Diesel Engines and internal Seals

[X-Roughneck]

Referring to the used DP market.  At what point, more so from a age stand point, when looking at used RV when should  a person start to worry about seals and gasket failures?  Even though the coach may have low miles at what point do the rubber seals / gaskets in the engine becoming prone to break down or failure causing the need for the engine to be yanked out and repaired? 

I see some mid 2000 range DP with low miles and great prices but I keep thinking about how sound are the seals and gaskets in that engine?  I am sure the labor cost in just pulling and replacing the engine in one of these beasts will set a person back $1000s.

Also, Is there a Diesel engine / trans combo out there in the RV community that is a huge favorite and also is there one that is consistently given not so favorable reviews?  For a guy just wanting to carry a average load and pull a small size car I would think that exceeding a coach with 400 HP would be way over kill, but then again, I don't really know anything about RV-ing and that is why I am here asking.

 

[Gary RV_Wizard]

Tough one to answer. The rubber seal parts are generally NOT part of the engine proper, and sometimes not power plant at all (suspension, axles, etc).  Good maintenance has  a hefty effect on that as well.  Most actual gaskets, e.g head or exhaust gasket will be fine for a couple hundred K miles. However, there are always a few years of some engine model that have more gasket problems than others. 


Note that the vaunted half-million or million mile diesel engine life applies only to the engine block & pistons, i.e. the time between a major engine rebuild.  Cooling system & pumps, starter, alternator, hydraulics, a/c compressor, etc. have about the same useful life as on a gas engine.  Belts and hoses also last about the same as on a car.


The Cummins ISB & ISC are popular work horses with a good rep. So is the Cat C7.  For larger coaches, the Cummins ISL and Cat C9 are excellent.

 

[Utclmjmpr]

If you must worry, worry about some thing important, like is the sky going to fall??
  Let the engineers worry about the things they are paid to worry about.>>>Dan

 

[Larry N.]

I would think that exceeding a coach with 400 HP would be way over kill, but then again, I don't really know anything about RV-ing and that is why I am here asking.

For the most part, coaches with the higher horse power have it to "pull their weight." For example the Beaver I used to have weighed a max of 50,000 lbs and had a 525 HP CAT C-13 engine. But the Ventana I have now weighs a max of 36,400 lbs and has a Cummins ISB 360 HP engine. It seems to be very slightly more lively than the Beaver was, and get a lot better fuel mileage (8-9 vs 5-6). So in the neighborhood of 100 lbs per horsepower seems to be typical.

If you were to examine both coaches you'd find that the Beaver was heavier, in part, because it used real, very heavy wood for cabinets and trim vs lighter substitutes using veneers (though very well done) in the Ventana. Air leveling AND hydraulic jack leveling on the Beaver vs just hydraulic jacks on the Ventana, and so on.

at what point do the rubber seals / gaskets in the engine becoming prone to break down or failure causing the need for the engine to be yanked out and repaired?

I don't have a definitive answer for that, but a lot of rubber items need to be checked after a dozen or so years. I suspect that's not too much of a worry in the engine itself, but hoses and belts, along with various things throughout the chassis may need checking -- it won't just magically come apart all at once, of course, and a lot may depend on the climate the coach has been in and the treatment it's received over the years.

Perhaps someone more knowledgeable will have some "magic words" here. (Gary had some above)

Is there a Diesel engine / trans combo out there in the RV community that is a huge favorite and also is there one that is consistently given not so favorable reviews?

Engines have been mostly Cummins and CATs for quite a number of years, and mostly Cummins for the last 8-10 years since CAT quit doing over-the-road engines, while nearly all DP transmissions are by Allison. I've seen occasional mention of one or two that are more troublesome than others, but that's not something I've retained.

I'd say overall condition and maintenance history are the main things, BUT floor plan is king. No matter how pristine the coach, you want a layout with features that you can live with for extended periods, otherwise you'll soon wish you'd done something else.

I see Gary has been here while I was typing, with his usual excellent comments.

 

[docj]

For a guy just wanting to carry a average load and pull a small size car I would think that exceeding a coach with 400 HP would be way over kill, but then again, I don't really know anything about RV-ing and that is why I am here asking.

From the perspective of physics, horsepower governs the speed you can maintain while climbing a hill and torque determines how quickly you can accelerate.  Of course, both factors are weight dependent so looking at HP/weight or torque/weight provides a better understanding of how how a particular coach will perform.

