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excess boost pressure


Spring
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So I will try to explain this the best that I can and hope that it makes sense.  I have talked to the local mack dealer, and pretty much everyone that I know and I haven't found an answer yet.  First off my 2008 MP8 will build and hold 42 psi of boost on a good long pull.  It sits at idle with around 3 psi.  Sometimes in between gears or if i gave to stop really fast (like throw it in neutral and slam the brakes) my truck will be at idle with 12- 13 psi of boost.  the real weird one is this, lets say I'm driving the truck down the interstate at 65 mph.  the road is either flat or has a very slight downhill grade with the engine brake turned off, cruise control turned off, not on the gas brake or clutch, just free coasting down the road, the truck build 12 psi boost and start to slow the engine down.  The effect is not as strong as the engine brake on the low setting but it makes a similar although quieter sound and does very slightly try to hold back the motor.  The actual speed doesn't matter, the grade doesn't matter, and the gear doesn't matter.  I did replace to boost pressure sensor.   This problem has existed for a while but now and then more recently during a driving regen the turbo will spool up so hard the the boost gauge goes to max and something rather load pops under the hood.  any thoughts or idea would be great.  I really don't want to start out by replacing the turbo, but I will if it comes to that.

-Thanks

Spring

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That does sound strange but if you are holding high boost pressure at very low engine speed or even idle, it sounds as if the engine exhaust system is plugged, or restricted. I don't know if your pressure gauge is electronic, or mechanical as have no experience with the newer engines but I'd replace your existing gauge with a mechanical one and verify the problem actually exists. Of course an exhaust system that is restricted will run hot, power will be down, and I'm sure with all the sensors involved there would be many codes thrown.

Dog.jpg.487f03da076af0150d2376dbd16843ed.jpgPlodding along with no job nor practical application for my existence, but still trying to fix what's broke.

 

 

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Rob

Everything is electrical but the problem is not a bad gauge.  I know this because I can feel the back pressure on the engine when it shows high pressure when not on the throttle.  You mention problems that come with a clogged exhaust and I have none of those.  If that were the case the regen system would d-rate the truck and it probably won't even move.

 

Mr Grumby

The test you ask about was not performed but it sounds like a simple test.  If the turbo veins were sticking wouldn't that cause problem during all engine operation?  If works fine while pulling and jaking

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Probably a sticking VGT nozzle ring or actuator. 

The pop is unsupported boost dropping backwards out the intake/air filter hsg. Possibly when the VGT frees and drops out. 

Thats a guess, but I’ll put my money on the turbo. I can unhook a VGT linkage on external mount ctrl modules and build whatever boost I want at whatever RPM the engines running. It does labor the motor. 

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Mack Tech

you sound pretty sure there, I am a truck driver that tries to fix his own truck when possible.  I do not know what a VGT nozzle ring is...  Is that a serviceable or replaceable part?  Also where might I find it?

Mazidyne

I appreciate you input also.  Fortunately my egr cooler is relatively new.

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  • 6 months later...
On 1/4/2020 at 10:10 AM, Spring said:

Mack Tech

you sound pretty sure there, I am a truck driver that tries to fix his own truck when possible.  I do not know what a VGT nozzle ring is...  Is that a serviceable or replaceable part?  Also where might I find it?

Mazidyne

I appreciate you input also.  Fortunately my egr cooler is relatively new.

Not easily serviced by the average Joe ! If that what it is ! The other thought is a sticking EGR valve, Less common however a possibility!  Turbo issues are most common ! as Tech mentioned High on the short list!

Edited by fjh
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3 hours ago, Spring said:

I know this old, but i finally found the issue.  The rod that operates the EGR valve was completely broken.  It was still actuating like it was supposed to, so it never threw a code.  This was not allowing the exhaust to escape properly and was building manifold pressure.

Great find Spring!

When we get this on our D13 "volvo MP8" it generally comes over with high NOx causing a code. The EGR cools the burn, no EGR + no cool cylinder burn = High NOx output from lack of exhaust mixing. The tier 4 final can't handle high NOx resulting from an EGR failure, it was designed to work along side the EGR system and can't work properly without it. Your 2008 interim 4 must be able to cope indefinitely with an EGR drop-out failure like you saw. Thanks for following up, it teaches us a lot.

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Yup good find for sure ! egr valves don't  seem to throw up codes  we had one a few years back stuck closed from the get go had all sorts of other egr related issues ! 

Very few issues with the valve it self usually input issues!

 

 

 

 

 

 

 

 

 

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