CM871 Idle Fluctuation

[Rawze]

(10-11-2017 )Dieselhawk Wrote:  Hi Guys,
I have the exact same problem. The first thing I did was replace the Delta P sensor but it made no difference so I swapped it back. I also swapped the EGR temp sender and again no difference. If you disconnect the delta p, egr valve or egr temp sensor and run the engine it runs fine. So I guess the EGR system switches off in the instance that there is an active fault relating to the EGR. After this I found some coolant in and around the EGR valve and cooler so I had to replace the cooler. This fixed the coolant issue however the rpm fluctuation is still there every 5-10 secs. When I monitor the EGR system with Insite the EGR valve sits at 35% open at idle which isn't right as it should be at 0% however the commanded percentage is also 35% so I believe the ECM is telling it to sit at 35%. The delta p pressure slowly builds to about 5-10pka then drops back to 0kpa when the revs drop. The cycle then repeats over and over. At the same time the VGT and commanded VGT builds from 70% up to 90% then drops back to 70% and repeats. I know from looking at another truck this should sit on 70% with little fluctuation anf the EGR valve should be at 0%. I have disconnected the turbo exhaust outlet to rule out any restriction in the DPF and this made no difference. I'm starting to wonder if I need to try another ECM on it. The egr valve and VGT both seem to be doing what they're told by the ECM.
Anyone have any ideas?

You did not mention what model engine. It is not likely the ECM itself.

* truck could have the wrong program in it.

* could be a mis-reading IMAP sensor.

* Could be a bad mis-reading exh. BP sensor.

* Could be clogged tubes/passages for Delta-P or a cracked mounting plate (seen that before).

* Could be excess soot/crud buildup in the head past the intake manifold (seen that before too).

* could be a mis-reading rpm sensor in the turbo (seen that before).

* it could be a bad cam or crank position sensor or a loose tone wheel.

* could be a leak in the EGR or CAC system, or a restriction in the EGR cooler. I hope for your sake you didn't put on an after-market EGR cooler, that is the kiss of death of the engine.

* Could be a bad metering or timing actuator (cm871), or a leaky injector. This is the most common problem for fluctuating idle on CM871.

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BTW: the EGR temperature sensor is called the "Magic Plug". When you unplug it, you get a check engine light, the engine goes into its factory non-egr (and protection) mode, the EGR system is instantly switched off, egr valve is closed, and DPF system shuts down too. I know of some guys who drive their trucks like that for a week or 2 at a time, them plug it back in and do a regen, the go again. Don't blame them, though it is not any kind of long term kind of fix. A decent de-mandate with proper programming is what is needed if someone is going to do that.

- This goes right back to my bone of contention with the Iniste software. It will not show you enough settings to find a problem like that very well. It leaves the mechanic guessing and throwing parts at the engine. How much you want to bet I could find the problem if it were here and we were looking at it with C~!term.