor, coolant temp sensor,map sensor, crank sensor (most of the time) and pick up (most of the time) because it needs those in open loop (cold) aswell. it is possible that one of them could be freaking out at certain temp ranges but generally i check elsewhere first. it sounds like you did that already though.
when it throws the fit just unplug a sensor one only thought and drivge it. see if it does any different. the pick up and crank sensors are the exception. it needs them. it will just dummy in a number for the missing input. won't be a picture of power and response but cn be helpfull. i've done this many times myself. it can dummy up a map and tps sensor. it may do the same for the engine coolant temp but it's usually hard to tell a difference since it need it for timeing and mixture.
i do know that these ecu's were bad to short when the o2 died and you had to replace it to fix but not ussually the case thank god.

 

Thanks for all the info, will give that plenum gasket some invesigating. This has been an on and off battle for over a year now. Hopefully i will eventually get it found!

 

good luck on it man.