Exhaust pressure- CM871

[Marajin]

(03-05-2016 )Brewster64 Wrote:  I bought a used 08 and the truck has some serious gremlins. I've done the EGR tuneup, changed some sensors and plugs, and had a few repairs done.. here's the thing, the truck has been very inconsistant. Fuel economy is all over the map ranging from 5.2 to 7.8mpg, load and terrain don't seem to matter whatsoever. Driveability is all over too. Engine doesnt idle great either, surges if you bump it up. Truck will at times pull well at 38 - 40 PSI man press and shift and drive fine, other times it'll pull to almost 50 psi but isn't running shifting worth a damn. In both these instances it may or may not be having an overheating issue. At times both oil and water temps will spike fast when pulling, I can pull it back with the fan. Other times it does fine and just climbs slowly to about 220 oil temp and 210 water temp.(Normal) At other times truck seems to to derate where it will only boost up to 28 PSI, BUT it runs, drives and shifts great, with no heating issues at all. The other day i noticed a small amount of soot on steel line that goes from exhaust manifold to thermostat housing, so I tried to remove line out on the road but the manifold side was not coming out. I ordered a replacement line at my local cummins shop so I could feel free to break it when I got home.Got home, and attempted to remove line,a lot of peneterating oil and heat but still wouldn't budge, just skipping by on flange wrench.I removed the top of the line so I could slide a box end onto it and hmmm, I can wiggle the bottom flange about 1/16" in and out of the bottom fitting that wont budge. Finally removed bottom fitting(And it was super tight all the way out) to find that someone had crossthreaded it and it never had sucked the flange tight.( Read major pressure loss) I repaired the threads , replaced the line, I had already replaced the exhaust pressure sensor. I was originally going to remove line to make sure it wasn't soot plugged, it was clean. I haven't run with it yet, but the truck now idles smooth and a quick bobtail around seems like its running correct, I may have found my red flag! Lets hope so. Remember it's not always software, don't overlook the hardware..

41-42 PSI or more boost is too high for any CM871 engine, no matter what the HP is and will start to cause damage. .. but 50+ psi boost is extreme overboost and can crack the head, cause injectors to leak, cook the motor, damage and overheat the turbo, crack pistons, and do a lot of other engine damage.

* it can be caused by -- A bad/misreading Exhaust gas pressure sensor (the tube your fighting is the feed-line for it).

* it can be caused by -- A VG turbo that is failing and is now unstable.

* it can be caused by -- A blocked or failing EGR circuit where the Delta-P sensor is reading too low, or a bad/misreading Delta-P sensor. i have also seen cracked crossover tubes or cracked mounting blocks do this too.



* it can be caused by -- A bad/misreading IMAP (Intake Manifold Pressure Temperature) Sensor.

* it can be caused by -- BAD ECM Programming. Especially if you have a bad EGR delete by one of the big-name companies out there.


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An external pyro and boost gauge is your best friend in letting you know where your fuel mileage and power is going.


This future is a very possibility for you,..
If you were driving it under medium to heavy torque loads and fighting that much overboost, then I see an engine failure in your near future. Give it a month or possibly three. That is how long it takes for combustion gases to drill a hole through, and into the bottom of that now cracked piston you may just have in your engine there after driving it like that.






Hopefully you got lucky,.. but i would not bank on luck if you drove it like that for very much. - I would schedule some downtime, do an injector leak-down test, and if any injectors were leaking, pull them all and boroscope that thing to ensure it didn't get damaged.

Tags:

isx boost, overboost, excess boost, bad delete, cracked piston, bad exhaust gas pressure sensor