I have a brand new '05 2500 with the 6-speed. The oddest thing happens when I'm coming up to a stop light (or any stop really) when the engine is warm. As I push in the clutch to come to a stop, the engine cycles up from about 550 rpm to about about 700 rpm. Then it comes back down slightly to a normal and steady level. What gives??

 

Did you check to see if any codes come up? The fuel control actuator can cause some surging but check for codes. Maybe somebody will chime in with a better idea.

 

If you are leaving it in gear with the clutch engaged as it appears you may be doing in your post (550 rpm) it is just the computer trying to recover the RPM's to where they belong at 700. You are in essence lugging the engine and the computer is feeding it more fuel to compensate for it and when you push the clutch in it Revs until it catches the fact that it has overshot 700. try pushing the clutch in before the RPMs get so low and i think you will see that the problem stops.

 

Thank you! I will try that. I wasn't getting any codes and I'm new to diesel so I was hoping there was some logical explanation that didn't include a trip to the dealer.

Thanks again and I'll let you know what I find.

 

This isn't much help but fwiw my '03/6-speed has always done the same thing. Seems to only happen if you're coasting w/ clutch depressed for a couple seconds.

 

Doing the above Coasting with the clutch in causes other problems by putting additional stresses on the transmissions inner workings by multiplying rotational forces somehow. I would be very cautions about doing that. I have owned 8 Semi's and have been told that coasting with the clutch in can and does cause big problems.

 

Could you elaborate on this please?

 

Thats because theres a transmission break on them. We dont have one. Coasting with the clutch in will not harm anything on these things. The worst thing it does is put additonal where on the throw out bearing.

Now back to subject of this post. It is normal for the engine to surge a little when pushing in the cluch. It will do it every time with out exeption. You can minimize it but not eliminate it. If it realy bothers ya the only thing you can do is if stopping from a high speed let off the throttle a little but not completly just a split second before pushing in the cluch. From a slow speed push the cluch in before you get the engine down to idle speed. if you are moving slow and you dont push in the cluch till after the engine has dropped down to idle speed the computer will be feeding the engine some additional fuel to keep it from stalling this is equivalent to pushing in the cluch when accelerating. It will cause the idle surge to be worse. Efectively what you are trying to do in both casses is take the load off the drivetrain before pushing in the cluch. I dont even notice it anymore unless for some reason i am looking for it. Like i said you can not eliminate it but you can minimize it buy changing your driving habit.

 

Thank you. It doesn't bother me at all...just wanted to make sure that my new baby wasn't broken!

 

Sorry to burst your bubble but you are wrong. there is no transmission brake as you referred to. you are referring to what is actually a called a clutch brake and the clutch break only engages when the clutch is disengaged and the pedal is in its last 1" of travel after it is totaly disengaged and its purpose is to aid in getting the transmission to stop spinning to get it in gear. On large trucks, NO, NOT YOUR CTD the clutch brake is "bumped" to allow you to put the transmission in gear and that is the only time the clutch pedal should actually be pushed to the floor. If it is used any other way the ears will be torn off the cluch brake and will render it inoperative. I suggest you talk to a good transmission shop as far as coasting in gear with the clutch in because i think you will be suprised to find out that you are wrong about that as well because it does cause damage.

 

Your truck loses throttle position when you push the clutch in. It loses it because you broke the RPM to MPH relationship so it has a bit of a time finding TPS versus speed. Once it realizes it must be coasting or clutch in, it will return the idle to the correct idle speed. Auto trans trucks shouldn't have the same problem, only manual trans.

This is what I was told. Hopefully someone will correct me if I'm wrong.

 

the ecm doesn't need to see the clutch signal either for this to happen. if you have cruise on and it is just about to transission from coasting to throttle, you could float the gear out into neutral with no clutch and the ecm will cancel cruise when it determines that there is too much engine speed increase too quickly [in neutral, clutch slipping, wheel spin on ice...] when it is idling and vehicle speed is above about 15mph, that is normally not a speed where the engine should be trying to find idle, so it goes through its process of trying to figure out if it is in gear or not, and you get that 2-4 hundred rpm hunting. pop it into neutral under 15mph and it will hunt once when it starts to fuel after not being fueled [the fuel is basically off like when shut down], and the ecm needs to catch the engine again before it dies, so the engine drops to 500rpm or so and then bounces to around 900rpm then stablizes at the 750rpm idle.

this is pretty much normal on this engine. it has been discussed here and on the tdr before. some still feel that it is a coding fault but i don't see it as that at all.. you all that are seeing this sure watch the gauges more than i do. mine does it too, but i usually don't go into neutral until under 1000rpm in 3rd gear so at that speed, the idle is a valid input to the ecm so it does the one hunt then stable...

 

I agree that it is completely normal. I have driven several 3rd gen 6 speeds that all act the same.