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Powertrain Q & A: Dec 2003

Originally Published in MotorHome Magazine

Q.  Please explain the difference, if there is any, between
winterized diesel fuel and No. 1 diesel fuel. I have been under the
impression that there is no difference, but others say I’’m wrong.

–Hal Leroy, Bend, Oregon

A.  As temperatures drop, diesel fuel thickens and begins to
gel and form wax crystals, which can clog up the fuel filters and
prevent the engine from running. That is the main reason why diesel
operators seek other fuel options in cold winter climates. With regular
No. 2 diesel fuel, this “thickening” occurs around 10 to 15 F.
“Winterized” diesel fuel starts out as No. 2 diesel fuel, but has
additives mixed in to modify it for winter use. There is no universal
standard for winterized fuel, so the mixture varies between regions and
refiners and distributors. Winterized fuel can contain such additives
as pour-point suppressants, anti-gelling compounds, cetane improvers,
additives designed to absorb and emulsify moisture from the fuel (so it
won’t freeze in the fuel lines) and No. 1 diesel fuel (to “thin out” the

The farther north (or at higher altitudes) that diesels are
operated, the colder it will probably get. Therefore, winterized fuel is
more likely to be needed and more widely available in these areas.

No. 1-grade diesel fuel is considerably thinner (less viscous) than
No. 2 diesel fuel and is not intended for regular use in roadgoing
diesel engines. It has less energy in it per gallon and provides less
lubrication to the engine that’s burning it. It should be used only at
very low temperatures when other measures fail.

Both winterized and No. 1 fuel should be used only when they are
actually needed, since they provide lower fuel mileage and generally
cost more.

Q.  I’m interested in purchasing a used motorhome capable of
10 mpg or more. I know that mileage varies widely — one friend of mine
has a 1988- model 29-foot coach (Chevrolet P-chassis, 454-cid engine)
that gets just 6 mpg, while another with a 2000-model 26-foot coach (I’m
unsure of the engine) says that he averages 10 mpg.

What year did more efficient engines come out? What size Class A
coach gets this economy (10 mpg or better)? What engine should I be
looking for (454, 460, V-10)? Is there a particular manufacturer’s
product that I should be looking at?

–Warren Buckmaster, Canyon Lake, California

A.  What you are searching for — 10 mpg or better from a
large, gasoline-powered Class A motorhome – is essentially the “holy
grail” of motorhoming. Instead of asking me, you should be asking that
friend of yours who claims 10 mpg what make and model he has; most
coaches of this type typically get between 6 and 8 mpg.

Engines are continually being refined and made more efficient.
However, as for determining what year engines became more efficient for
large, heavy vehicles like motorhomes, my pick would have to be 1892,
when Rudolph Diesel developed the engine design that still bears his
family name. Most of the large Class A motorhomes that get upward of 10
mpg are diesel-powered. Smaller coaches tend to be lighter and have less
aerodynamic drag, but the coach’s size or manufacturer’s brand name
isn’t paramount in regard to mileage. Rather, the most important factors
for better mileage are lighter weight, better aerodynamics, more
forward gears (with overdrive ratios) and a turbodiesel engine.

Q.  Robert Gibfried’s situation, as outlined in the
September issue (“Octane Problem?”), is very similar to one I
experienced with a 27-foot 1985 motorhome on a P-chassis with a 454-cid
engine. After chasing down all of the same possibilities mentioned, I
finally dropped the fuel tank — and found that the main fuel-pickup
rubber line was loose on the metal pickup. It was held in place with a
spring clip that had become loose. After replacing the rubber hose and
clamps with new line and worm-gear clamps, I had no further troubles
with fuel starvation.

I hope this helps someone.

–Roger Fox, Penn Valley, California

A. This problem would probably show up during a
fuel-pressure test under full load, which I recommended, and would also
be noticeable if you removed the intank pump. It’s nice when there’s an
inexpensive repair like this. Thanks for writing.

Q.  Several recent letters and replies about how to resolve
overheating issues on P-chassis motorhomes equipped with 454-cid engines
have left out an important step. While radiator and engine flushing,
fan clutches and thermostats are all important, an often-overlooked area
is dirt and road-debris buildup in the radiator and condenser fins and,
most critically, in the space between them.

It doesn’t do any good to have fresh clean coolant, a new
thermostat, etc. if there isn’t enough airflow passing through the
radiator to activate the fan clutch. The space in question is quite
large and, because of the high airflow and the highway speeds at which
motorhomes generally are operated, it can become packed with dirt,
debris and insects rather quickly.

I have the fins of my radiator and condenser and the space between
them blown out with compressed air every couple of years along with my
regular cooling-system maintenance; I’ve yet to suffer any overheating
problems with a 10-year-old, 35 foot Bounder towing a 3,000-pound dinghy

–Alan King, Oxford, Michigan

A.  A lot of crud can get trapped in the area between the
radiator and the air-conditioning condenser. Although I have mentioned
the need to clean it, I haven’t stressed it recently in this column;
perhaps it’s time to do so. Thanks for writing, Alan.

