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Old 11-02-2011, 12:26 PM   #1
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5.9 Cummins fuel consumption

I'm new to Monaco and to diesel pushers, although I'm not new to motorhomes. Our new (to us) 1999 Diplomat 38A is our 4th motorhome. Nor am I new to the 5.9 Cummins engine, since I have one in my Dodge 3500. I've read numerous reports of folks getting 10+ mpg in 38' Diplomats with the 5.9 coupled with the Allison MD3060 tranny, pulling toads (toweds? A term new to me!). We just returned from our first outing in the Diplomat, drove some 4,200 miles pulling a Jeep Wrangler, and never saw more than 8.4 mpg with a range of 7.2 to 8.4. We were in both flat and hilly country and the 8.4 mpg, of course, came in flat country. I drove a steady 60 - 65 mph, mostly 65 yielding an engine speed of 1800 rpm. Engine is smooth, no smoke, proper boost, etc., etc. Should I expect better mileage from this rig, or am I reading exaggerated reports of good mileage?
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Old 11-02-2011, 01:02 PM   #2
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Hello kwkerch, I have the same rig and also pull a Jeep Wrangler. I stay round 1800 RPM with the occasional 2000 to 2200 range depending on traffic conditions and I'm getting around the same mileage you're reporting. The generator doesn't seem to have any effect on my mileage.
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Old 11-02-2011, 02:11 PM   #3
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I have a fleetwood discovery 37v with the 300 Cummings. I pulled my Tahoe 1100 miles and averaged 9.8 to 10.4 mpg.
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Old 11-02-2011, 02:27 PM   #4
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I think Your results are pretty normal for the speeds that You are driving and the Towed Jeep. Slow down a bit to 55 MPH should get You another 1-2 MPG.

We did achieved a 10.5 MPG average with Our Rig on a 6,500 Mile Trip , but the Normal seems to be in the 7.5~8 MPG range.

Ours is the C 8.3 Cummins so a bit bigger, pushing a slightly heavier coach!
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Old 11-02-2011, 04:49 PM   #5
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slow starts

You can save alot by starting out slow ,no jack rabbit starts if you can keep rpms at 1500 you will save alot.
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Old 11-03-2011, 09:48 AM   #6
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I have a Cayman 2009 and after 3,000 miles, my average is 8.4 MPG towing a 20' trailer at around 65MPH. I found that slowing 10MPH made a big difference.....
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Old 11-03-2011, 10:21 AM   #7
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Mine is a home built RV. it weighs 13-14k empty.
94 F-700 190 HP 5.9L/6-Speed manual trans
Total weight 19K-20k (RV+trailer with Jeep)
It get 10-10.5 at 60-65Mph
9.5mpg @ 70mph
in the campground before leaving east.

on I-40 crossing texas
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Old 11-04-2011, 11:54 AM   #8
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The coach I had befor this one was a Fleetwood Southwind diesel pusher with the 5.9 in it. I loved that engine!

I got 9.5 to 10 mpg at 60-65 mph, and that dropped to 7.5 to 8 at 70-75 mph.
Our coach was only 35ft, no slides, and we never towed anything with it.

Adding the extra few feet on your coach and pulling the toad it sounds like you are right where you should be for milage.
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Old 11-04-2011, 06:42 PM   #9
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Quote:
Originally Posted by dwkerch View Post
I'm new to Monaco and to diesel pushers, although I'm not new to motorhomes. Our new (to us) 1999 Diplomat 38A is our 4th motorhome. Nor am I new to the 5.9 Cummins engine, since I have one in my Dodge 3500. I've read numerous reports of folks getting 10+ mpg in 38' Diplomats with the 5.9 coupled with the Allison MD3060 tranny, pulling toads (toweds? A term new to me!). We just returned from our first outing in the Diplomat, drove some 4,200 miles pulling a Jeep Wrangler, and never saw more than 8.4 mpg with a range of 7.2 to 8.4. We were in both flat and hilly country and the 8.4 mpg, of course, came in flat country. I drove a steady 60 - 65 mph, mostly 65 yielding an engine speed of 1800 rpm. Engine is smooth, no smoke, proper boost, etc., etc. Should I expect better mileage from this rig, or am I reading exaggerated reports of good mileage?
Slow down to 60 and you will be surprised at the increase also put it in the economy mode when on flat or semi flat terrain. We had a 99 Dip that I chipped from 275 to 325+ and it also increased the fuel milage. Another thing is check your speedo and odometer they may both be off especially if someone has installed 275 tires, check with your GPS on next trip.
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Old 11-05-2011, 07:23 AM   #10
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Chipping Up

Walt, one of the items on my list is research on process, cost, and results of "chipping" the 275 HP to 325 HP. Living in Colorado, I could use more power through the mountains. What is involved in the process, what does it cost, and is there a significant increase in performance? I have to keep in mind that we are required to have an emissions test here every year. Does the "chip up" effect emissions?
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Old 11-05-2011, 03:40 PM   #11
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Originally Posted by dwkerch View Post
Walt, one of the items on my list is research on process, cost, and results of "chipping" the 275 HP to 325 HP. Living in Colorado, I could use more power through the mountains. What is involved in the process, what does it cost, and is there a significant increase in performance? I have to keep in mind that we are required to have an emissions test here every year. Does the "chip up" effect emissions?
DW when I decided to chip it was 2001 and after searching the best at the time and safest seamed to be BD out of Canada because they used a 99 Monaco Dip as a test bed for their chip with the results run on a Dyno the best I can remember it claimed over 330 hp and a increase on torque from 660 to 830 I think but i am sure it was 800+, the chip along with a boost fooler really woke the little engine up if fact it got off the line at 29,000 lb a lot better than my Dyn ISC 350 at about 32,000 lb I did have a Pyrometer to watch the exhaust temp in the hills but a shift down to keep the RPM up was all that was necessary to keep the temp in check, I hate to tell you but they are not making that chip any more and the ease of installation was that all you had to do is unplug the pig tail from the fuel control and plug it into the BD box and plug the BD box pig tail into the fuel control. The best I remember my fuel milage went up about .5 mpg and the down shifts on small hills were no longer happening, I checked with the chip unplugged and then plugging it back up about three different times on a trip to the Casino on the MS coast that we made a couple times a month at the time and the increase in fuel milage and HP was steady proven each time. Talk to these guys I think you can trust them for info I always did and see what they say is now avaliable for your Coach. Cost at the time was $500+ and I dont know what effects on emissions. http://www.bd-vfi.com/
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Old 11-05-2011, 03:59 PM   #12
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We have a '99 Discovery with the Cummins 275 ISB. I can averge 10 mpg towing a Chevy Colorado 4 door. Do your self a favor and do some reading on the lift pump. Ours is in the shop for the lift pump with no word on the injector pump. "OUCH"
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Old 11-05-2011, 05:30 PM   #13
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My ISB over 50,205 miles has a average of 10.238 MPG.
Always has pulled a 2,600 lb trailer or a 3,300 lb toad.

55-58 MPH gives the best sometimes 60 is good.
Start pushing 65 and up it will drop fast in MPG.

Banks Powerpack can give a good boost in power. But it is without their claim of 10% increase in MPG. After installing it I had 0 change in MPG either up or down.

The increase in power was noticed from the OEM 23 PSI boost to the Banks 34 PSI
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Old 12-05-2011, 07:09 AM   #14
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what ended up being the problem with your lift pump? curious since i have a cummins ISB 275 also 74K miles.

thanks, joe
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