Hey all,

My 98 1/2 ton 5.9 is having a problem that doesn't sound unique, but it might have a twist. When first started and running at idle it's fine for a few minutes, but then it starts to get rough and really suck in the air. It will stumble and idle down and die if I let it, I can rev it up and it runs, but if I let off it will go right back to sucking air and it will die. If I turn it off and right back on again, it will idle with no further problems. I've removed and cleaned the TB, and replaced the AIC. Any ideas, TPS maybe?

thanks

 

I have a K&N FIPK and i learned the hard way during the rain (acts just like your prob)lol...it don't like rain (sucks it in and boy does the engine start to shudder and stall)... so i just dump two things of stp dry gas and keep the filter cleaned...purrs like a kitten...

You already cleaned the throttle body and aic, i dunno about the TPS tho...

 

interesting. have you got a tach ? normal operation is for cold idle to be about 1000 rpm, and then as it warms up it gradually drops to about 600-700. the IAC controls most of this by acting as an air valve, since the throttle plate is closed. if you mash the gas or manually open the throttle, then the plate opens and the tps tells the pcm what you're doing.

my first guess would have been to replace iac, but you already did that.
i'm guessing that changing tps will not help, as its not supposed to be involved in the idle, except sitting there doing nothing at baseline voltage.

did you reset pcm after changing iac ? try that. disconnect neg battery terminal and turn key to start for 30 sec.

 

Hey dhvaughan,

Thanks for the reply.

Yes, I do have a tach and the idle does exactly as youreplied it should, starts out at 1000 cold, warms up and smooth as can be at about 650, then after a few minutesthe idle gets a bit rough, the air intake starts to sound like a jet engine, and the idle gets very rough as it drops to zero. I had the neg cable offbut I did not turnthe keyto thestart position after replacing the IAC while I cleaned the TB. I just tried this:

1. Start it up and keep it at 2 grand for 2 minutes.
2. Let it idle, started to get rough after 3 more minutes, pulling lots of air, the usual.
3. Put it in drive, idle increasedand was smooth for 1 minute, then started to get rough.
4. Put it back in park and it idled down to about 200.
5. Turned it off and right back on. It started right up smooth as can be, the only needled that dropped over the next 6 minutes was the fuel gauge.

I can get it to do this all day long, it just needs about 15 minutes between tries.

 

Hey DodgeRAmPower,

Thanks, I'll try anything at this point.

 

I forgot to mention that the check engine light has never come on.

 

Does the '98 have an EGR? Check the PCV also.

 

when the truck starts up its in open loop... it hasnt warmed up yet and isnt reading sensors. once it warms up it will start to read things like o2's. try cleaning them.

i'm probably wrong... i usually am. but its worth a try i suppose.

 

Good point timwag, the o2's are the only sensors I think I haven't checked yet, because I can't get them off. I'll try to break them free tomorrow and see how they look. I'd hate to start replacing parts to see what fixes it, but the aic is new, and voltage checks of the map and tps seem fine. Anything is worth a try.

 

just to be doing, try resetting pcm first. no cost.

after that, yeah, like tim said, i'd replace the pre-cat 02 sensor. don't worry about the one after the cat, as it doesn't do anything except monitor the cat. get a can of pb blaster from local auto parts store or wally-world and spray the heck out it for a couple of days to loosen it up. then reset the pcm to force it to relearn all the sensor readings. i'd just replace it, as opposed to trying to clean it so that you know you've done something.

it sounds like the high pitched whistling is the IAC closing up too far.. there's a few other sensors involved including intake air temp, map, engine temp, etc. do you get a check engine light, or have any stored codes ?