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Old 07-10-2014, 10:23 AM   #1
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Different things to do to operate a Cummins Diesel vs. A Gasser

I am beginning the process of upgrading to a diesel pusher. I have owned 3 gasser motorhome, 1 class C and 2 Class A's. And have probably 75,000 to 85,000 miles in a gasser. But have never driven a diesel. So I am not a novice to RV and my current rig is 38'.

I am looking for details on operational differences. Thanks in advance, lots and lots of good people and topics on this website.
P.S. Any comments on things to be looking for in the purchase decision will be helpful. I will be buying used......2006 to 2010 most likely, and 95% chance it will be Newmar.
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Old 07-10-2014, 10:30 AM   #2
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well u can put away the ear muffs no engine noise when going through the passes WFO
the power band/rpm will take a little getting used to.
DONT BUY one with LESS than 400HP. air brakes are different than hydraulic
Try and buy a unit with a 2 speed ENGINE brake verses a EXHAUST brake
Newmar are nice units 2009 dynasty are real nice, American revolution also
lots of nice units.look carefully
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Old 07-10-2014, 10:33 AM   #3
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older cummins 12 valve, with P7100 injector pump and 6 sp Alison
almost 14 mpg on the road towing. very cost effective to operate low maintenance and easy to work on . diesel oil change 7500 miles, but uses 3 gallons oil.
usually with a cummins diesel, drive it like you stole it.
cummins are wind up diesels. throw it on the governor and it will run forever.
but never lug it..
cat diesel are just the opposite and will lug all day long but don't like high rpm..
I do like my diesel as I left home in washington state, drove to McCloud Ca and home and didn't have to stop to refuel. quiet in the cab, and no motor heat..
much nicer ride. . . . good luck.
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Old 07-10-2014, 11:28 AM   #4
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I do not agree on the 400HP limit but won't make to big a deal about it if 400 is what you want. I drove a 18 wheeler with two trailers behind for over a million miles without 400 HP and they served me well.
One of the differences between diesel and gas is the air brakes. Read and learn all you can about them. Having the air drop below the set limit will cause the brakes to lock up.
Diesel engines do not produce vaccum and the engine compression will slow you differently than what you are used to with the gas engines. (the reason for Jake Brakes)
The fuel requires a bit more attention during extreme cold temps as it may jell. The fuel stops will sell you a different blend during the months when the temps drop, however the MH's tank capacity may allow you to fuel in the southern States where they don't do this and unblended fuel may take you into area's that it is needed and you will not be covered.
A gasoline engine can be idled to warm up, a diesel needs to be working!
The most recent diesels require the use of DEF. It has some temp and time restrictions that require a small amount of study to do it properly.
The in cab heater requires the heated water to circulate the length of the coach, so keeping your feet warm during extreme,s may require some extra thought.
OIl changes may be quite a bit more expensive, as well as filter changes. I believe mine holds around 40 qt.
The Freightliner shop charges almost $30 more per hour to work on my MH VS a truck.

Chances are the chassis is rated to carry more weight and can tow more than the gas models you are used to so it most likely will stop differently and require a different brake system than you now have on your toad.
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Old 07-10-2014, 12:57 PM   #5
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Originally Posted by Luckiest Dreamer View Post
Chances are the chassis is rated to carry more weight and can tow more than the gas models you are used to so it most likely will stop differently and require a different brake system than you now have on your toad.
I did not know that about needing a different brake system when I upgraded to my DP.
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Old 07-10-2014, 04:45 PM   #6
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Here is my experience of going from a Ford V10 to a 400HP Cummins.
I immediately noticed how quiet the DP is. With my back bath and bedroom doors closed I can't hear the engine at all.
Throttle response is very smooth on the diesel. Perhaps due to the weight of the vehicle but is just accelerates very smoothly. No jack rabbit starts when you step on the gas.
The air bag suspension provides for a smoother ride.
I went from the 36LA to a tag axle DP and the handeling is difference is remarkable. The Phaeton doesn't have any semi push and side winds are just not an issue. It tracks as well as my Honda and requires about the same amount of effort. Very easy to drive.
I love the air brakes. They don't require near the effort as the 46LA and feel more solid. It will take you all three stops to get used to them.
Engine braking. Whether Jake or exhaust they are the best thing ever invented. Not only used to control speed going down grades but useful while in town or driving in stop and go traffic.
The ability to fuel in the truck lanes. Never have to worry about getting in and out with a toad. Get the Good SAMs Pilot/FJ charge card. Pump and go at the truck lanes at Pilot stations.
These are just some of the differences I noticed.
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Old 07-10-2014, 08:06 PM   #7
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Originally Posted by kb7uxe View Post
older cummins 12 valve, with P7100 injector pump and 6 sp Alison
almost 14 mpg on the road towing. very cost effective to operate low maintenance and easy to work on . diesel oil change 7500 miles, but uses 3 gallons oil.
usually with a cummins diesel, drive it like you stole it.
cummins are wind up diesels. throw it on the governor and it will run forever.
but never lug it..
cat diesel are just the opposite and will lug all day long but don't like high rpm..
I do like my diesel as I left home in washington state, drove to McCloud Ca and home and didn't have to stop to refuel. quiet in the cab, and no motor heat..
much nicer ride. . . . good luck.
you actually believe you really got 14MPG?
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Old 07-10-2014, 08:17 PM   #8
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I question that too !
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Old 07-10-2014, 10:53 PM   #9
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14 mpg in a diesel motor home? Towing? Downhill...with a 30 knot tailwind...maybe.

