CM870 High Pyro/Low Boost intermittent under load

[nmazpeterbilt]

I read a previous thread that was somewhat related to this but I don't feel exactly the same. I have a CM870 all stock. It is 500 hp. It is a Recon that got put in 2 years ago. Whenever there is high ambient temperature and under load the pyro temp will climb up for 900 to almost 1100 pretty quickly and at the exact same time the boost pressure reduces to about 26 pounds. It will stay there for 10-30 seconds and then pyro comes back down and boost goes back up. This is accompanied with some black smoke. During the winter did not have the issue. Do not get any codes or warnings. The pyro is an external unit, not coming from the ECM reading.Trying to get this figured out while it is under warranty, but of coarse cannot be replicated at the shop. They did replace the Barametric/temp sensor thinking that may be it, but that didn't resolve the problem.

[tree98]

Sounds like the famous 870 emissions derate.

https://rawze.com/forums/showthread.php?tid=3798

[nmazpeterbilt]

I have read through a lot of those articles, seems like most of them have to do with low ambient temp or with a tuned engine. Neither of those apply, it does seem to be the derate, but what is causing it to happen?

[tree98]

Copied from the thread I linked Rawze said this.......

There are a few reasons for this derate.

Most of the time on the CM870's as a result of someone disabling the EGR in some way and not re-programming it (or not programming it properly) for such things. In this case, the EGT's are usually higher then normal. It sounds like yours are lower than normal.

It can also happen if the ECM fails one of the fueling, metering, or fuel shutoff solenoids. Usually, you get an intermittent/occasional code for one of these components this in the fault history, but on rare occasion, you only get the derate without the code. This can cause lower then normal EGT's as well, as the injection timing could be off.

Could also be a malfunctioning turbo controller. They are air operated on the cm870.

[Rawze]

it can also be the trubo speed sensor feedback is exceeding the rated turbo speed and derating it too.

That can be for many reasons including blocked up exhaust muffler, intake air filter restriction, after-market charge air cooler that someone has installed, or the turbo itself might have damage to the turbine blades, making it spool faster than normal with less boost.

[Bengy88]

I’ve seen what rawze is saying cause exactly your issue with a clogged air filter and a damaged turbo…. Also I wonder could your egr valve be opening just enough to loose boost? Is your boost gauge mechanical or electrical? We’ve all seen false boost and egr pressure readings cause issues like this as well. There are really a lot of factors that can play into this. The black smoke points away from a derate. Black smoke comes from a boost air issue.

[nmazpeterbilt]

WE have still been fighting with this. There isn't an issue until ambient temperature gets over 90* then the hotter it gets the more we have it. It is a mechanical pyro. There was an issue with the turbo actuator, we had that replaced. Replaced the sensors mentioned here and had EGR checked. Can't figure out why it is doing this. Have had mechanic in the truck with Insite and never can figure out what it is doing that is causing this.

[Rawze]

(09-27-2023 )nmazpeterbilt Wrote:  WE have still been fighting with this. There isn't an issue until ambient temperature gets over 90* then the hotter it gets the more we have it. It is a mechanical pyro. There was an issue with the turbo actuator, we had that replaced. Replaced the sensors mentioned here and had EGR checked. Can't figure out why it is doing this. Have had mechanic in the truck with Insite and never can figure out what it is doing that is causing this.

Insite is fairly useless when it comes to silent derates. If ti just keeps eluding everyone, then you might have to use the Calterm software and see what is holding the fueling and/or boost back. The most important settings to watch would be the combustion control path owner, the fueling control state, the aux. emissions control states, etc.

ref: http://rawze.com/forums/showthread.php?t...3#pid58693

[Peashooter67]

Check your egr temp sensor. I started having very similar issues and my egr tem sensor was reading all over the place and when it would spike way high it would cause the same issue.

I have straight exhaust so I could hear it before I saw it on the gauges.
The exhaust note would change then the manifold pressure would reduce and as the manifold pressure went down the egt would sky rocket.

I just learned this part, may be true with yours may not be. With insite my egr temp sensor is listed as ambient air temp sensor. I have a thread asking where this ambient air temp sensor is and no one has the answer.

So I starte unpluging sensors until I found it. Try unpluging egr temp sensor and also look at your other sensors.
Inlet temp sensor
Intake temp sensor
Intake pressure sensor
Barometric pressure sensor.

Also your exhaust back pressure sensor if it’s starting to go bad this will also happen.