Should the 300HP ISB Cummins be avoided?

We are considering getting a 2000 ? 2002 36? diesel pusher with an ISB Cummins. I have read that when Cummins came out with the ISB 300HP in this era, it had problems (besides the block 53 and the lift pump problems). Is this true? Should the 300HP ISB Cummins be avoided? How much difference in performance would you expect between the 275HP and the 300HP.

We just tried out a 2003 Bounder with the 300 HP Cummins.  It was a great coach but the power was anemic.  It couldn't get out of it's own way...

I have been satisfied with my 275 ISB 5.9.  No problems with the 53 and lift pump.  I purchased it new in 2000 and have 79000 miles on it.  It gets me up to the speed limit and stays there until I climb a hill, however, I am towing approximately 6000lbs.  When I spoke to Cummins regarding increasing the HP from 275 to 300 or 330 they said the only difference I would see would be increased fuel consumption.

It is my understanding the increase in HP is only an adjustment to the computer, everything else is the same on the ISB.

I do have a 6 speed transmission with the 275 ISB in our 36' Mh and we do pass a truck or two now and then.

I suspect you will be happy with the 300 ISB.

enjoy the trip and keep it under the speed limit so you can sight see rather than watching for radar.

Russ, WB3FQI/6

The main thing is your weight.  The newer motorhomes are longer and heavier.  The next main thing is the torque you get out of the engine, a diesel is all about torque.  I've spoke with and read posts on many of these forums and most folks weren't happy with 300 hp.  When you see what the torque is it's probably somewhere around 600 ft lbs.  This isn't much especially when pulling a toad and being loaded close to max.  Personally I'd want to see something close to 1000 lbs of torque.  Depending on year this means something on the order of 360 to 400 hp. 

Be sure and drive it, and remember when test driving it usually doesn't have any weight in or attached to it.

......and remember when test driving it usually doesn't have any weight in or attached to it......

Good point. The one we drove was empty (water/gray/black water) with only a quarter tank of fuel. I knew then that when full and pulling a toad, it just wouldn't be up to the task.

Keyman said:

We are considering getting a 2000 ? 2002 36? diesel pusher with an ISB Cummins. Should the 300HP ISB Cummins be avoided? How much difference in performance would you expect between the 275HP and the 300HP.

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We have very limited experience on diesel pushers. We recently had this experience. Hope it helps in you quest for answers.

We just made a 3300 mile trip in our 2008, 39' Cummins 6.7L ISB 350 HP, 750 ft lbs torque, Allison 3000 6 speed. We towed a Honda Accord.

Most of the trip was as good as I could have dreamed of. Set the cruise on about 62 MPH and seldom did we see it drop to 55 MPH.

I remember one hill (not the highest hill in the country, last part was 6% grade for 2-1/2 miles plus more miles of lesser grade) I wished for more power. This was on I-25 going south from Trinidad, CO to NM (Raton Pass). We were coming up on the beginning of the 6% grade when we noticed something in the northbound lane that caused us to disengage the cruise control, Then noticing we were loosing speed fast we mashed it to the floor. We dropped to 42 MPH on that hill. I do not remember what RPM we held of know if we could have shifted down to get more RPM & more HP or not. I just know, once it reached 42 MPH, it did not gain any until we started cresting the hill.

We also read somehwere, on some diesel pushers the engine fan could use up to 75 HP when climbing long grades & the temperature was rising.. We had been climbing for a few miles.

More of what I consider good news. We averaged 9 MPG for the whole trip & 10 MPG on maybe the last third.

There is no question that the ISC or ISL engines outperform the ISB, but that wasn't the original question. And the bigger engines also out-guzzle the ISB at the fuel pump.  Horsepower isn't that much different, but the low RPM torque is 50% or more greater in the bigger engines and therefore they accelerate and climb hills better.

