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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8726
Experience:  I am ASE Master Truck Certified.
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I have a 3406c only 15 psi boost I have turn screws all the

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I have a 3406c only 15 psi boost I have turn screws all the way out both, replaced all fuel lines and filters,still no change ,it has a little black fuel smoke on low side of trans however nothing on high side,timing is pinned correct all nozzels are good and truck runs as fine and smooth as silk no power on high side if I hit overpass it falls, brand new frc installed and turn all in,no change.fuel and air screw almost all out.i hooked fuel pressure gauge up today 38psi at low and high idle however fuel to the floor bobtailing it drops to 20psi to 25 under full throttle, any ideas
Thanks for asking your question. I will try and help you out.
It sounds like the fuel pressure is low. Normally the pressure should stay between 30-40 psi. When it's low, the plungers in the fuel system will not get completely refilled and cause low power and low boost.
Also, I assume your engine has an air to air cooler. I would also have it pressure checked for cracks. I have seen many crack and cause low boost.
I would also check the fins on both sides of the turbo. Also check the turbo for excessive end play.
Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 7 months ago.
No black smoke when floored,and I know fuel makes boost so in your opinion fuel transfer would likely be the problem?
I would suspect the fuel transfer pump or a restricted fuel supply or return line. Either fuel line can cause low boost and power if restricted.
Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 7 months ago.
All new fuel lines and returns will change transfer pump in am and let you know
Sounds good.
Let me know how you make out.
Cat Man and other Medium and Heavy Trucks Specialists are ready to help you
Customer: replied 7 months ago.
Changed transfer pump today.
Still the same.
No black smoke no more boost.
Fuel pressure holding at 38psi.
Thanks for the information.
If the pump did not help. I must suspect wear in the governor.
The governor has flyweights and races inside. When they wear they flyweights will hang and not allow full fuel.
Customer: replied 7 months ago.
Ok great
You are very welcome my friend. I am happy I could help.

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