RPM 's Bounce

[Neighbor]

Got a 06 cm870 inframed and de-mandated from a member on here with only 40k on motor. My problem is when i push in on clutch or throw it nuetral to coast down hills my rpms bounce between 700 to 800 3x, every time. I can hear the engine doing this not only the cluster. At high idle its fine no problem. Last week i replaced all the metering and timing actuators, got a good deal on them. Injectors where replaced 4 months ago oem recons. I read from a previous post maybe to look at the turbo actuator arm even thou the turbo was replaced during the inframe oem. Im going to look at the tps sensor, and at that air controller the sends air to the turbo actuator. Any other suggestions would be appreciated. And already did cut out test on my lap top with insite, never no codes. I can tell you that pretty much all the sensors are probably old and never been replaced. Thanks in advance to any ideas.

[Signature620]

I notice a lot of cm871s do this when rolling in neutral. Could it be a slight glitch in the ECM when it has a vehicle speed signal at idle? Someone here might be able to shed more light on it...

[Rawze]

my truck and engine has never done this. I have a CM871.

My metering and fuelling actuators and entire IFSM including fuel pump, electric pump etc... are all still factory-original components with 1.4 million miles on them now.

I inframed my engine at 980k miles due to injector cup starting to leak. During that 980k miles, my engine started to get a bit of a rough idle at times in it but never had a bouncy rpm like that. It would stumble a bit, especially when cold, then a bit less when warmed up, but was not smooth and perfect like when the engine was new.

After replacing the head, engine ran quite a little bit smoother. I did not change any fuel system components until some time after, so I was running all the old parts. It became obvious that the head itself was the cause of the little bit of rough idle that I was seeing before its replacement. - Would have never guessed that the head itself could cause a bit of rough idle. Only thing I can think of is a tine bit of imperfect valve seat wear after it got that old when it was cold?, and less when warmed up?. ... Maybe the wear in the cam bearings allowing more variance in the cam/rocker positions?.. Maybe a combo of all this? -- who knows.

It was running quite smooth at 1.03 million, had injectors started to get a bit leaky, so I replaced them all. It did not seem to change the fuel mileage or how it ran when I did so.

I know this is likely the exception and not the rule of thumb, I baby the h#ell out of my truck, but it does make someone curious. Just stating all of this here for reference purposes only. makes me wonder if you maybe a reman or machined head could contribute to rough or bouncy idle after an inframe?.. or was an OEM brand new head installed onto yours when it was inframed.


-- The next question would be... If you put a spare ecm with factory programming in it ... does the issue go away?. If so, then it could be a sitting in the custom program somewhere that may have been set too sensitive or perhaps is not quite right. It being a CM870, gotta keep in mind that you cannot just use CT (Calterm) to copy the program in and out of the ecm as easily as you can with newer engines. You can copy it out, but you have to make a program based on an Incal factory file put something back in *(using only insite). ... hence the suggestion of trying another whole ecm instead of flashing it back to stock just for testing purposes. If it were a CM871 or newer ecm, sending programs in and out of the existing ecm would not be an issue.

If you suspect the turbo/controller, maybe put a gopro-camera under the hood, pointed at the actuator arm and take it for a test drive? -- See what the turbo is doing?.
you also mentioned it has an inframe recently. Those CM870 turbo's, the exhaust housing on them sometimes has to be clocked after mounting it up. here is some info on this: http://rawze.com/forums/showthread.php?t...8#pid54418

[Chamberpains]

All my cummins have done this. My old cm870 and the new cm870 long block with all new actuators, new ecm, programmed by Mommaburt, overhead ran correctly, cam timing verified, new fuel pump, new turbo and actuator, new actuator control module and system 100% good, replaced the speed sensor... basically everything i could imagine and everything i could gather from here was done. Except fully rebuilding the IFSM. Other than that I chalked it up to to what Signature620 said.

My cm871 does the same thing but I haven't verified everything I did with my cm870's so I can't vouch for the nature of the beast diagnosis yet with it.

[Neighbor]

Ya it was a new head with new cams and rockers also, i have gotten a code twice in the last 12 months for a speed sensor but never looked into that. The programming has me wondering, i requested it stay at 500 hp. But after waiting and 3 days to get it tooned Mr. H and Mommaburt decide to change it to 565hp torque i think at 1850. Truck has an 18 speed with 3.73 rears. I guess i should get with Jerry to remote in and look at it. He said he was going to get with Rawze during is off time this winter and get his own cm870 tune because we waited so long. I just assumed it would be ok and probably still is. Mr. Hag treated me really well like family. I need to learn how to pull the file and maybe have Rawze double check it. As far as the clocking the turbo my problem started about a month ago, inframe was done in July last yr. Only thing that wasnt replaced was the egr and oil coolers, and compressor. Even had the radiator and cac replaced, couldnt reuse the the turbo yts rebuilt something was wrong with it so we got one from cummins.

[JimT]

I think I've seen the same or similar issue with my cm2250. If I have the engine up over 1400rpm then let it drop back to idle it will bounce a couple times when it gets down to the set idle rpm. It's not massive, sometimes only about +/-10rpm. And if I wind the engine down while still in gear then go back to neutral when it's close to the set idle it doesn't do it. Not sure exactly when this started, but I think it was present before my inframe. My first thought was maybe a leaky injector. I've done cylinder cut-out test, injector performance test, rail pressure test and everything was perfect. Only thing I haven't done is pull out and try individually testing each injector.

I'm not sure exactly what or why, but at 1400rpm I can hear a difference in the fueling. Quieter below 1400, louder (more horse power or torque? or just a different fueling pattern?) at 1400 and up. Been that way since the programming was done last February. Related or not, recently I've noticed that the line isn't so clear cut sometimes. Sometimes it will kick in at a lower rpm or continue down to a lower rpm instead of the strict above or below 1400rpm point. Seems more likely to do this when it's still cold, though it still occurs when warm.

I bring that second symptom up because I think it goes hand in hand with the first and further points to an injector not acting right or being slow to respond, despite not being able to narrow it down during tests.

[Neighbor]

Performed the turbo actuator test, air pressure was built up to 120 psi, what happens every 2 or three times during the extended phase. Air leaks out the bottom of the diaphram for about 3 seconds then stops, but the arm stays extended. Did that test about 10 x same response every time. Is that normal? Also found code 0241 erratic speed sensor. Removed sensor and cleaned and knocked off some rust around tone ring.

[Chamberpains]

What make and model truck is this in?