I have a 1999 5.9 and I have noticed my RPMs fluctuating all over the place. It only seems to happen in the mornings and only within the first 5 miles of driving the tach goes bonkers and the check engine light comes on. but in the evenings on my way home it doesnt malfunction at all. Would it have to do with possible moisture in an electrical connection? It has happened to me 3 times and all 3 times it has rained the night before or within the prior 24 hours. Looking for a place to start checking out stuff. I have stopped at a local NAPA and Advance auto to have them put a scan tool on it to find codes and have yet to come up with a code. Thats what really confuses me?? So needless to say I am stumped

 

Mine did it when my cam position sensor was going haywire

---AutoMerged DoublePost---

Or a bad crank position sensor on your year of truck

 

Wouldnt that throw a code tho scott? Would I be best off to replace the crank and cam position sensors?? How much are they and how hard are they to replace?

 

If I remember mine did. It may also be a bad APPS. Or its out of calibration. Your truck reaeds rpm via crank position sensor is below and to the right of the injection pump

 

How big of a pain in the a$$ is it to replace the crank position sensor? Im gonna try and find someone who has a better code reader..

 

so the RPMs went haywire again yesterday morning and after work i had a buddy put his smarty on it to check for codes and once again it came back all clear no codes detected. It only seems to do it in the mornings. I am baffled cause it doesnt affect the performance of the truck.