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Powertrain Q & A: Sep 2003

Originally Published in MotorHome Magazine

Q.  We bought a pre-owned 1987 34-foot Class A coach on a GM
P-chassis with a carbureted Chevrolet 454-cid engine. Our trips are
infrequent, one or two a year, but we drive some 15 to 20 hours
straight. Usually after three or four hours, the engine starts stumbling
or backfiring when going uphill.
The problem is inconsistent; it may climb a long hill without protest
and then, on the next small incline, I’m reduced to 30 to 35 mph to get
up the hill without the engine stalling. All instrument indications are
normal. The engine uses one quart of oil every 3,000 miles. Sometimes
the episodes last for 5 to 10 minutes and sometimes for hours.

On our last trip we replaced the fuel pump, spark-plug wires and
inline fuel filter. At our destination we replaced the spark plugs, the
plug wires (the first shop laid them against the exhaust manifold) and
the small carburetor filter and had the carburetor cleaned on the
engine. Leaving for home, after about four hours on a 50-F day, the
problem resurfaced. At our next-to-last fuel stop, I filled the tank
with 90 octane instead of the usual 87. This was late in the evening,
and we had only one slight problem. At the last fuel stop, I filled with
89 octane and had no problems.

The cost difference between 87 octane at $1.39 and 90 at $1.61 is
substantial and other Chevy owners I talk to seem to have no problems
with 87 octane. I believe the problem is heat-related. Have I missed the
simple answer, just the grade of gasoline, or should I keep looking for
other causes?

–Robert Gibfried, Derby, Kansas

A.  Gasoline companies have done a great job of convincing
the public that buying higher-octane fuel is the solution to all their
problems. Actually, higher-octane gasoline only helps reduce detonation,
commonly called pinging or spark knock.

Intermittent stumbling or backfiring are classic symptoms of fuel
starvation and are not caused by octane that’s slightly too low. I think
you will find that the electric fuel pump back at the tank (yes, these
chassis have both a mechanical and an electrical pump) is failing. To
verify this, have a fuel-pressure test performed while the engine is hot
and under full load when it is running rough.

Q.  I have a 2001 Jayco Eagle 31-foot motorhome that I enjoy
tremendously. It has a switch near the entry door that is supposed to
turn the electrical system on and off to save the battery when it is not
in use. This was working great until last year. Now, that turns it off
only while you are holding the switch in. As soon as you release it, the
system comes back on.

I have been pulling the fuses to get around the battery dying when
not in use, but I am afraid that this is going to stress the fuse block
if I do it too often. Is there any way that I can get the electrical
diagrams for this motorhome? Any help you can give me to solve this
problem would be appreciated.

–Arnold Neitzke, Brighton, Michigan

A.  The latching portion of the relay isn’t latching, so
you’ll have to replace it. Here’s a generic trick I use when trying to
find a relay: The relay should be located near the coachbattery box.
Have an assistant operate the relay and follow the clicking sound to
find the relay. You should be able to get another relay at most RV
repair shops and supply stores.

Q.  We have a 1991 Winnebago 34-foot Class A motorhome on a
Ford F-53 chassis. Last year, while returning from a trip in the
mountains, I lost my brakes coming down a pass. After getting it home, I
took it to my mechanic and had all the rotors, all the pads and all the
calipers totally replaced with new parts (not rebuilt) and had the
brake fluid changed in the complete system. After picking it up, I had
great brakes. I just returned from a trip, and while going through
Denver at rush hour I lost my brakes again. They came back after about
one hour.

Looking at the brakes, I see all the lines run in the frame rail
away from hot exhaust pipes. It looks to me like the master cylinder
would be the closest to a heat source, and I also noticed that all the
brake rotors have a shield on the back side of them. This looks to me
like a bad idea for cooling needs, on the rear dual wheels anyway.

My wife thinks that we should get another motorhome after this last
loss of brakes, but I could not trade it in, knowing that someone else
would be in danger, plus I love my coach.

Could this be a heat problem or a power-brake booster problem? Do
you think that they need air ducted to them like a NASCAR brake system?

