COMMON FOX COOLING
ISSUES AND UPGRADES
FANS
Fox Mustangs came with either mechanically operated clutch fans or electric units on fourcylinder and turbo cars. Mechanical fans are fairly reliable, but they’re driven off the water pump
which costs some horsepower. They also use a declutching mechanism that allows the fan to
freewheel when the heat load is light. At lower temperatures, the fan has a higher slippage rate
and turns slower. At higher temperatures the fan slips less and turns faster. Over time, the clutch
drive tends to wear out, allowing the fan to freewheel all the time. This can lead to overheating,
particularly at idle.
To test if it’s worn:
• ith the engine off, grab a blade by the tip and wiggle fore and aft, checking for play at the
W
center shaft. There should be none.
• ith the engine cool, idle the engine and push a rolled up newspaper against the hub — not
W
the blades. You should be able to stop the fan when the engine is cool but not once it’s fully
warmed up.
• Listen for the fan to engage and make more noise as the heat load increases past the threshold.
It should also disengage and get quieter as the engine cools past that same threshold.
If the fan clutch fails any of these tests, replace it.
Fox Mustangs used three different types of clutch fans:
• flex blade, ’79-’81
• seven steel blades, ’82-’85
• nine-blade plastic fan phased in during ’85 model year.
The plastic fan is generally very reliable, but check at the base of the blades where they attach to
the hub, where they can develop fatigue cracks. Replacement fans are widely available through
vendors like Latemodel Restoration and even when combined with a replacement clutch assembly,
shouldn’t run you more than $120.
Note: The ’79-’93 Mustang water pump spins in reverse rotation, so the fan/clutch must be
directional, too. The ’78-and-older Mustang V-8s and engines use conventional belts that rotate
in the normal direction.
Electric fans are popular for the sake of efficiency and space, and were used on ’94-and-later
Mustangs. They’re a good retrofit on Fox cars, especially high-powered street-strip or road race
cars which require maximum cooling for short bursts and minimal parasitic loss of power. If
you’re planning on installing an electric fan, it’s a good idea to incorporate an override manual
on/off switch. Also, use a relay rated for the circuit, to deliver the required current. On most mild
street Fox Mustangs, unless the car originally came with one, electric fans are probably overkill.
UNDERDRIVE PULLEYS
Many 5.0-liter Fox cars were fitted with underdrive pulleys as an inexpensive way to reduce parasitic loss, freeing up horsepower and thus increasing
horsepower to the rear wheels. The downside is that the pulleys turn the water pump more slowly, reducing the coolant flow through the engine. Some
owners have reported cooling problems as a result. Was this because their cooling systems had other problems? Installing an electric fan (or two small
fans) may help.
40 FOXMustangMagazine.com