Cummins ISX CM871 Technical discussion

you need insite for it, but if you had the DPF cleaned out and have the aft-treatment injecotr working good, then that is not likely costing you any significant fuel mileage loss. Just make darn sure your fuel line going to the doser injector is not leaking (it should give an alarm for this, but not always). 177k miles is a low mileage for those kinds of problems. usually they do not set in until 200-250k+. I am thinking that its something else that is costing you fuel, unless it is still a bad IMAP or EGR valve itself possibly. 8500 hours on the motor is a lot tho. must have had the snot idled out of it at one time. My truck had 427k miles on it before it had 8500 hours. I drive 55-60, and have never idled it once, so that should give you a ballpark on how much it has been idled.

Do you top off you tanks really hard?,...If so, it could be costing you fuel mileage. As you fuel expands in the tanks from being heated by the motor, If there is no more room in the tank, it will bleed out the top of the tanks. That's what causes those black streaks down the sides of the fuel tanks. Also, if you top off, then climb a steep grade shortly after, then the same thing can happen as the fuel gets pushed to the one end of the tank where the bleeder tubes are.

Also, are you fuelling the same places every time?,...I found that if I fuel in ogden,UT my fuel mileage always sucked, but 20 miles up the road, and my fuel mileage was much better. Can anyone say 'Pump Error'?. I had a guy who uses a stick to measure his fuel levels tell me about this problem. I started paying more attention to it after that.

I have taken a couple turbos apart that the face bearing and the inner shaft were totally carbonized from trucks idling hours endlessly. This can cause the turbo speed sensor to read erratic if it gets bad enough, as well as cause the turbo to spin slower than it should, costing you fuel from excess back-pressure. Usually you can check for this if you remove the speed sensor, then spin the turbo slowly by hand until the notch in the shaft aligns with the hole, and check to see if there is carbon build-up inside the notch of the shaft.

I also wonder what the fuel mileage was when the engine was new, or fairly new when it wasn't being idled. This can sometimes tell some of the story as well.

Your fuel shut-off solenoid could be weak. They have been known to cause fuel mileage loss through the fact that it heats your fuel unnecessarily combined with slight to moderate pressure drops.

Is your fuel mileage noticeably higher in colder weather?,...maybe your fuel is getting hotter than it should. Your tank temps should be watched closely if this is the case. Max inlet temp for fuel to the IFSM is 70 deg c/ 158 F. If its running close to this, then that will definitely cost you fuel mileage. Some trucks have an option for a fuel cooler because of this.



Just some more thoughts on it,,...Rawze

 

A reference for you and others,...

going down the road,...

My fuel rail pressure reads between 18.5 and 19.0. It goes UP to 19 when I plant my foot in the accelerator as RPM's climb.
It also reads 1.8 with the engine not running and the electric pump is active.


My fuel temp reads 57-59, no higher.


Calterm fuel rail parameter,... [ _Fuel_Rail_Pressure ]

Calterm fuel temp parameter,... [ _Fuel_Temperature ]


What is the AV1????.?? on the ECM of your motor? I think that some engines might just be programmed crappy. I have seen a lot of variation in EGR settings among different engine cals. Some for no apparent reason it seems.

 

There is always a solution to abnormally low fuel mileage. It just has to be found.

They say that whenever you look for something, it will always be in the last place you expect it to be...

Science agrees with this totally, and supports that theory,...

because when you find it,....you stop looking. LOL