I have 2008 volvo vnl 780 with d12. Turbo is not boosting. I beleive it is a fuel problem. I have changed boost censor,

Hello, My name is***** am a Volvo/Mack master tech. Allow me to try and help you out here. You had said that you checked the engine brake. Can you tell me exactly how you had checked the ex brake? Thanks

Are you sure that the epg control valve is not sending air to it while the key is on or engine running? Is the ex temp raising above normal? like a plugged muffler? Do you have PTT?

And it is running fine all the time just lacks power? Has a cylinder balance test been ran on this with the PC? The other thing I have seen and checked is the preheater block. If there is studs coming out of both sides then it has the heating grid inside. At times when the egr cooler fails it will put coolant out of the cooler into the crossover pipe in front of the valve cover and will soak the preheater coils which will start to collect soot which will restrict air flow into the manifold. I haven't seen any issues with the VCB solenoids sticking on them. If it had an egr valve sticking under a load you would have a fault for that, either a sid 146 or 266. If you pop the cac clamp off of the cold side at the black cac hose you can lift it out of the way and look down the mixer for any build up

Okay, If the preheater is good with no restrictions then that will rule out your intake restrictions. You had said earlier that there is no cac leaks so that rules out that portion of the 4-strokes. On these D12D's we have had a handful of trucks having a very similar issue that needed the injectors replaced. The needle control valves inside will wear and its weird how it happens so soon. This is usually a last resort after everything else is ruled out of course. The unit running good will not signify an overhead issue so I would not be concerned about that. The only other thing I can think of at the moment is the turbo mounting gasket. They had restrictor plates in them that were made out of ceramic that would break and they would fall down inside the turbine housing and chew up the ex fins, If that was checked when the epg housing was off I wouldn't worry about that. If all of that is fine then I would replace the injectors in it. You have to have good combustion to have good boost pressure.

The injector needle control valves being worn out will not cause a starting issue, That is when the cups leak or the tip of the injector is cracked. These injectors were built when it was just low sulfur fuel, Then in 2007 the ULSD came out, sulfur being a lubricant was dropped down even more and when a proper amount of additive is not introduced it will cause wear over time. Inside these injectors there is also a spill valve that can wear out which when it is bad it will cause the injector to dump some of the supply fuel on that stroke back out of the return. The stumbling is often seen on the d13, d11, and d16 Volvo engines. That is a totally different injector and different injection pressures. It wont smoke because the ecm is adjusting the fuel for how much boost it has, which says that the ecm is not the cause. If the ecm was causing for less fuel it would smoke under a load because you would have more air than fuel, The only ecms I have ever seen bad on these d12D's is for a injector driver, one for egr valve fault, and one that had a 5v supply circuit shorting out. I have been working on these engines since 1999 when the D12B's were out. We have had to put injectors in them as well.

One other item I would check as well. I know you said the boost pressure sensor is new. That has a lead that runs into the vector box which still runs thru the engine harness. I would unbolt it and take a pressure regulator and a small hose that will fit around the tip of the sensor and apply air to it. Use a gauge regulator and match the gauge to the sensor, try and reach up to 20 psi and see if the matches the reading. This will help in checking the integrity of the harness and make sure the return circuit is good on the sensor, Just so that the ecm is getting the right info from the sensor.

We as a dealership do not have a way of checking them like that. We can only run a balance test and load the unit up to check. plus just cutout test. We have a injection pump shop close that claims they can check them, so far over the last 18 years we have been sending injectors to them they say that they are junk. One time we sent a brand new injector off the shelf(known new) and they said it was bad. Unless they take the injector apart and use a micrometer on all of the parts and a magnifying optic eye, and microscopic tip tester, they really cant tell anything. The problem is on all these injector systems anymore there is really no tolerances. No room for error.

Well that is also checking before the overflow valve, Did you have any air in the return flow?

Check the spring in the overflow valve then, See if it feels like it is hanging up. It should have steady spring pressure on it. Either way if its getting non-aireated fuel supply to the engine that is above normal it will not cause a low power issue. Did you check the pressure at that point you connected the hose to?

So you had 70 psi at the top of the cylinder head at the fitting? or on the filter housing at the sensor? It should be checked at the head location as well, so you can isolate the complete fuel delivery

Here is a complete overview of the fuel system on that engine. It may have a standard fuel filter housing on it without the primer pump. The location and the direction of the fuel remains the same.