Engine/Transmission (1998.5 - 2002) Weird idling

[NPloysa]

Recently on my '99 my idle has been acting up.



It only does it after a long or hard run, say running around town, then parking at the store in the parking space while I let it cool down. I see my tach jump about 50 down, then back, every couple seconds, and can hear it, almost like air in the system. It does not do this all the time, but when it does, it is only after a run.



It never does it during warm up in the mornings, and never after short few mile runs to the store or whatever.



I am on my second lift pump and am running safely within the #s. I replaced my fuel filter about 2000 miles ago and it seemed to go away, then came back. I then went to a BHAF. It seemed to go away after that then came back. Pressures were and are still good according to my guage (not digital).



Always does it on idle, never on throttle. While it is *burping* I can touch the peddle to 900 or 1000 or even higher rpms and it goes way. I hadn't noticed it for about a week and a half and it came back tonight.



I am puzzled. I just bought a Blue Chip FMS and don't want to make anything worse before I figure out what it is.



All my injector lines, fuel filter, etc, is all tight.



Any ideas?



TIA,



Nick

 

[csevers]

Two ideas I have, one is the Crank Postion Sensor it is known to affect tach readings. The other is are you using fuel additive? I had the same almost( mis )weird idle for some time, several years ago and it came and went with out me pin pointing it untill I started to notice it with a certain cetane additive and the over use of it by me. I quite using the additive/changed dose to the recomended one and the mis/weird idle went away and I have not had the weird idle since.



Craig

 

[BarlowCummins]

I have the same problem with my 02 and it would be nice to know what is causing it. I do use an additive but my truck had this problem way before I even added it.

 

[NPloysa]

Thanks

Thanks for the help guys.



I do not currently use additives and while driving today it did not do it. Guess I will hope it goes away.



Appreciate it!



Nick

 

[Ncostello]

My 1999 does exactly what you describe. I have replaced many many items such as sensors and have checked all wiring and fuel lines. Cummins dealer has acknowledged the miss and has determined that its different cylinders misfiring... at different times. I originally replaced my injectors through their advice after the first time they hooked it up to the INSIGHT computer program. But on the second run of this after the new injectors we saw it was different cylinders misfiring this time.



So as soon as I get the chance, I'm gonna put a used VP44 I got on there. Its only got 10K miles on it so I'm assuming its OK. I've been waiting around to find someone who can run it on a test stand and make sure I'm not wasting my time.



Only other thing I might do is a valve adjustment. Thats free.