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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Engine heating up when pulling trailer
Message-ID: <>
Date: Wed, 15 May 2002 20:25:27 -0400

On 15 May 2002 22:12:23 GMT, (HLBRSMA) wrote:

>>I have a 17 ft. Scotty trailer and am pulling it with a 1994 Silverado
>>pick-up. I don't have a towing radiator. I just have the standard one that
>>came on it. It runs around 235 degrees when pulling and with the A/C on.
>>Other than a bigger radiator is there anything I can do to reduce the heat?
>>It's not over heating but I'm not wild about it running that hot. It can't
>>be good for seals and gaskets.
>I been there,done that. Start with a bigger radiator that has 4 rows of tubes
>instead of the 3 rows you have now, then add a good sized transmission cooler
>bigger is better. I once had this problem and nothing will solve your problem
>other the aforementioned retrofits I tried a myriad of cooling fans from many
>bladed to flex fans. That brings me to 'Flex Fans' do not ever use flex fans
>while towing because when you are going up steep grades and your engine is
>going full bore and your tow vehicle is going very slow the fan straightens out
>and you have no cooling at all. Stu

I've had very poor results with 4 row radiators.  The problem is, the extra
air path restriction impairs the air flow sufficiently to negate the extra
cooling surface.  Many fans, particularly if the shroud isn't tightly fitting
and reasonably air-tight around the edges, can't generate sufficient
differential pressure to drive a 4 row radiator.

The first thing to look at, of course, is the radiator itself.  If it is limed
up, the first thing to do is have the radiator boiled out (or do it yourself).
A related problem is tube blockage.  If the vehicle is very old and has had
engine work, even something as simple as a thermostat change, then it is
probable that some of the tubes are clogged by debris.  One customer vehicle I
worked on had over half the tubes clogged with shards of silicone seal!  The
mechanic had apparently not bothered to clean up his scrapings after removing
the old sealing.

I had a chronic overheating problem with my 68 Plymouth Fury.  I tried the 4
row radiator and the problem got worse.  This is identical to my experience
with Datsun Z cars and a couple of Ford pickup trucks.  I decided to
instrument the vehicle and discover why.  I measured the pressure drop across
the radiator as well as the inlet and outlet air and water temperatures and
the air velocity.  I found that a new OEM trailer package radiator (3 row)
radiator provided almost twice the heat rejection capability of the
aftermarket 4 row radiator, all because of the added restriction to air flow.
As an experiment on the worthless 4 row radiator, I carefully clipped every
other fin on the front and back rows.  (not possible to get to the center two)
This markedly improved things, indicating that a 4 row radiator might help
provided the fin count is REDUCED, the opposite of most radiator shop

The cause of the overheating in my Fury was not obvious until I started
measuring things.  The rear springs had fatigued and dropped the rear end a
couple of inches.  This change in attitude caused high pressure air to build
up under the car, slightly pressurizing the engine compartment.  This pressure
mostly negated the air flow from forward motion ram effect.  I found the
static pressure under the hood to be about equal to the dynamic pressure in
front of the radiator at 60 mph!

After testing with air shocks, I had my rear springs recurved which almost
completely eliminated the pressure buildup under the hood.  My second change,
opening some vent holes from the inner finger to the wheel wells and some
additional vents between the engine compartment and the vented area beneath
the windshield, solved the problem completely.  I have NO overheating, even
with the stock 2 row radiator and even when pulling a utility trailer.

There are some other things that can be done.  Adding an oil and a
transmission cooler will help but ONLY if they are mounted in clean air that
doesn't disturb the flow to the radiator.  On my MH, I added a small
transmission fluid cooler and mounted it over to the side of the radiator and
equipped it with a small electric radiator fan acquired from the junkyard.  I
put a thermostat on the fan so that it doesn't run until the fluid temperature
reaches 200 deg F.  Since I didn't have an overheating problem to begin with,
this fan rarely runs.  Mounting the cooler in the radiator air flow does
little other than shuffle the heat around a bit and is quite ineffective if
there is already an overheating problem.

Of course, turning off the AC when overheating is evident will make a major

Another thing that will help is to make sure there is little leakage between
the fan shroud and radiator mount.  There was almost an inch gap between the
radiator and shroud on my Fury.  Simply sealing that gap with adhesive foam
and metal tape made a significant difference.

Finally, there is the fan clutch.  Most folks don't know it but there are
several different clutches for a given engine (at least for the Chevy engine
in my MH and the Mopar engine in my Fury) that outwardly look identical.  The
difference is what temperature the fan cuts in at and how much power it
couples to the fan.  The more power, the faster the fan turns.  In my case, I
found that while GM didn't list a MH fan per se, the most heavy duty clutch
was listed for a full size van with towing package.

A major complication is that the major aftermarket houses such as AutoZone
don't distinguish between the various clutches, or at least they didn't for my
vehicles.  I paid the extra money to get an OEM clutch from the dealership.

These clutches couple power via a viscous silicone fluid.  This fluid can
slowly leak out, decreasing the power coupling over time.  The clutch may seem
fine (spinning the fan at idle) but won't couple enough power to really spin
the fan up when hot.

An unexpected byproduct of upgrading the fan clutch is that unless your engine
has dual belt or serpentine belt drive to the water pump,  belt life will be
negatively impacted.  My MH only has a single belt drive but I found that buy
using NAPA's highest grade belt, I could get a year out of it.  That cheap
sh*t Kelly Springfield that AutoZone sells would last no more than a couple of

One other thing to watch out for.  At least AutoZone, and maybe other parts
houses, is selling a non-thermostatic clutch as the standard grade unit.  The
non-thermostatic unit does not vary the coupling according to temperature and
will make a marginal system overheat.  These can be identified by the lack of
a bimetal spiral on the front of the clutch.  At least for my vehicles, I
could get the thermostatic clutch by specifying the extra-cost "premium" unit.
I abandoned this chinese-made junk for the OEM part after seeing how poorly
they were made.


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