overheating problem 96 accord

Hi, all.
1996 Accord EX-R Vtec
5-speed
320,000 kms

I have a somewhat perplexing overheating problem. Starting last summer, I
noticed the temp gauge would slowly rise at highway speeds with the A/C on
and while I was pulling a heavy boat (it didn't overheat for 4 years
previous with in this condition). I shrugged it off as being something I
should expect to happen under those conditions. To make a long story short,
as of now, on moderately warm days, the car will overheat with just me in
it, no boat no A/C. The temp gauge climbs only slightly, but putting on A/C,
climbing a hill or obviously, pulling the boat worsens it to the point where
I need to throw the cabin heat on full to keep the needle from climbing.

This is what I have done/noticed during the past year. The rad seems to be
in fine condition ( I've had worse) and the coolant always looks as new. I
took it into Honda and they diagnosed the sensor that turns on the fans (the
same two fans that the A/C opereates but this switch turns them on when the
car is running even if the A/C is off) as faulty. (I had already determined
this as the first time I noticed a severe overheat condition was when I left
the car running for 4 minutes to get a coffee. After returning, the needle
was all the way in the red. If I hit the A/C button, the temp dropped
rapidly when the fans kicked on.). Being a student, I couldn't afford the
$50 for the sensor so what I did was short-circuit the switch, making both
fans run full time. The condition persisted.
I was convinced the entire time it was my [original] thermostat but Honda
told me it was fine. I changed it anyway and Honda was right, as the problem
is still there. Bottom line is, it's not the thermostat and the cooling fans
run al the time and the car overheats.

I have read on these N/Gs that there are often faulty temp sensors. I'm sure
Honda checked this out. I also believe this to not be the problem as the
symptoms seems to point to an actual high heat condition.
So what are my next possibilities? The only two I can think of is restricted
flow through the cooling system and/or a rad that needs replacing. Can
someone recommend to me how to check for either? IS there such a thing as a
cooling system flush that removes built up residue? Advice on changing a
rad?

Thanks a million in advance,
MajBach

 

You didn't mention if you bleed the cooling system after checking the
switch and after replacing the thermostat. For some reason if you
have trapped air in the heater core or elsewhere you may find that the
water pump will cease to do it's job correctly. I've not seen the actual
scientific proof of this but most often this can lead to an overheat. Bleed
according to the service manual for your particular model, or bleed so that
all air is out. Let us know how you bleed it, just to be sure.