[peterz]

First _ I realize that idle problems can be as common as oil leaks on these trucks : ) I've ruled out the common stuff- HTL's, cleaned the coil pack connections New Plugs, air filter, fuel filter etc- The truck starts and runs great (for a 210k 98) after a a few cycles to school and the store and some stop and go traffic it starts to run rough not a shaking misfire but if you hold your hand under the exhaust it is miss firing- like a cough- no check engine light - my original thought was a bad tank of gas or injector- but I've run through several now- ran some injector cleaner through also- Now I'm thinking maybe a vacuum or evaporative problem? I read somewhere that a purge / vacuum valve is in line with the fuel system? Also perhaps a bad O2 sensor- they heat up over time? The thing is that most of the time she runs smooth as silk then the coughs- I thought also the idle control valve might need cleaning but that seems unlikely as the problem is intermittent and idles rock solid when not coughing- Any thoughts or directions of pursuit appreciated

Peter

 

[Rich998a]

It could be anything, or more likely a couple of things. Have you removed the IACV (stepper motor) and had a look to see if it is carboned up? What gaps did you use on the plugs, and what type did you use? Too many possible issues to list.

One thing which helped my idle was to open the inspection plate for the flywheel and check the 35 teeth on the reluctor ring just to check they were not bent or broken. What I found out was that some of the teeth had reduced the gap to the point of making contact and wearing the end of the crank sensor! Others were too far away from the sensor. I knew there was a problem as Allyv8 had demo'd his Picoscope and the sensor waveform was a little iffy, but with one person turning the engine by hand and the second visually checking and carefully bending the teeth, I mananged a perfect idle for about 2 months.

I think a combination of heat and stress bends the teeth over time. That, and the teeth being bent during engine replacements can be the only reasonable explaination really. If they are not 110%, then the engine is never going to idle and run properly.