CM871 rocking back and forth at idle

[grainhauler]

New guy to the forum and fairly new to the industry. The truck is a 2008 model that my employer bought a couple months ago. The motor rocks back and forth at idle just enough to get on my nerves, the rpm holds steady and it sounds ok. As soon as I apply throttle the rocking stops. Other than the idling issue the motor seems to perform well. The motor was overhauled about 70,000mi ago by the previous owner and was demandated at the same time according to the service records of the owners local shop. Any ideas as to what is causing the issue and if it is related to the demandating? Thanks in advance for any information or thoughts

[Starlight]

Sounds like a top end is due. Valves, injectors and jake adjustment. Checking for leaking injectors would go along with that.

[Rawze]

A bad delete program?

[Old Driver]

Have you had your actuators replaced? Mine had a bad wobble side to side and once the actuators all got replaced it smoothed it out immediately.

[grainhauler]

No, the actuators havent been replaced. Is there any way of testing the actuators?

[pearce trucking]

Rawze has a video. Search youtube. It should come up.

[grainhauler]

Found it, Thanks

[Pete379]

I second what Rawze said about the programming. When I purchased my Pete with an 870 it had a tune that caused it to act the same way as you are describing at idle. Maybe it's worth a pm to unilevers he got mine taken care if. If I remember correctly he said that it was getting too much fuel and then trying to cut itself back which caused a shake at idle almost like it had a performance cam in it or something

[Rawze]

(01-29-2018 )Pete379 Wrote:  I second what Rawze said about the programming. When I purchased my Pete with an 870 it had a tune that caused it to act the same way as you are describing at idle. Maybe it's worth a pm to unilevers he got mine taken care if. If I remember correctly he said that it was getting too much fuel and then trying to cut itself back which caused a shake at idle almost like it had a performance cam in it or something

big-name tunes are like that. What they do is fool the ECM into over-fueling the cylinders against its will badly and it makes them "lope" a bit. All it is doing is wasting fuel and nothing whatsoever is gained by it except to indicate an idiot who has no clue what he was doing tuned it!. - The result long term is this...

1- The ECM knows something is wrong with the injectors over-fuelig the cylinders.

2- It then tries to back off the fueling to all the injectors because the cylinder balance data reports the cylinders are firing too strongly but don't know why.

3- Often, but not always, the fuel mileage readings in the dash get screwed up and read about 2-3 mpg higher than if you measure gallons to miles at the pump. This fools tyhe driver into thinking the fuel mileage is higher than it really is by this totally sh%it tune now.

4- The cylinder balance manager can only back the fueling off by a small amount using injector trim corrections and cylinder balancing corrections.

5- The cylinder balancing data then gets skewed all the way into to the left for all 6 cylinders and against the limiter. The engine then looses its ability to balance cylinders completely.

6- The engine "lopes" and does all kinds of unkind things to itself internally, but it is usually only felt at idle, though it is doing it all the time.

7- The output power and torque is now unknown by the ECM because it no longer knows the correct fueling into the cylinders.

8- The ECM can no longer protect or properly control combustion temps and pressures at all. This sets it up for an eventual failure if someone drives it hard a lot.


-- The sad part is that the engine will appear to run ok for the most part by the driver. The driver will brag about fuel mileage increases because they never bother to check the readings in the dash against calculating it based on gallons put into the tanks. He will then go tell all his buddies about how great that expensive big-name tune was and brag about that fake "cam lope" like it was something glorious. He will then learn later (after finding this forum and post) the hard way after an engine failure or 2 that all the money he wasted is what brought his engine to a pre-mature death.