The Twin Cam cooling system has been a trouble spot for many Fiat owners/mechanics, primarily because of a basic misunderstanding of the system. Some criticize it as being overcomplicated, but I've found that with the proper maintenence and procedures, it can be quite trouble-free.
Next thing to inspect is Hoses. Factory hoses last a long time, and usually
die prematurely due to oil soaking. Replace any oil-soaked hoses (fix that
oil leak sometime too!), and look for signs of 'pregnant' bulges on every
hose. New hoses are available from Fiat, but in a pinch you can usually
take the old one down to the Parts house and see if you can match one up...
You might end up cutting a foot or two off the new one, but you can usually
make something work. See the Parts Substitution area if you want some
common Gates matches for Factory hoses.
While you are checking hoses, make sure that the small hose to the recovery tank is clear- It's usually a good idea to remove and flush out the recovery tank, and then use low-pressure compressed air to make sure that the path from the radiator neck to the tank is clear. It's vital that you do this, or the bleeding method I describe below will _not_ work!
OK, with radiator and hoses theoretically squared away, there are only a few items left- Let's start with the Thermostat.
The Three-Way thermostat used on later Twin cams has been cursed, modified, and in some cases simply done away with... David Voss likes to replace the 'Tee' on the top of the cylinder head with an early-style single outlet part, and put a Chevrolet stat in the head itself. I'm going to encourage him to write an article on this procedure.
Assuming the three-way is what you've got, you can test it. These stats are in excess of $50 new, and really do last a long time, so they are worth the time it takes to diagnose them. (you will need a Pot you don't care about eating from anymore, and a Candy Thermometer) - Immerse the stat in hot water on the stove, and inspect it periodically with a set of Tongs - Along around 180 degrees, you should start to see movement in the Internals - Depending on whether the stat is a Savara, Wahler, etc it might be a little harder to see the inside of the Stat, but in any case you should be able to see the valve opening up internally. By 190-195 degrees, the stat should be fully open (good sized gap appearing internally for water flow). It's hard to do with a pot of boiling water in front of you, but try to make sure that the stat opens smoothly. If it 'jumps' along it's travel, you might think about investing in a new one.
With all these items checked/replaced, only one item remains, and that is the electric Cooling Fan. However, we need to bleed all the trapped air out of the system, before we can check the Fan Switch, etc!
I disconnect the left 'Radiator to Tee' hose at the radiator, and slowly fill the system. Make sure the Heater Valve is open. Patiently fill the system, and listen for escaping air - If you fill too fast, you will create air pockets that will complicate the bleeding procedure.
Once you have the system as full as you can get it, attach the hose to the radiator again. Fill the radiator and recovery tank to the top, and have at least a gallon of coolant mix handy for the rest of the procedure. Start the engine (If Carbureted, get it off fast idle as soon as you can), and let it IDLE. As the engine warms up, you will see the first big pockets of air escape - keep the radiator topped up, and watch the temp guage - Once you get past about 180 degrees, install the radiator cap. Let the engine continue to warm up, and when you start seeing 185-190 on the temp Guage, start feeling the lower hose (radiator to thermostat). If the stat is doing it's thing, it should gradually warm up, but don't panic if it is not. Keep idling (this can take 10-15 minutes in some cases) and watch the guage. If you start seeing 195+ on the guage, shut the engine off and don't touch anything! Wait 5-10 minutes (during this 'quiet time' you should see and hear air escaping into the recovery tank), and then start the engine again. Typically, while the engine sits, it warms the thermostat to the point that it opens right away on a restart. Once you've verified that the stat is open (lower hose and bottom of radiator nice & warm), you should get an electric cooling fan at about 190-195 degrees, that kicks back off again at about 185.
If you experience problems with this procedure, you can let the engine cool for 15-20 minutes, carefully remove the cap, and make sure that the system is full enough- I've only really had this procedure fail if I was too hasty with the initial fill, and did not get enough coolant in the system to start with.
If your electric cooling fan did not kick on at 190-195 degrees, there are a
few possible causes - Before you start looking at possible electrical
problems, make sure that your Thermostat is opening! (again, when the stat
is open, the lower hose and lower radiator tank are approximately as warm as the top radiator tank)
This being the case, the next step is to remove the two leads that go to the Fan Switch (some cars still have the little polarized plug) and put a jumper wire between them. With they key on, the fan should run continuously. If the fan runs with key on and the Switch bypassed, you need a new fan switch.
If the fan does not run at this point, unplug the Fan itself from the harness and apply 12 volts to it directly.
If you still do not get a fan, the motor itself is Zorched and needs replacement before you proceed. If your fan does run with direct voltage, you now have the unhappy task of tracing the circuit from the Fan Switch to the Cooling Fan relay, and from the relay to the Fan. Don't be tempted just to rig a bypass straight from the battery to the Fan, as this will overwork your charging system and is a kludge anyway... The fan circuit is simple, and if worse comes to worse you can wire a new one and bypass the cars' wiring harness.
There should be some upcoming articles that support some of the problems you might run into, but for the time being you can send us some email or post to the Forum.
May you have Cooling System Happiness!
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