SHO cooling system maintenance

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Thanks mostly to Gary Morrell for compiling this information.


There have been 2 OEM thermostats for the SHO, E9DZ-8575-A, and E9DZ-8575-B. The B suffix is a later, improved unit that is supposed to better withstand the corrosive breakdown of the antifreeze, more on that later. Dealer list for this part is about $27. There is also an aftermarket alternative in the Robertshaw 492-180 (180F) and 492-192 (192F). The 192F application is closest to the stock 190F calibration. These pieces look to have all the right hardware for the SHO and are only $9 each, one third the dealer price. The gasket (actually an O-ring) is either an NEA-813 or a Felpro 35278. Another thermostat alternative may be the Robertshaw Extra Performance 330-195 (195F). I've checked the mechanical fit but haven't actually tried it in the engine. This unit does not have jiggle valves, but does have a small gap in the valve interface that should function in a similar manner. Unless you have lots of engine mods or have the access and knowledge to appropriately change your engine calibration, I'd recommend the 192F Robertshaw part. A lower temp thermostat may richen your fuel mixture a bit, but at the expense of increasing exhaust emissions, dirtying your oil faster, and shortening the life of your cylinder walls and catalytic converters. The minute performance gain is simply not worth the end stage effects. If you're having overheating problems, the solution is likely to be more complex then a lower temp thermostat.

Jiggle Valves

These are the little numbers at the edge of the thermostat that look like captive pins in holes. Their purpose is to allow air bubbles out of the head and into the radiator, where they eventually bubble out into the recovery reservoir, even when the thermostat is closed. As such, the thermostat should be mounted with the jiggle valves at 12 O-clock.

Draining the cooling system

Draining the radiator is easy, but the engine block drain plug is darn near impossible to get at, and it's very difficult to undo heater hoses to backflush the heater core if they've cooked onto their fittings for a few years. You may want to consider paying to have this done if you can verify that the heater core et al. is in fact done properly.

Filling the cooling system

The SHO is a tough system to fill completely from empty, if you just pour coolant in, the air trapped in the highest point of the system will prevent you from getting the last few quarts in. This can create some excitement during the first warm-up as what little coolant there is in the heads begins to flash into steam. Here's how to get around that: look under the throttle body and you'll see a hose entering the throttle body vertically, just under the throttle plate shaft, this is a loop that warms the throttle body and idle air passages to prevent ice formation in freezing weather. Loosen the clamp from this hose and work it off the metal tube that enters the throttle body. Position the hose next to the metal tube, then wad up a towel and put it under the hose to catch the drips. This is the highest accessible point in the cooling system and will allow air to escape as the system fills. Now, SLOWLY fill the system thru the radiator neck, I prefer to predilute the antifreeze 50/50 with water, that way I don't have to keep track of how much of each that I've added separately. About halfway thru the filling process, give the lower radiator hose a few squeezes to dislodge any bubbles that may be trapped at the water pump inlet. When coolant starts dripping from the disconnected hose, reconnect the hose and clamp. Now you can replace the radiator cap and fill the recovery reservoir to about halfway between the COLD and HOT fill marks. Start the engine and idle it until the upper hose is hot and feels pressurized, all the while, checking for leaks.

Over the next few days, check the recovery reservoir with the engine cold to see if it's low. It generally takes several hot/cold cycles for the bubbles and dissolved gas in new coolant to work their way out and into the reservoir.

Cooling System Sensors

There are 3, they are:

1. The Engine Coolant Temperature sensor or ECT. This 2-wire sensor measures the coolant temperature and transmits it as a voltage to the EEC-IV module. The ECT is a negative temperature coefficient (NTC) device: its resistance decreases as its temperature increases. The voltage on the sensor's Green/Yellow wire will vary from 4V at 32degF to about 0.3V at 250F. The system is fairly linear, so:

coolant temperature in degrees F = ( volts at sensor - 4.5 ) / -0.0165

If this sensor fails open circuit, EEC will think that the engine is always dead cold, a short circuit fail would indicate severe overheat. The latter failure mode may cause EEC to stop the engine, I haven't gone looking for a software strategy for this. The ECT sensor is the single pin device that's threaded into the coolant manifold near the thermostat housing.

2. The temperature sender for the dashboard gauge. This is a one-wire sender that relays coolant temperature information to the dashboard gauge as a function of current. This sender is also an NTC device. It should read about 75 ohms at 50degF and about 10 ohms at 220F. Like most Ford "gauges", the temp sender/gauge combination is not meant to be an instrument, it is an enhanced idiot light. Under most conditions, with everything working correctly, the temp gauge should run in the middle of the NORMAL range. This equates to about 185F at the thermostat outlet. This sender occasionally fails to a mode that makes the dash gauge read lower then it should. It is located in the coolant manifold to the left of the thermostat housing. This sender has no EEC-IV function or connection. It's part number is part number is F1SZ-10884-A, and the cost is minimal, if you're replacing the thermostat, you should probably replace this as well.

3. The coolant level sensor. This sensor is located in the recovery reservoir and lights a dashboard lamp if the recovery reservoir is near-empty. '89 and early '90 models used a float switch, late '90 and later models use an electronic sensor that changes resistance in the presence or absence of coolant. The latter type is prone to false readings due to gunk build-up in the bottom of the recovery reservoir and on the sensing contacts. This sensor also has no EEC-IV function or connection.

Corrosion and coolant breakdown

The SHO's combination of aluminum heads and iron block sets up an electrolytic effect that tends to acidify the water/antifreeze mix over a period of time. This effect was responsible for the short life of the early A_suffix thermostats. Despite the new B-suffix thermostat, Ford now recommends flushing and replacing the coolant every 12 months / 15K miles. I second this recommendation, especially now that I own a bunch of stock in Prestone and Zerex. ;-) This doesn't mean that you have to load up the environment with used antifreeze; the recycling systems that are currently in use filter the old stuff, adjust the water/glycol balance, buffer the acids, and replenish the corrosion inhibitors.


I personally prefer Ford Heavy Duty Antifreeze/Coolant, although most stuff on the market meets the auto-maker's new car requirements. Stay away from coolants that are not specifically formulated for aluminum/iron combinations. I can't recommend Sierra for 2 reasons: first, propylene glycol/water mixtures have about half the heat transfer capacity of ethylene glycol/water mixtures, and second, I don't care for their advertising campaign: ALL used antifreeze is highly toxic, due to the metal contamination that it picks up while in the engine. Additionally, to my knowledge, no auto manufacturer has approved propylene glycol coolants for use in their vehicles. 'Nuff said.

Radiator maintenance

Thanks to Paul Chapin for this info on how to crimp the radiator if it starts leaking from a seam : 1. If locking-type(vise grips) are used to squeeze header tabs against tank, install a hex nut on pliers adjusting screw. 2. With jaws of locking-type pliers closed and locked, turn adjusting screw to position jaws against shank of a 10.9mm(27/64-inch) drill bit. Tighten hex nut on adjusting screw against handle to lock adjustment in place. 3. Squeeze header tabs down against lip of tank base with locking-type pliers while rotating pliers toward tank.

Joshua Teixeira adds that he was able to use these instructions to correct a leak on his '95, but had to set the pliers at 3/8" "since 27/64 was too loose to really do anything".