CM871 Idle Fluctuation

[multitask]

Hi everyone,

Have an 871 and every 3-5 seconds the rev will drop alittle and make a huffing sound.

This is what I've done so far:
Egr tune up
Changed IMAP and exhaust back pressure sensor
Cleaned all piping using carb cleaner
Cleaned egr temp sensor and delta P using carb cleaner (hope that wasn't a bad idea using the spray?)

Yesterday I removed the egr pipe that's between the egr valve and Venturi but with the sensor still connected I started the truck and she idled perfectly!!!! (All sensors connected and no faults active).

Could it be the egr or delta p sensor causing the problem? Is the egr valve meant to be closed during idle? Was getting exhaust gases coming out.

I've also tried running the truck with the egr sensor unplugged and she runs like a dream, but Rawze did tell me once that will derate the engine etc

What's you're recommendation gents?

Thanks

[Fergie]

I don't believe the Delta p can be cleaned. I'd get a new one.

[Waterloo]

Don't clean the sensors, replace the sensors.

[Ragindirt]

I'll keep an eye on this trend, have same scenario but only just started with erg tune. One sensor pulled and plugged to all get.

[multitask]

Thanks guys

[Nilao]

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.

[Dieselhawk]

(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?

[Brock]

Is this at full operating temp? Cause the engine uses egr to warm up the engine or to maintain temp when in colder climates.

It also uses vg position to cause the back pressure into the egr to warm the engine/ keeping up to temp.

[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.

====

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.