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I've noticed that my truck occationally will "choke" under acceleration (moderate to heavy throttle) and I'm not sure what it may indicate. First off, it happens very infrequently; maybe once a month. When it happens, the engine seems to 'cough' for a quick second and then all is fine. It only happens once at a time and then I won't do it again for a long time. When this happens, there is a slight effect upon acceleration (slows down) but once it clears its throat all is fine again. With all the talk about injectors being bad, I'm always a bit nervous.

On another note, I very rarely push the throttle down as I'm cheap and not in a hurry to wait behind someone but when I do get on it the sounds of the engine/transmission seem very...flat and confused. The noise is nothing like a gas motor under full power. My truck makes noise but the tranny is shifting like crazy and just sounds...different I guess. (Yes, I'm relatively new to diesels.) It seems to pick up speed plenty fast enough but it does not hold in a gear for very long at all like a gas motor. Does this sound right to the rest of you?
 

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is it like a "chug-chug" in rapid succession?

Most likely its turbo bark, which can be very bad for the turbo. Try to let off the throttle a little more gradually and it wont bark.

Ben

EDIT: I didnt see this was on ACCelleration. I thought it was on decel. It could still be turbo bark tho...the LLY's do funky things soemtimes on the 4-5 shift... Do a search for "4-5 shift burp"

Mine used to do it if the lift pumps were not on...could also be simple air bubbles in the fuel. Ever since I shimmed my pressure releif valve, it has not done it... supposedly there is a new reflash out for stock trucks that fixes the "burp" tho.
 

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Discussion Starter #4
The only mods on my truck are a cat-back MBRP exhaust and a drop-in K&N filter. (Simply replaced stock filter element.)
 

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Its the EGR thats causing your problem. GM is aware of the problem and has tried to fix it by reflashing the ECM. It can and will happen with a stock truck but it will be worse with power modules and larger exhausts. The problem is when you accellerate quickly the intake pressure exceedes the exhaust pressure and if the EGR is open at that point the turbo boost bypasses the intake and takes the route of least resistance wich is the egr port. Basically reversing flow through the egr and robbing all the air intended for combustion. If you dont have a cali truck just unplug the egr.
 

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This is at the center of the thread I started HERE. But I am not sure I see the dynamic you describe. Seems the exhaust pressure would always be higher than intake pressure, but have little to base this on.

If it wasn't, how would egr ever flow in the correct direction?
 

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Exhaust pressure is only equal to the backpressure of the system, and the turbo "sucks" in air on the compressor side.....wheter or not it is fresh or exhaust!
 

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My truck did this stock or with programmer at about 70% throttle until the blocker plate went in.
 

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The ecm only checks exhaust backpressure once at startup. If its insufficent at that point it throws a code for insufficent back pressure and disables the egr. At any point after startup the backpressure falls below the intake pressure the ecm does not disable the egr. The fix that gm has used was to program the ecm to command the egr to open less. This seems to me to render there egr practically useless in decreasing emmissions. The probably will redo the whole egr system in the new trucks if they havent already. Maby a quicker egr response with the ecm constantly monitoring the backpressure.
 

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The ecm only checks exhaust backpressure once at startup. If its insufficent at that point it throws a code for insufficent back pressure and disables the egr. At any point after startup the backpressure falls below the intake pressure the ecm does not disable the egr. The fix that gm has used was to program the ecm to command the egr to open less. This seems to me to render there egr practically useless in decreasing emmissions. The probably will redo the whole egr system in the new trucks if they havent already. Maby a quicker egr response with the ecm constantly monitoring the backpressure.
I am having issue with your post, just about everything in it.

What do you mean "checks backpressure"? And where would the backpressure sensor be?

With EGR, you are suggesting GM has altered the functionality of federal Clean Air Act equipment. Is there a recertification to go along with this change.

Please keep in mind I am just asking questions here, perhaps you can address them. Also, with your statements, you sound like you have some unusual quals, could you say how you come across this information?
 

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i am not sure how the ecm knows that the backpressure in the exhaust is higher or lower than the intake pressure but it detects the pressure when you start the engine and if the exhaust pressure is lower than the intake pressure then it throws the insufficient exhaust backpressure code and disables the egr. They know if the exhaust pressure is less than the intake pressure the air will flow to the point of least resistance which if the exhaust pressure is lower than the intake then the air will bypass the intake and flow through the egr port out through the exhaust basically dumping all the turbo boost just as if an ic hose had blown off. If you check the reflash info gm has reprogrammed the ecm to command the egr open a lesser percentage than origanally programmed. The egr is still operative but it has to be less effective because less exhaust gas is being recirculated. That doesnt mean that the trucks would fall below epa standards, but it would be interesting to see if they would. They may have just decided to compensate in other ways if they did not redesign the egr system, such as a new catalist for the new trucks. Anyway i dont know what gm has to do to make there trucks compliant but that is what is happening with the egr.
 

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...If you check the reflash info gm has reprogrammed the ecm to command the egr open a lesser percentage than origanally programmed. ...
How would a person go about doing this?
 

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I would imagine that there must be a sensor in the EGR valve (or connected to it) that determines if the pressure on the engine exhaust side of the EGR valve is higher than the pressure on the engine intake side of the EGR valve. Otherwise, would it not be possible for pressurized engine intake air to flow backwards through the open EGR valve and out into the exhaust system?
 

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