Q. Do you have any opinion on the new lower-emission diesel
engines? Cummins has gone to exhaust-gas recirculation (EGR), as have
all other manufacturers except Caterpillar; however, EGR apparently
causes the engine to run hotter and less efficiently (they are seeing a
3- to 5-percent fuel-mileage loss in commercial truck operation). Cat is
using advanced combustion emission reduction technology, which is a
different approach and requires some new after engine exhaust treatment
to meet the requirements.

What affect will this have on Class A motorhomes in performance and operation?

–Jim Herman, Berlin, Maryland

A.  There’s likely to be very little change noticeable to
the driver because motorhome diesel engines don’t operate as closely to
maximum output as do heavy commercial diesel engines. Many people
likewise thought that gasoline engines would become unusable due to
emission controls, but they have evolved to the point where they now
last longer, run better and get better fuel economy than in the “good
old days.”

Q.  We tow a 2002 Honda CR-V behind our coach and have
noticed that after a couple of days the dinghy’s battery goes dead. I
also have a Brake Buddy unit that compounds the problem because of the
current it uses. When we get to the campground we invariably have to
jump the Honda to get it started. Is there something that can be done?

–B.D. Miller, Diamond Bar, California

A.  Have a shop run a charging line from the motorhome’s
alternator to the dinghy’s battery. This should be done in a fashion
similar to routing a charging line from a tow vehicle to a trailer. Wire
size is not as critical because heavy current is not drawn from the

but use 10-gauge and include a circuit breaker in the line.

Q.  Our 1994 E-350-based 29-foot Four Winds Class C
motorhome has 82,000 miles on it. Some time back, I installed a
transmission-temperature gauge in the output line (going to the
transmission cooler) and verified its accuracy. When I first installed
it, the temperature would rarely rise above 190 F and virtually never go
above 210. Yesterday, on the way home from a trip with ambient
temperature about 90 F and freeway speed at 70 mph, the temperature
stayed at 230 F for most of the trip.

Do auto transmissions run hotter as they age, due to more slippage? I suppose I should also check the accuracy of the gauge.

–Ed Jacklitch, San Jose, California

A.  Most of the heat from an automatic transmission is
generated in the torque converter, due to slippage. However, your
transmission has a lockup torque converter that is controlled by the
computer. The lockup clutch within the torque converter on these models
does have a history of chattering, slipping and failing. That is why
there are now extraheavy- duty aftermarket converters available for

However, if the torque converter clutch were slipping enough to add
this much heat, it already would have failed. It is possible that the
signal to lock up is not getting to the clutch, which could cause a
major increase in temperature. The most likely culprit is the engine’s
cooling system. As vehicles get older, the radiators clog up —
internally with mineral deposits and externally with dirt and crud in
the fins. This often happens at around 10 years of age, and could result
in heat being added to the transmission fluid (instead of removed) as
it goes through the heat exchanger.

Q. We own a 30-foot 1993 Class A motorhome on a 1992 Ford
F-53 chassis with the 7.5-liter V-8 engine. We have owned it for 10

We installed the Banks PowerPack and are happy with it. The problem
we have is that the fan clutch will pulsate on and off for hours; on
for 8-10 seconds, off for 8-10 seconds. It doesn’t seem to matter what
the temperature is outside, or the speed we are driving. On level ground
it will cycle on and off for 8-10 seconds at a time. The temperature
gauge is always below the midpoint, so it’s running cool.

In the 10 years we have owned the coach, we have replaced the fan
clutch twice. I don’t think this is normal. We have 42,000 miles on the
coach. What can I do to fix this?

–Steve Zurek, San Diego, California

A.  Fan clutches do deteriorate over time; replacing two,
unfortunately, is not that unusual. There is really nothing you can do
to adjust a fan clutch. However, try replacing the thermostat, which
should be changed occasionally anyway and may change the cycling of the
clutch. If that doesn’t help and you do replace the clutch, try a
heavy-duty aftermarket unit, such as a Hayden.

Q.  I was reading about Walter Cornell’s high-temperature
transmission-fluid readings (“Transmission Temps,” April) and want to
share my experience. I had the same problem with my 28-foot 1991
motorhome (Chevrolet P-chassis) when I used synthetic fluid in my
automatic transmission. I drained the transmission and torque converter
fluid and replaced it with conventional GM transmission fluid. Since
then, I’ve had no more high transmission-fluid temperature readings.

–C.J. Baker, San Juan Capistrano, California

A.  This is an unusual report and I’d like to hear from
other readers if they have experienced similar or opposite results. The
only way I can see this happening is if the synthetic fluid caused the
torque converter clutch to slip more than normal.

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