–Larry Ledom, Colorado Springs, Colorado

A.  A brake-booster failure would cause a major increase in
required brake-pedal effort to stop the coach, and it would stay broken.
The fact
that the brake pedal went to the floor but began working normally
after about an hour would indicate that the failure was caused by
boiling brake fluid. It only has to boil in one place anywhere in the
hydraulic system. What usually happens is heat boils the old,
moisture-laden or-low quality offbrand fluid in the calipers.

That may not be the case here, since a Ford dealer replaced it a
year ago. But just in case some old fluid from a container that had been
open awhile was used, I’d have the fluid flushed, using fresh Ford
heavy-duty brake fluid. There’s also a slight chance that the master
cylinder is beginning to fail, but usually you can pump the pedal back
up right away if it’s going out.

Ducting could help if it was done right and if it didn’t scoop up
water and direct it onto the brakes, but your best bet is to install
heatshielding anywhere that the brake hydraulic system is near a heat
source. Factory-installed shields on the brakes keep splashed water and
other road spray off the rotors, which helps prevent warped rotors and
loss of braking in puddles and heavy rain.

Q.  I read your response to Tim Fitzgerald concerning his
hard-starting problems on his Cat engine. You were right about fuel
delivery, but most likely it’s not sucking air. I have worked for a Cat
dealer for 35 years (Ziegler, Inc. in Des Moines, Iowa), and these later
Cat engines have been driving us nuts with hard starting. We have found
that some of the problems are related to fuel delivery because owners
are not using Cat filters with a 2-micron paper rating. They complain
the Cat filters plug easily, so they switch to an offbrand that does not
clean the fuel sufficiently for Cat injectors. The Cat filters were
doing their job, which is keeping particles larger than 2 microns from
getting to the injectors and partially plugging their tips. Failure to
remove these particles leads to hard starting.

If they change their clogged injectors, their engines will not only
start fine, but will gain 5 to 15 percent more power, as our
tests have shown. When testing and comparing filters Cat uses a
Nominal Rating System; the other brands use an Absolute Rating System
that is not the same. Some of these brands claim they have 2-micron
filtration, but Cat’s testing roves they do not meet its standards, with
some llowing more than 10 to 20 times the micron-sizearticulates to
pass through the filter and enter he injectors. Cat has information for
this type f testing on its fuel systems, including ublications nos.
PEDP9127, PEHP9522-01, EHP7046 and PELE0728.

I hope this helps some readers.

–Rod McClanahan, Melcher, Iowa

A.  Thanks for writing, Rod, and sharing your experience.
While I agree that clogged injectors can make an engine difficult or
impossible to start, injectors are very expensive, so I strongly
recommend that owners have them tested before replacing them.

Q.  My problem pertains to a 1982My problem pertains to a
1982 Fleetwood Rallye on a Chevrolet P-chassis that has a 454-cid
engine. I purchased this coach in 1998. About two years after purchase,
the motorhome began developing an unusual symptom, generally after being
on the road for 50 or more miles at highway speed and in hot weather,
usually above 90 F.

The engine would rev to higher rpm for up to 10 minutes at a time.
Other than the noise of apparently higher rpm, there was no change in
speed or performance. After awhile the engine would slowly work its way back to normal rpm.

These symptoms occurred while driving at between 1,000 and 6,000
feet elevation, perhaps even higher. I had the carburetor overhauled and
it didn’t seem to improve the situation. I then had the transmission
overhauled, and it seems to have helped a little. The problem does not
seem to occur as frequently or for extended periods of time. (It used to
last 10 minutes or more; now,

about a minute or less.)

The transmission-shop owner said, “It can’t be the transmission.”
Everyone I’ve talked to is stumped. I’d like to get the problem solved.
Everything else on this coach works as it should. Got any thoughts?

–John R. Catsis, Chandler, Arizona

A.  Your description definitely seems like the normal
operation of the fan clutch. When the thermostatic spring in the center
of the fan clutch senses enough heat coming through the radiator, it can
cause the fan speed to greatly increase until the temperature comes
down. This results in a roar that sounds like a downshift of the
transmission. This can be verified by driving the coach with a test
tachometer temporarily connected to the engine. Most repair shops will
have one available for this use. If you hear what sounds like a big jump
in engine speed, but the rpm is virtually unchanged, it’s the fan.

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