CM871 fuel mileage

[superbudy208]

Got a Question about fuel mileage on a 08 pete 387 with the CM871, fresh service all new filters new imap and baro and averaging 5 mpg in the last 5000 miles always loaded about 70k does this seems bad??

[SquareOne]

What are your driving habits? Whats the trans ratio as well as the rear ratio? Are you keeping the boost 10-12lbs and lower most of the time? Are you lugging the engine down or are you letting it turn up 16-1700+rpm between shifts while loaded or climbing?

All these factors will greatly affect fuel mileage.

What do you haul? Type of trailer? How much deadheading?

We need much more info to give an accurate answer. At first thought, yes 5mpg with 70k does seem very bad unless you're pulling some ridiculously non-aero load.

[superbudy208]

Ok I thought so, I try to cruise around 68 to 70 @ about 1450 rpms, the truck has a eaton ultra shift 10 speed auto, with 3.36 rears and 22.5 tires, I try not to lug it, it runs the hills well at high rpms, I have a 53 ft flat bed and have been to hauling a lot of vinyl fence that is pretty square and most of the time it shifts around 1500 rpms and will down shift around 1200, I don't really go over 1500 very often it don't seems to like running that high rpms, I've tried running 63, 64, and found the mileage was the worst its been

[superbudy208]

Also i dont have a boost gauge installed yet or egt

[Rawze]

(05-09-2021 )superbudy208 Wrote:  ...I try to cruise around 68 to 70
...

If you slow down a bit... to 63~ish it would help tremendously.. and stop using the cruise control. - Learn to keep the door shut more and don;t stop once you pick the load up.. narrow down that window of driving speed vs. average speed. Ideally you want your driving speed to be within about 7 mph or so above the overall pick-up-to-delivery average based on the total miles.

I.E.> if a UPS, fedex, or other load averages 50 mph (50 mph is an industry standard for average delivery times LTL), calculated from pickup time to its delivery time, based on its total miles (most of them do) ... Then you should be able to drive it at ( +7 to +8 mph above this average) 57-58 mph down the highways to get it there. We do this a whole lot ... and we are very, very good at it. if you find yourself driving +12 mph or maybe even +14 mph faster then the total mileage average calls for.. then that is simply poor planning, poor driving habits, or some other issue of one sort or another that can be eliminated so that you can push for better fuel economy and higher profits.

(05-09-2021 )superbudy208 Wrote:  ...I try not to lug it...
1500 rpms and will down shift around 1200, I don't really go over 1500 very often
...

your absolutely torturing that engine... and it sounds like the Auto-S$itter is programmed horribly to limit power output by lugging the engine everywhere its whole life. - That engine was made to run between 1100 and 2100 rpm ... Anything below 1500 or under while heavy load is torturing it in the bottom 1/3 of its working range though, and beating the h$ell out of the liners and bottom end. The way you describe driving it is exactly how people torture an ISX and then wonder why they have big problems and a severely shortened life out of one. Not keeping it in the higher RPM ranges when heavy is how people loose liners and end up with problems out of an ISX. - and like you claim... if if does sound of "feel" like it does not want to run above 1500, it is because no one has run it there much ... which is horrible.

That engine will last much longer if you use those higher rpms, making less wear over time vs. high torque. It is higher torque and lower RPM that wears the ISX and other 15-litre engines the most. A person could get into your truck and drive it for the next several years at 1700 rpm everywhere it goes, drop[ping to lower gears to keep the rpm up when pulling, and the engine will last twice as long as someone always trying to keep rpm to a minimum, and below 1500, working it hard with high torque demand.

-- That thing, when pulling hard against a big hill or mountain, really anything more than about 40% on the fuel pedal ... should shifting be in the 1500 - 1850 rpm range... not down below 1500. - If it complains, like you said,... its because no one has driven it enough up there ... Replace the crank shaft damper if it is more than 5 years old, or has more than 500k miles on it... and BREAK IT IN UP THERE IF NEED BE ... and stop trying to torture that thing everywhere cuz it makes you "feel better" ... that engine will do it, as it is hard on it at high torque below 1500RPM!.

