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Old 01-13-2011, 12:55 PM   #1
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Good afternoon all,
I'll be buying a used 2003 - 2006, 38' - 40' Diesel Pusher in the next few months. I've been told by a few RV Salespeople to stay clear of the Cummins ISB engines, as they are simply Pick-Up truck engines, and won't do the job, especially if pulling a Toad and travelling in mountainous terrain, both of which I intend to do. Are they telling me straight ? Also, can anybody tell me which Cat engines coorespond with the ISB's, if any ?
Thanks !

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Old 01-13-2011, 01:28 PM   #2
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Make sure you do not buy a MH with any ISL motor made between Oct 2005 and April 2006. Check this recall

Cummins did not fix the problems in these motors they just reprogrammed the enine ECU to make it put out less power. The bad connecting rods are still in the motor. Verify the engine serial number against this recall. Outside of this recall the ISL motor is great.

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Old 01-13-2011, 01:28 PM   #3
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IMHO they are giving you good info regarding the ISB. I live in the west and do a lot of mountain driving. I had an ISB in a 38' MH pulling a 4,300# toad and it was seriously under powered in the mountains. I now have a heavier coach with the ISC and the difference is very noticeable. Grades that I was barely able to maintain 35 mph with the ISB, I can now pull at 45 mph or more. 650 ft lbs of torque compared to 1050 makes quite a difference.

Of course others may disagree, but keep in mind the mountain driving part.
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Old 01-13-2011, 01:38 PM   #4
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actually the ISB did not start life as a pickup engine that is just plain BS
It earned its keep long before dodge contracted to have them in a pickup truck.
it had some issues like was already mentioned

my folks have a 04 bounder with an ISB and he gets good mpg, but it is imo under powered for a 38 footer, it does maintain 65 just fine but with not much left in her
its right at 26k lbs before the toad, if i remember right

again its just me
but 38 to 40 foot you should be looking at an isc or isl engine
TQ TQ TQ is what carries you through the mountains and makes life loaded up traveling that much better,

i would try to drive similar coaches in weight with all three engines
and you will see right away
the difference in 600 ft lbs of tq and 1100 or 1200
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Old 01-13-2011, 01:49 PM   #5
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The 5.9L B-series Cummins traces its history back to the early 1980s when Cummins and Case created a joint venture to develop a diesel engine for use in Case agricultural and construction equipment. The result was the B-series Cummins in inline 4 cylinder (3.9L) and 6 cylinder (5.9L) configurations. The B-series engine was first used by Case in 1984, so if one wants to hang labels, the B-series Cummins was actually developed to be a tractor engine, not a pick-up truck engine where it was not used until Dodge first offered the 6BT as an option in 1989. (As an aside, that was not the first application of diesel engines in Dodge trucks - they had offered a Mitsubishi diesel before 1989.)

Having said that, the B-series Cummins in 6BT, ISB, QSB, etc. variants has been used in applications varying from all types of off-road construction equipment to marine, electrical generator drive, military, school bus, RV, medium duty truck and, yes, Dodge pick-up truck applications. Spark ignited gaseous fueled variants are used in the oil industry primarily in gas compressor drive applications.

You may well decide that the ISB is on the small side for a 38' to 40' motorhome application, but it's not really accurate to characterize this versatile engine as merely "a pick-up truck engine".


P.S. - The piston pin "issue" cited in an earlier post dealt with the ISL engine, not the ISB.
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Old 01-13-2011, 02:12 PM   #6
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Gentlemen, Don't sell the ISB short. It may be short on size or power compared to the ISC and ISL BUT you must remember it's all in the application of the engine that matters.

In a 34 to 36 RDP platform it works just fine.

See the article 2011 Tiffin RED ....

There are other applications for the ISB in the 28,000 GVWR weight rating that this engine makes a whole lot of sense for as well. You want to go 32,000, I'd say an ISC is where you need to be and anything much greater than that the ISL will come into play .... and then there's the ISX ...

Cummins will tell you to bring it on ... they have specific engines for every weight rating on an RV that you can think of.
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Old 01-13-2011, 02:22 PM   #7
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And if all else fails, there's the 3000 BHP Cummins QSK 60.

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Old 01-13-2011, 02:56 PM   #8
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I have an ISB in a 34' Newmar. On my last trip I got 8.73 mpg over 2760 miles towing a 4x4 extended cab ranger with 3 motorcycles (dirt bikes) and traveling 60-65 mph. It does slow considerably on the long steep climbs, but even there I manage to pass a lot of the semi trucks. I don't how much difference the extra 4 or 5 feet will make but obviously it will make some difference. If you can handle being a little slower on hills then I think you could be very happy with the ISB, but if you like to keep your speeds up no matter what the road then the bigger engines are the way to go. Good luck.
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Old 01-13-2011, 06:07 PM   #9
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Originally Posted by michguy View Post
Also, can anybody tell me which Cat engines coorespond with the ISB's, if any ?
Thanks !
The comparable CAT engine is the C-7 or 3126, which is about 330-350 HP, but has better torque than the Cummins ISB (860 lb-ft on the CAT vs 600 lb-ft on the older ISB's).

