PLEASE help me! VNL670 with an ISX CM2350 4.4 MPG!

[VNL670]

(06-26-2018 )Rawze Wrote:  Just for reference there hammerhead, on a 2350 with egr still active, a couple psi higher than a demandated truck if it is using a lot of egr gas.


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What is the rear ratio of the truck? What is the mileage? - I did not see it mentioned.

Like hammerhead has said,.. has anyone verified that the truck has the correct program in it based on the engine ser#?. I.E.> Make sure someone did not "up the hp".

* Has anyone bothered to look at the decomp tube and verify it is not partially clogged? Reference: http://rawze.com/forums/showthread.php?t...07#pid1807

* Has anyone bothered to see if the Doser injector behind the turbo is leaking fuel?

* Has anyone bothered to pressurize the CAC and check it for leaks? -- How about pressurizing the entire engine between the intake filter and the back of the turbo and checking it for leaks as well?

* Does the truck have an after-market CAC unit on it? - Many brands of after-market CAC uints are very restrictive and cause fuel mileage losses.

* Does it have an after-market EGR cooler on it? -- Same thing, they can rob you of fuel mileage and in fact many have even caused engine damage due to excessive egr temps.

* Anyone bothered to pressurize the fuel tanks and check the lines for leaks?

* Does it pass a leak-down test? Reference: https://youtu.be/eH9GRkeozTM

* Anyone bother to flow-test or even inspect the the SCR unit for clogging?

* Anyone bothered to do valve adjustment?

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It the mean time ...

Like Hammerhead said, slowing down to 63~ish will help -- A LOT!. So will driving by your boost gauge and limiting boost to about 15 psi or so as a good start. I am not saying this as a replacement to any fuel mileage loss problems the engine or truck may have,.. but it will definitely help mitigate losses. This is especially true while you figure out how to get the fuel mileage back up to normal levels. Better driving habits always lead to higher profits and less down time.

Hi Rawze. I rear axle ratio is 3.420MT40-14X*C and the current mileage is 429306. I updated the ECM from EF10064.34 to EF10064.39 & with no improvements to the MPG. I just wanted to make sure everything is correct and not tampered with in the ECM. And in regards to the SCR screen, Yes I had a similar problem which I fixed by replacing the DEF doser valve, but mine was not as bad. Now the turbo injector/doser I have not took apart yet & I will do that after I check the CAC with soap & water on Monday or Tuesday. I will also see if I can get it pressure tested from the turbo as you mentioned. The CAC on this VNL is OEM & so is the EGR cooler (it's red like the engine). Please keep in mind that the highest EGTs I get from this truck even during REGEN or PARKED REGEN, is 720 degrees fahrenheit. I did get under the truck for fuel leaks, but did not find any what so ever. Not even a minor leak or even at the engine. Now the mechanic that built my fuel pump and switched out the ceramic rods to metal rods, did the injector test and found no leak, so he replaced the fuel rail sensor and that issue was fixed. But i will have him do it again just to make sure. I honestly don't know if the SCR was checked for clogging or not. I will put it on the list along with an overhead valve adjustment.