So... this means that you need to re-learn how to drive that thing much better... and YES it will save you fuel if you learn to drive it at higher RPM when pulling ans LESS BOOST! and better gear ratio .... instead of torturing it with a huge bunch of torque at low rpm. there are about 50 or so people a year that come on here, many learn this the hard way after an engine failure, fretted liner, or blown head gasket due to that kind of torture ... and heave to re-learn how to drive their tucks.

and oh yeah... DRIVING BY YOUR BOOST GAUGE by keeping it below 20-PSI or lower if you can.. and using higher RPM (1500 - 1900) and actual power with a lower gear range instead of higher torque and lugging it below 1500 when climbing a hill.

(05-09-2021 )superbudy208 Wrote:  ..
I've tried running 63, 64, and found the mileage was the worst its been

This generalized bulls$it of "I get worse fuel mileage by slowing down" is just that... something is wrong with how your driving it (too aggressively at lower speeds),.. possibly your lugging it terribly and creating a lot of inefficiency, and/or the engine itself needs to have some issues solved ... because slowing down and it consuming more fuel goes against the laws of psychics and thermodynamics. - Slowing down DOES NOT CONSUME MORE FUEL unless something is sucking it up improperly. Possibly...

A). Because you have horribly-spec'd rears for a 10-speed and driving a pete 387 in its Aero-sweet-spot of 63~-ish mph. Your geared wrong and doomed to lug the engine at 1300 or so with high torque if you drive it in top gear at 63~ish with a set of 3:36 rears. Those rears are terrible when considering a 10-speed is in front of them. No way to split the gear and fix it like you could do with a 13-speed.... even tho this still would not be optimal. BETTER SPEC REARS is the answer.

B). You have an Auto-Sthitter tranny which is going to take away roughly 0.6 mpg for every mile you drive, simply because a decent driver can out-shift that thing and save that much fuel all day long.

C). Depending on the health of your emissions and egr systems, if it often found that when its in bad shape and in dire need of an EGR-Tune-up .. the fuel mileage will go down when you slow down... (seeming against the laws of physics itself) ... simply because the engine is not breathing well and because sensors like the IMAP and exhaust pressure sensors are giving incorrect feedback. -- PERFORM an EGR TUNE-UP!..

This leads to the next couple questions ... Is this thing De-Mandated(deleted)./.. and if so, by who and what was done mechanically and who's programming is in it??

-- Sound to me so far, your journey towards owning that truck has just begun... I grantee you I could get a hold of that truck and get it into the 7+ mpg range If I drove it, worked on it, and took your freight. - It is a matter of knuckling down, solving all its issues, Having the discipline to SLOW THE H34ll DOWN A BIT to narrow the avg. speed vs the drive speed to less than 7 mph difference during the whole driving shift ... and making any changes to get me there like putting a set of 3:79's or 3.91's in it and re--programming that tranny to let her run up to 1850 like it should be, etc..

-- If I could do it ... so can someone else....

However, I would have never purchased a truck with horrible rears like that, especially with a 10-speed Auto-shitter in front of them. Whoever Spec'd that truck initially had their head up their arsse,. or was one of those fleets who could give a damn about the engine, was willing to lug and torture it to death in attempts to slow their company drivers down. - its like shooting yourself in the foot before the race and then wondering why your slow and dizzy half way down the track. Fleets are notoriously bad for spec'ing trucks like this these days.

I haul 80k Water loads for Nestle out of Pennsylvania across I-70 ... even in those big hills and 80k loaded... I still break 8.2 - 8.4 MPG. My engine never sees anything below about 1500 the whole time I am in those hills.

I haul loads of Wire from South-wire out of Kentucky that is in excess of 76-78k... down into Georgia across Mont-eagle mountain... AND I BREAK 9+ MPG CONSISTENTLY WITH THOSE LOADS.... so yeah...

So as far as I see it ... 5 mpg is horrendously bad for an aero truck, even with flat-bet freight and that 70 MPH every where you go does nothing but fund these guys personal islands!.



I Don't know about any one else.. but i think they are rich enough .. they don't need my money too... I think i'll learn how discipline myself to keep that money in my own pocket instead of wasted fuel.


- Drive it right.. Drive it by a boost gauge.. Use better discipline... keep the door shut and keep it moving so you can go slower and still make the delivery .. Fix its issues, Make changes for better fuel economy, etc.etc.etc... because the BIGGEST fuel mileage issue at the end of the day ultimately ... IS THAT NUT BEHIND THE WHEEL! (yourself).

[Waterloo]

I'm one of those past luggers, I had to relearn how to drive too... Now in the high 6's with a mid roof pulling a van. If I had a flatbed, I could do 8+ all day long.

