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Thread: 998 6.2L Fan Clutch/Overheating Issue

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    Default 998 6.2L Fan Clutch/Overheating Issue

    Hi Friends,
    Based on the threads I have researched here, the cooling systems on the 998's can have numerous gremlins to chase. I have an issue that doesnt seem to map on to any one threads so far, so I would love some input. My issue started as an intermittent overheating issue, here is the scenario: '85 998, 6.2 orginal motor in what appears to be well maintained good order. I could drive the machine some days and the fan would cycle on at about 215 (Per the Gauge) and cycle off at about 195. When running, the thermostat seems to open at 195 as expected and the temp will stabalize there for a while, then slowly creep up until the fan actuates and the temp goes back down. So far so good, however other times, the fan will not cycle on, causing the temp to climb to 240, at which time I shut it down and wait till it cools down. This makes the vehicle obvioulsy untrustworthy, so I started trouble shooting. First thing was to replace the overflow tank cap, then the thermostat, and replace most of the fluid (though I have not flushed the system yet). These improvements made no change. Then I replaced the cadillac valve figuring that I might have an intermittent issue with the valve solenoid. When I did this, I noticed that the old valve seemed to be hooked up incorrectly(based on the TM drawing) so I hooked it up per the drawing. Upon testing, I blew an o-ring in the fan clutch and spewed fluid. (so maybe there are few possible schematics??) There is another thread here which noted a similar issue, the cause being a bad NOS cadillac valve so that is a possibillity. Anyway, Id like to hear if anyone has experienced anything similar. I might replace the TDM and the Thermo switch, however, neither of those seemed to be the issue using basic trouble shooting procedure. Thank you in advance!!

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    There are several online videos about troubleshooting these problems along with a complete flow chart in the -20 manuals specifcally to address overheating issues. Your right that there are several different problems that could be causing it but follow the guidelines in the manuals and get back to us.

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    Does the coolant ever bubble over when you are at 240? Or other obvious signs of overheating? Rough guess is a TDM acting up and not dropping power when it should. Next time it happens I would take a multimeter and test the 4 wire plug for power to the Cadillac valve. Then check to see if the temperature sensor breaks continuity when it’s hot. At least then it would tell you if it’s electrical or mechanical.

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    You might want to pick up a temperature gun and make sure you don't have a funky gauge.

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    I did the tdm on mine but the real problem was the temp sensor on the back of the thermostat housing, its like $90

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    Most common failure is the thermostatic switch in the water crossover. Then the TDM and lastly the Cadillac valve solenoid.
    The Cadillac valve body I have never seen fail, it’s just the solenoid that screws into the body that fails.
    AFAIC, your issue was/is the thermostatic switch....seen this issue many times.

    As far as the plumbing of the Cadillac valve, if done wrong, you will blow lines off, sounds like maybe yours was plumbed up fine. The TM pics are wrong in many cases, so be aware.
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    Yes, the fluid will definitely boil over. The gauge seems to be fairly accurate. I would have thought TDM or switch also, but when you unplug the TDM , the fan still doesnt engage always. So, it leads me to believe that the TDM or switch are good, or at least not the only thing that is bad, considering that the fan should default to engaged once 24v is cut.

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    Thanks RetiredWarHorses, Yes the pics are bad and yes, I was correct the first time I installed the CV. I'll be switching it back and pulling the fan and fan clutch this weekend to service the O-ring, I'm also replacing all of the lines and fittings for good measure. Its odd to me that the fan won't engage even when I kill the power from the TDM. Im now wondering if I got a bad NOS CV, I've asked to exchange the unit, we'll see if the seller steps up. Im still leaning to that as the problem, but Im going to order the switch anyway. Thanks again!

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    Quote Originally Posted by TG5446 View Post
    Thanks RetiredWarHorses, Yes the pics are bad and yes, I was correct the first time I installed the CV. I'll be switching it back and pulling the fan and fan clutch this weekend to service the O-ring, I'm also replacing all of the lines and fittings for good measure. Its odd to me that the fan won't engage even when I kill the power from the TDM. Im now wondering if I got a bad NOS CV, I've asked to exchange the unit, we'll see if the seller steps up. Im still leaning to that as the problem, but Im going to order the switch anyway. Thanks again!
    Measure the resistance on the CV. IRC it’s about 75 Ohms. (it’s in the manuals) Easy to do even with a $5 VOM from Harbor Freight.
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    Thanks Norm, Ill be checking that now. Good call.

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