Could you average 14 mpg? Absolutely no way, no how.




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Old 07-10-2014, 11:16 PM   #10
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14 mpg in a diesel motor home? Towing? Downhill...with a 30 knot tailwind...maybe.

Could you average 14 mpg? Absolutely no way no how.
My first coach was a National Tradewinds with the 3126B Cat. I got an honest 12 MPG. It wasn't 14 but pretty close
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Old 07-11-2014, 12:42 AM   #11
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Wow......some pretty bad info on this post. "Cummins a wind up diesel"???? Certainly not true on a modern Cummins, maybe an old 5.9. And yes, you should be looking for a Cummins 400 ISL. They are one of the best motor home engines ever built and are in the model years that you're looking at. A smaller engine in a semi with 18 forward gears can be made to do anything, but when you're limited to just six speeds, the HP and torque comes in handy.

There is another post running where someone is asking a similar question. The biggest difference between the engines, is the gas engine you wring it's neck when climbing and can't hear yourself talk. A diesel likes to settle in at it's torque peak and just keep going, quietly.

My favorite description.....a gas engine accelerates and a diesel gains momentum!
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Old 07-11-2014, 03:25 AM   #12
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well, it was 13.8mpg towing the samuri. I believe it would have been better but the tank sender on the side was leaking. seal was bad.
it was mostly flat. went from washington, to white earth reservation in minnesota,
then mt rushmore and home.. this was with some very hard tires,
minimal amount of water, and no hurry.
I have since put new tires, ride is much better, replaced the samurai with an 86 CJ7.

I suspect my milage of going from Roy to ocean shores ( about 125 miles, ) with the stop go and hills and the heaver toad, is less..
I can get there and back on 1/4 tank of fuel.
so I believe it's still at or just over 10 mpg..
but on the open hiway it's better.

diesel = long hiway trips or 2000+ miles or so..
gas= 100 mile or shorter trips.
100,000 miles for a diesel is like 20,000 for a gas..

cummins 5.9, P7100 pump, alison 6 sp.
so yes, 13.8 mpg.. it can be done.

Dan kb7uxe.
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Old 07-11-2014, 03:38 AM   #13
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Wow......some pretty bad info on this post. "Cummins a wind up diesel"???? Certainly not true on a modern Cummins, maybe an old 5.9. And yes, you should be looking for a Cummins 400 ISL. momentum!
Thank you for correcting me.
my only diesel exp is with my john deere and the 5.9 cummins. the instructions say that if you run it at 1500 rpm full throttle for more than 1 minute, it will void the warranty. so I would say, no , you should not lug the 5.9.. so while it may not be true on modern diesel, it seems to be what works for the old 5.9 and old me. ..
again, Thank you for your advice and correcting me.
my comments are based on my personal experience and knowledge.
albeit as little as it may be.
oh yea, I also have a straight pipe on the exhaust..

so I guess you prolly should not pay any attention to any thing I say..
i'm probly wrong, just ask my wife..
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Old 07-11-2014, 06:26 AM   #14
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A few quick thoughts Diesel vs Gas (I've had both):
- Much improved ride
- Dramatically quieter going down the road
- Generator away from the bedroom
- Generator quieter, more powerful, and more efficient - and it'll run forever
- Plan $1000 a year for scheduled maintenance unless you can do it all yourself.
- Do it yourself maintenance is easier as components, especially engine, are easier to access on a diesel chassis.
- Tires are more expensive
- Stiffer chassis allows putting out the slides without leveling the coach. Some will say this is a no-no but we do it all the time on our Winne and have never had a problem. Makes it really nice if you pull into a rest stop or roadside for lunch.
- Pass Through storage bays
- Using the truck lanes at the fuel stop - the pump flow rate is easily three times that of the auto lanes.

That's my experience - yours may differ!
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