[Rawze]

I think that one of the big problems that people do not realize is that the fuel mileage they get is directly correlated with the amount of engine wear and when the engine will likely need an in frame/rebuilt.

- Control that #1 expense (fuel mileage) like a hawk .. above all else and that #2 expense will drop dramatically (maintenance and repairs). Someone wants more profit for their loads... all they gotta do is look in the mirror.

On the rare occasion that my truck drops into the 6 or 7 mpg range... I start pulling things apart, testing everything I can think of, replacing sensors, pressure testing things, and freaking out. That s$it is gonna get fixed right away, because to me, all i see is a "drip, drip,drip" of money oozing down some imaginary drain, keeping me awake at night until its found.

[superbudy208]

WOW thank you for all the INFO RAWZE this is why I'm here!

When I slow down to 63ish the mpg's get worse, so that's why we try to run 68 to 70 is not to lug the engine end the mileage is not that much worse usually around 4.8 to 5 mpg's , the last tank at 63 was 4.44 mpg's, and 95% of the time the truck is loaded with the most deadhead being around 100 mile if that so far.

When were running the hills it does a lot better at the 1500 rpm range and try to keep it up there in the 1500 rpm range, and yes the balancer was the first thing I replaced and performed a overhead before I ever hit the road

I will get the boost gauge in this week and may have to run the hills in 9th gears about 60 to get the rpms up around the 15 to 1600 rpms range, it will run the hills well at 75 mph in the 15 rpm range, it will shift on hard hills around 1650 rpms but it don't seems very happy about in then jumps the rpms down to the 1300 or so range. and yes the mileage going down with the speed was a shocker we've been trying different speeds an watch the mileage

I will start looking for a better set of rear gears what gears would you recommend??
i do understand the auto use's more fuel, but once your on the highway it stay's in 10th or 9th if we hit a hill and loose speed and the door does stay shut unless its for fuel about every 900 miles or so depending on the price along the route's.

the EGR valve and cooler were replace not long before I got the truck the IMAP was plugged up so replaced that also found the BARO sensor was plugged and the first full tank after replacing the BARO was the best at 5.6 but has gradually got worse each tank, and I made sure all the ports on the exhaust differential pressure were clean and clear, but I have not checked the exhaust back pressure sensor yet. I do have a nexiq code reader and did notice when I pull the data there are some important things missing like intake air temp and intake manifold pressure say not available so I'm kinda leaning towards some tuning issues also on top of some other isssues.

No the truck is not de-mandated but the ECM and the fuel control module has been replaced when the engine got an in-frame 285k ago, and yes this is my first truck and yes its just beginning, and yes after every run I try something new and spend my off time working on it and that is an issue I enjoy it but coming to a loss on the fuel mileage at the moment, i never really thought about the spec's of the truck when I bought it! (stupid me) but I did not know I've been working on the on and off for the 5 years or so mostly service work, never any issues like this, so its been interesting to say the least.

Where are you located i would love to bring my truck to you and go over it! this fuel mileage is making this tough!

[Rawze]

Those 70+ mph speeds HAVE TO GO!.. learn to driver it 63~ish with better gearing and far less boost all the time.

What program does Insite say is in the ECM?.. and what is the engine ser# (the ESN#)?. Maybe a post pic of the data-plate if your not sure?

and what hard parts were installed during the inframe... that would be my next concern, as many places get the part#'s wrong and then your engine can suffer efficiency loss or other issues.

Also.. was the head re-worked?, or was a new OEM head installed?. Check the drivers side back corner near cylinder#6 .. If it is an RX or PX + part# and build date should be stamped into the head there.


will it pass a pressure test?...heck.. will the entire engine, intake, etc. pass a pressure test?. Anyone bother to pressurize the entire engine to 30 psi, front and back of turbo and check for any leaks?

Anyone did a second overhead valve adjustment 50k after the inframe?

Since fuel mileage is low, anyone bother to throw a set of wedges in it to see if one of the cams might have slipped?

A lot of unanswered questions there... and all related to fuel mileage.

- Those 386's (and 387's) are also prone to getting a fuel leak right over the exhaust or above the driveshaft and drip fuel while going down the roads, where no one can spot it without a trained eye .. Slowing down obviously will make lower fuel mileage if fuel its pouring onto the highways all day long. - Just another thing to add to the list of checks.