RPM 's Bounce

[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