I'd say the 300 hp ISB is fine up to a coach weight of around 26,000 lbs, even if towing a 4000 lb toad in addition. Beyond that you may begin to wish you had a bit more power.  The 2000-2002 36 footers you are looking at are probably in that range, but check the gvwr and gcwr.

You should avoided it if you would like to have a lower MPG diesel to spend $$$ for fuel on.  

I have had a ISB 5.9L 275 HP 660 torque with the 53 block for 7 years now. It has had no problem even pulling a HHR toad going up Mount Eagle TN. 6% hill at the 55 MPH speed limit in 5th gear at a steady 2,100 RPM.

It had no problems in the big hills in CO., WV, PA, MA, NH, NY, OH, IN, IL, IA, NB, GA, KY, MO and many other states.

This summer my average MPG is 10.9 keep on a spreadsheet. The ISB does take 17 qt. on a oil change. Probably the least amount of any Diesel engine on a oil change. And you have to change it every 15,000 miles.  

Lift pumps have went out on some, but what percent? Lift Pumps cost around $170. Easier to change it then a fuel pump inside a gas MH fuel tank.  


A 400 HP 1,000 lb torque MH may pass you going up a long hill, but you will still get to the top with a smile knowing you saved on fuel cost and enjoyed the scenery more. ;D

I expect the ISB Cummins in this era to be 275HP or 300HP. I thought that I had read somewhere that when Cummins boosted the ISB to 300HP they created some problems. I was wondering if this was true or what the problems were. Of course, like everybody else I would like to have the higher HP. But if there?s a problem with the 300HP I suppose the 275HP would be good enough.

The short answer is I know of no reason to avoid the ISB 300.

If anyone from Cummins happens to be reading this forum (and I surely would be if I were them????), I hope that they will jump in here and give us some engine expertise and comments.  It would seem to be in their best interest to layout some facts and figures to help us address this question.

thanks, Gary

There's nothing like horsepower.  I would rather have more than I need - than not enough.  But this is from someone who rides a bike with 502 HP.  So there ya have it. Call me crazy.

;p)

How do you know if you have the 53 block...and what were the issues with it.
I have a ISB 275 built in feb 1997, esn 45478746 with 62000 miles on it.  On my 36' discovery it seems to have tons of power loaded up and towing 3000lbs.
Where do I check what tranny I have?

From what I read, the "53" block was used in 1998-2001 and about half the "B" engines made in that time frame have it. Less of a percentage in 2001, because the problem was fixed and no further weak "53" blocks were manufactured.  My source says the  "53" block came out of a foundry in  Brazil, and it is marked with a large ?53? cast into the block under the PS pump.  Some of the blocks also had a 53 by the turbo oil drain. If you can?t find a 53 prominently stamped on your block, you don?t have one.

A datum attributed to Cummins says that about 2% of "53" blocks will develop cracks and require replacement. I don't know if that is accurate or not. I have also seen it said that the cracking problem occurs mostly in Dodge pick-ups, because of the way the engine is installed. Don't know if there is any truth to that either.

Neal said:

I remember one hill (not the highest hill in the country, last part was 6% grade for 2-1/2 miles plus more miles of lesser grade) I wished for more power. This was on I-25 going south from Trinidad, CO to NM (Raton Pass). We were coming up on the beginning of the 6% grade when we noticed something in the northbound lane that caused us to disengage the cruise control, Then noticing we were loosing speed fast we mashed it to the floor. We dropped to 42 MPH on that hill. I do not remember what RPM we held of know if we could have shifted down to get more RPM & more HP or not. I just know, once it reached 42 MPH, it did not gain any until we started cresting the hill.

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I have had a couple of adventures on that 'hill'. The last time was in an 01 Suncruiser with a 454 V8 and 4 speed automatic. About 3/4 of the way up headed south, it downshifted into 1st gear. If it was any steeper, I would have run out of gears. I could have disconnect the toad, though.

mrschwarz said:

I have had a couple of adventures on that 'hill'. The last time was in an 01 Suncruiser with a 454 V8 and 4 speed automatic. About 3/4 of the way up headed south, it downshifted into 1st gear. If it was any steeper, I would have run out of gears. I could have disconnect the toad, though.

