ISX fuel mileage help

[jmickulin]

New to the forum and looking for some suggestions. In March I bought a 2007 Peterbilt 387 with an ISX Cummins in it. I pull a reefer from Idaho to Arizona, then to Colorado and back to Idaho. The truck gets terrible fuel mileage and I am almost at a loss with what else to do with it. When I bought it and ran it on the first trip it got 4.5 mpg. I took it in and had an overhead done which brought it up to 5.1 mpg for the next trip(better but not great at all). After a couple trips and a little research I unplugged the EGR just to see if i saw any improvements. Besides my reefer not being covered in soot, the first leg of the trip i got almost 6 mpg. Unfortunately, on the second leg of the trip I was down to 5.5 mpg, and right back to 5.2 mpg by the time I made it home.

I found this forum and started doing more research. I saw a post that talked about the IMAP sensor and cleaning the intake tube out. I pulled the sensor and it was completely plugged off. I cleaned the intake and went and bought a new genuine cummins IMAP sensor and replaced the intake temp sensor while I was there. Well, first leg of the trip I managed 7.9 mpg and I was JACKED!! But, like each time before, the second leg of my trip was down to almost 6 mpg and by the time I fueled again at home it was 5.4 mpg. I have been on 3 more runs since and I am sitting right at 5.1-5.2 per trip. This last run I plugged in the EGR again just to see what would happen with a functioning EGR and new sensors and it was right back down to 4.7 mpg.

I don't understand why every time I do something I get a temporary gain in mileage and then it just drops right back down no matter what I do. I talk to guys pulling reefers with ISX's and none of them get near as bad as mileage as I do( I know its hard to compare 2 different trucks). The truck has a 13 speed and 295/75 22.5's and I need to check my rear end ratio. I run 100 psi in all tires except the steers in which I run 110 psi. I keep the rpms under 1450 and try to drive by the boost and pyro gauge. What's your thoughts/suggestions on this? What are my next steps?

[Rawze]

Is this an 870 or 871? -- sounds like possibly an 870? if you had a separate intake temp sensor.

870 is notorious for egr valve sticky and problems because they suffer heat issues / poor design. - unplugging egr valve will not guarantee it will stay closed, it may drift open after a while. Also 870 will give turbo derate and make turbo overheat, shorten its life (boost will spike 36 or so psi then fall back to about 31-32). -- It takes proper programming to prevent this.

for troubleshooting it ...

only way to switch off egr temporarily and get engine into proper non-egr mode is to unplug the egr temp sensor, not the egr valve like some social media morons would have you believe.

If you unplug the egr temp sensor and have egr piping pulled away from intake manifold, egr gas flow should stop right away. if gas is still coming through, then the valve is not closing all the way. I think about all 2007 and newer versions of isx (870, 871, 2250, 2350) with egr this test can be done to see if valve is closing properly. Flow will not usually be totally zero, but it should be very little compared to when it is plugged in.

When is the last time you replaced the Delta-P sensor? -- the exhaust back-pressure sensor? - cleaned out the cross-pipes for the Delta-P?

[jmickulin]

Yes it is an 870. I haven't done anything yet with the Delta-P sensor or exhaust back pressure sensor. If those sensors are like the other two I already replaced they have probably never been done. I will pull the EGR pipe away from the intake and unplug the EGR temp sensor and see if that stops the flow. Do you think proper programming is something I need to look into as well for mileage? As far as I know the truck is still set at all of its stock settings and hasn't been tuned or anything.

[Rawze]

mommaburt or unilevers would be the 2 people on here trust-worthy for decent programming of a CM870. Although there are many places that claim to -- It is very rare indeed to find someone who can program the 870 correctly otherwise, and your problems will only be compounded.

I would try to solve your problems before any of that sort of thing is done to begin with though .. there is no program that can fix hardware issues, especially issues that cause lower then normal fuel mileage etc.

[Chamberpains]

Also your exhaust back pressure sensor and piping and housing I need to be free and clear of soot. Replace the sensor and clean all the soot and garbage from the openings. There is a little stainless steel tube that runs from the exhaust manifold to the back pressure sensor housing. Make sure that is free and clear. Run a pipe cleaner through it.

What are your boost levels getting to? Do you own Cummins insite? If not that should be way up on your list of things to purchase. You're doing a lot of elevation changes in your route out there in the west. This could also be a barometric pressure sensor issue. You need Cummins insite to be able to monitor what your sensors are putting out and reading.