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Dieselhawk · 10-11-2017, **(Subject: CM871 Idle Fluctuation )**

(09-14-2017 )Nilao Wrote: At idle the egr valve will be cracked open to allow egr to enter the intake. This lope at idle is the egr opening and closing which starves the engine for oxygen. I think the delta-p sensor is probably toast and not allowing the ecm to get an accurate reading on the flow through the egr Venturi.

Taking the pipe off between the egr valve and venturi is interrupting the flow of exhaust gas to the intake which allows the engine to run on 100% clean air, thus the lope goes away due to the lack of exhaust gas interfering with combustion.

Just my opinion on why it goes away if you unhook that pipe.

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?