As for how much is "enough", we happen to have a MH that Beaver designed on purpose to be "over powered".  It has the engine that powered Beaver's heavier chassis in a lighter, 34,000 lb one.  With a 12 liter CAT the result is a MH that is a lot of fun to drive.  It accelerates in an almost car-like manner and climbs the steepest hills on the interstates at >50 mph.  Do we need that much power?  Of course not.  But do we enjoy having it?  Immensely!

Today, most "lower end" DPs use the 6.7 liter Cummins ISB engine which is the same one that is used in the RAM pickup.  Large block diesels aren't really available unless you're prepared to pay >$500k.  All of them achieve the objective of moving the coach; some simply go up hills faster than others.

Joel (AKA docj)

 

[Neal]

To answer your question. It all depends.
I'll tell my experience, and blow up my luck.
We own a Hesston tractor, 35 years old, with a 5.5 liter diesel engine. Probably Fiat, maybe Iveco.
So far we have only replaced one hose, maybe 1" diameter x 4" long.  I think that was caused from the two pipes it connected together were only maybe about 1" between the two ends. All other rubber is original.

 

[Gary RV_Wizard]

Diesel motorhome manufacturers use 100 lb/hp as their figure of merit for adequate perfromance.  They typically use a big enough engine to achieve that. Higher end models or those with pretensions of  higher performance have a slightly lower ratio, while cheaper rigs may exceed that slightly. The weight in this formula is the GVWR, so a coach that has a max weight of 34,000 lbs would typically have diesel of about 340 hp.

This is not at all high performance; your car probably has more like 10 or 15 lbs/hp. That's why a car accelerates faster and passes motorhomes on even modest grades.  You will NOT be super-impressed with the performance of even a 400 hp diesel unless somebody squeezes it into a tiny 6000 lb rig.

Note that the factory generally use the motorhome GVWR as the design weight. If you tow a car, even a 3200-3800 lb model, acceleration and hill climbing are reduced further.

 

[docj]

Diesel motorhome manufacturers use 100 lb/hp as their figure of merit for adequate perfromance.  They typically use a big enough engine to achieve that. Higher end models or those with pretensions of  higher performance have a slightly lower ratio, while cheaper rigs may exceed that slightly. The weight in this formula is the GVWR, so a coach that has a max weight of 34,000 lbs would typically have diesel of about 340 hp.

This is not at all high performance; your car probably has more like 10 or 15 lbs/hp. That's why a car accelerates faster and passes motorhomes on even modest grades.  You will NOT be super-impressed with the performance of even a 400 hp diesel unless somebody squeezes it into a tiny 6000 lb rig.

With all due respect, the acceleration of an RV is far more governed by the torque of the engine rather than its HP.  Torque is the rotational analog of force, so by analogy from Newton's Second Law in which F = ma,  we have τ = Iα, torque = moment of inertia x angular acceleration. 

Manufacturers like to advertise HP because they can use tuning to increase the HP of a small engine.  Unfortunately, for diesels, the torque is largely a function of engine displacement.  A small engine, like the Cummins ISB, can be tuned to provide more than 300 HP but not much can be done to increase its torque which is in the 800 lb-ft range. By comparison, the nearly 9 liter Cummins ISL has torque in the 1,200 range whereas my 12 liter CAT delivers 1550 lb-ft of torque.

When comparing the acceleration of an automobile to a MH, the better comparison is torque to weight rather than HP to weight.  My 2014 CR-V has an engine which produces 179 lb-ft of torque in a vehicle with an GVWR of ~3,400 pounds, roughly 19 pounds per lb-ft of torque.  My MH has an GVWR of 34,000 lbs and the engine produces 1550 lb-ft of torque for a ratio of 22 pounds per lb-ft of torque.  I won't claim that my MH accelerates as rapidly as my CR-V but it's not all that much slower!

But take that same 34,000 pound MH and power it by an ISB with 800 lb-ft of torque and now the ratio is ~42 pounds per lb-ft of torque and it will accelerate far more slowly.