So the tradeoff is better torque, but the Cummins ISB gets better mileage, at least compared to my prior 2000 ISB, which got close to 10 mpg, vs my current CAT C-7 which gets about 8+ mpg. Both are 34' Winnie Journeys.

You choose. Either way you get there.
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Old 01-13-2011, 07:24 PM   #10
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I'd put the C7 Cat as more of a ISC competitor than and ISB, but it's not important. What is important is to buy enough horsepower to move the size coach you buy. If it is going to weigh 30,000 lbs (don't forget to include the toad), you want at least 300 hp (100 lb/hp) and more if you feel you need lively performance or can't stand to be passed on a hill.

My 2004 ISL produces 375 hp and we travel at about 37,800 lbs. It's fine on most roads and adequate on 5% grades, but I wish I had more power for those long, steep western grades.

Today's ISB is a 340 hp, 6.7L engine, so it can move a pretty good sized coach. 16-18 years ago is was a 190-200 hp 5.9L and not so good for a larger rig.
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Old 01-14-2011, 04:31 AM   #11
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Thanks to all of you ! I now have the information I need to make a good decision.
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Old 01-14-2011, 05:07 AM   #12
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We recently drove a friends 36' Monaco Cayman (ISB) from Ohio to the Jersey Coast area. There are some monster hills on the way out there. It drove great. I barely noticed climbing the grades and it got just a tad over 10mpg hand calculated. We didn't have a TOAD but I can tell by the way it was climbing the hills that even if we would of had one it would have been just fine climbing.

The 5.9L ISB cannot be beaten in longevity and serviceability at an extremely reasonable cost over it's service life. Parts like injectors, pumps, valvetrain, turbos, etc etc are in abundance which makes them very inexpensive to service/repair compared to other powerplants. Just about any decent diesel mechanic is very familiar with and can work on the 5.9 with their eyes closed. The 5.9L (ISB) dominates at the fairgrounds truck pulls.

If the floorplan works for your needs and the coach is in decent shape then I personally wouldn't hesitate considering one with the ISB. Remember, you're on flat ground more than you're on mountainous terrain. 500hp would be nice to climb hills but quickly looses its luster in fuel mileage AND costs to repair as time progresses.

Just my $0.02 cents worth.
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Old 01-14-2011, 07:59 AM   #13
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It all depends on what you are used to or coming from. If you're used to a large "gas" engine coach, then the B series cummins will feel much better than what your accustomed to. As stated it's not a P/U engine but a very capable powerplant used in a wide array of applications. Also while other P/U diesels are actually catagorized as "light" duty diesels, the b series cummins is catagorized as a "medium" duty diesel. You could write a book about the component design differences in them but I'll not go there.

The ISB is a fine engine and will please a lot of people, but in my opinion for a 40ft. coach wieghing in at 30,000 or more with a toad tackling the grades around say Yellowstone you will wish for more power. I was there last summer with my coach and felt adeqaute. It's a 40 ft. qaud slide with ISL.

As stated earlier, "torque" gets the job done. A B series cummins has over six hundred ft. pounds of it while the ISL series has 1200 ft. pounds. You can take all three, ISB, ISC and ISL with the same 350hp rating yet they are not the same as far as power moving the coach. Horsepower doesn't move you torque does. At the same time the larger engines are heavier due to larger and heavier components. They also cost more in repairs for the same reason and get less fuel mileage. I haven't driven an ISC equipped vehicle but I have driven many vehicles and heavy equipment with the B series. I own a MH with the ISL series and have driven many OTR trucks with ISM and ISX powerplants. In the end the ISB will get the job done with no question, but the ISC or ISL will get it done easier with more power in reserve if needed or wanted. Like the old drag racers saying "there is no replacement for displacement".

Cat make a fine engine also. I've driven many of these in OTR trucks and used them in heavy equipment. The "CAT" name carries a premium though. You will find that parts are higher priced as well as sometimes service. They will get the job done just fine as well. In OTR trucks an overhaul on a large Cat was usually 3k higher than an overhaul on a like sized Cummins.
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Old 01-14-2011, 09:59 AM   #14
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A ISB with a Banks PowerPack will get you up any hill and will pass those fuel stops the bigger boys need to stop for.
Just make sure the ISB MH also has the Allison 3000 6 speed.

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