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We have the ISB 300Hp/Allison 2500MH (5 speed) and there is a mode button on the transmission control panel.  Unfortunately, for me, I cannot find anything in the manual for the MH or the manual for the chassis or the manual for the AT that tells me what happens if I press the mode button.  When I press it, a red light comes on in the corner of the display but that's it.  I cannot detect any difference in the way the transmission shifts or performs.  For these long, 6% grades I keep wondering if I should do something with the mode button or just let the "computer" select a gear and "do its thing"?????  We have made it up to the Raton Pass in 3rd gear, about 50 or so which seems to be acceptable.  The big trucks are going about the same speed or less.

The mode button on the Allison transmission lowers the shift points by 200RPM to prevent "hunting" when driving in rolling terrain.  It does no harm to turn it on most of the time.  It will have no effect on level roads or when climbing steep grades.

Ned said:

The mode button on the Allison transmission lowers the shift points by 200RPM to prevent "hunting" when driving in rolling terrain.  It does no harm to turn it on most of the time.  It will have no effect on level roads or when climbing steep grades.

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Let's see if I understand: 

If I'm in "rolling terrain", on cruise control (say 60MPH), and I go up a mild hill, then the tranny will wait to downshift until the RPM is 200RPM lower than with the mode button "off"?  Then if I'm going down a mild hill, the tranny will shift up 200RPM sooner than it would have with the mode button off?  So the engine, on average, spends more time in a higher gear... maybe getting more MPG (how much could this be?), but lowering the average MPH?  Thanks for your indulgence.

RV Roamer said:

From what I read, the "53" block was used in 1998-2001 and about half the "B" engines made in that time frame have it. Less of a percentage in 2001, because the problem was fixed and no further weak "53" blocks were manufactured.  My source says the  "53" block came out of a foundry in  Brazil, and it is marked with a large ?53? cast into the block under the PS pump.  Some of the blocks also had a 53 by the turbo oil drain. If you can?t find a 53 prominently stamped on your block, you don?t have one.

A datum attributed to Cummins says that about 2% of "53" blocks will develop cracks and require replacement. I don't know if that is accurate or not. I have also seen it said that the cracking problem occurs mostly in Dodge pick-ups, because of the way the engine is installed. Don't know if there is any truth to that either.

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You've pretty much got it !!

For some reason, I don't think the 53's came out until 99 or 2000 though. Not sure how long they were around.

There is marking exactly where you mentioned, right near the power steering pump. the blocks were a thinner casting and thus cracked. The cracking was attributed to the constant warming up / cooling down of the block, not due to the horsepower or torque. That's why you see them cracking more often in pickups.

The good news is that they can be repaired. There is a method called "stitching" where it is re-welded in a stitch like pattern and apparently it's a permanent fix. I know of a couple of guy's who have 53's in their pickups who to date have been problem free. One guy in particular races his truck at the track !!

If you're really curious, just google 53 block, and you'll find a bunch of info and fixes.

taoshum said:

Let's see if I understand: 

If I'm in "rolling terrain", on cruise control (say 60MPH), and I go up a mild hill, then the tranny will wait to downshift until the RPM is 200RPM lower than with the mode button "off"?  Then if I'm going down a mild hill, the tranny will shift up 200RPM sooner than it would have with the mode button off?  So the engine, on average, spends more time in a higher gear... maybe getting more MPG (how much could this be?), but lowering the average MPH?  Thanks for your indulgence.

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You understand correctly.  You may see a small improvement in MPG but the big advantage is keeping the transmission in one gear longer and not keep switching between gears on rolling hills.  This will keep the temperatures down as well as the RPMs.