I'm not saying that everyone needs a MH that accelerates as quickly as a car, but it's not impossible to make one.  The problem for manufacturers is that large block diesels are rather expensive engines and most customers won't know that the 300 HP produced by an ISB won't deliver the same "off the line" performance as the same amount of HP in produced by a larger engine.  The two MHs will climb a hill at the same max speed because that's governed by HP (energy delivered in a unit period of time)  but if they get stuck behind a truck on the way up, the smaller one will have much harder time accelerating up to speed again.

Sorry for the lengthy tutorial, but torque is one of those "mysterious" concepts which is difficult even for physics students to understand.

Joel (AKA docj)

 

[X-Roughneck]

Good info.  I appreciate the thoughtful answers. We went out to PPL yesterday, and knew there was nothing really we were excited about looking at but it was a Saturday and we walked the lot just to kill some time.  There were some real, well used, "dogs" currently on their lot.  Found out that it is fairly easy to identify what is probably water damage. Lot's of them smelled rotten / musty!  I got to the top of the stairs and first thought that came to mind was NO WAY on just about everything I saw.  I am sure that there is such a thing as "gently used" out there, but I did not see it yesterday.  I am thinking I need to refine the used coach search and keep things in the 4 year old range at a max.  I have found out quick that looking at the brand new models have a tendency to WOW a person.  Their price tags are pretty scary to look at even doing some quick -30% math in your head and pull me back towards "used" reality though.

 

[Gary RV_Wizard]

The torque vs Hp debate has raged for decades, often with examples that are misleading at best. The two are inextricably related - at any given rpm more torque produces more horsepower - you can't have one without the other. An engine that produces 800 lb-ft of torque at 1100 rpms has a much different performance profile than one that produces 800 lb-ft at 2200 rpms.  The 800-2200 engine may actually produce a higher peak horsepower than the 800-1000 engine, but trying to compare the two is all but pointless.

The torque and hp figures advertised are peak values and they occur at some specific rpm.  The amount of torque available at a given rpm determines both the horsepower at that rpm and to a large extent the acceleration at that point.  An engine that produces high torque at low rpms will generally accelerate faster than a similar engine with less torque at that rpm, simply because most acceleration is done from relatively low rpm ranges (standing starts or hill climbs).  Diesels excel at low-rpm torque and thus produce a lot of horsepower at the lower rpm range where it is useful. Gas engines, especially small displacement types, don't produce rated horsepower until  much higher rpms, an rpm range where most of us don't drive or are afraid of the engine noise.

And then there is gearing at the axle and transmission, which alters how the engine torque & hp gets applied at the drive wheels. The only place where torque and hp have any real use.  This is measured with a dynomometer and is far more meaningful than engine specs.

The net of it is that amateurs can rarely make any meaningful conclusion based on advertised torque & hp specs. Powertrain engineering is far too complex for simplistic rules of thumb. The engineers try to come up with engine performance and gearing that delivers a useful amount of torque & hp across a wide range of rpms, but it's always a tradeoff of more in one place for less in another.

 

[docj]

I am sure that there is such a thing as "gently used" out there, but I did not see it yesterday.

We've been told by quite a few dealers and others that the real "cream puffs" don't stay on their lots for more than "days" (or less).  Many dealers maintain lists of customers looking for quality older MHs and they are called as soon as one appears.  The owner of the shop we use for service, repairs and remodeling told me the last time we were there "when you're ready to leave the road, let me know; I have contacts who deal in coaches like yours."

We bought our coach off of a private ad on RVTrader (or one of those listing services).  It looked as if it was in good condition and the owner had advertised that he had all maintenance records and documentation.  He turned out to be a very pleasant and honest person who was selling his MH because he had recently become a widower.  It was an exceptionally well cared for MH that came with everything it had when it was new, including the Beaver monogramed towels, which we immediately ditched!  (Who wants used towels except at a hotel?)

But I do want to stress that even as well cared for as our MH it didn't mean that there weren't issues brewing that no one could have foreseen.  For example, on our first major excursion a hydraulic line ruptured, the result of which was that our slide started opening while we were driving on an interstate in South Dakota!  It turned out that the MH manufacture had "underspeced" the hoses and this was a known problem.  Fortunately, a truck service center in Sioux Falls had us up and running again in a day.  I included this example as a way of pointing out that if you use your MH a lot, as we have, things will go wrong and need to be fixed, no matter how well cared for it is.

 

[Wizard46]

Most manufacturers show horsepower ratings because most people know squat about torque
Like my BIL, if it cranks  )