998 6.2L Fan Clutch/Overheating Issue

[TG5446]

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!!

 

[papakb]

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.

 

[cwajciec]

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.

 

[lowell66dart]

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

 

[TOM R]

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

 

[Retiredwarhorses]

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.

 

[TG5446]

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.

 

[TG5446]

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!

 

[NormB]

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.

 

[TG5446]

Thanks Norm, Ill be checking that now. Good call.

 

[ken]

First unplug the switch on the crossover. This will make the fan run all the time. Then test drive and see if it still overheats. Since it is a 6.2 you need to eliminate the concern of cracks in the block at the rear cylinders. The combustion gases will cause the coolant to overheat if you have this problem. If it stays put at 195 then the switch is bad.

 

[911joeblow]

Could be blown head gaskets or cracked cylinders as ken said. A chemical head gasket tester kit can be had for little money and with a sniff of the coolant will tell if you have combustion gasses in the coolant.

 

[TG5446]

Ok, So Ive been dancing around the possiblity of cracked cylinders or blown head gasket, mostly because I just dont want to admit it to myself. However, a couple of questions, How common is the issue on a low mileage engine, there was no loss of coolant prior to the boil over, and no white smoke in the exhaust. Other than the overheating, the thing runs excellently. The other weird thing is that the fan doesn't cycle correctly, so even if I unplug the switch and TDM, the fan isnt engaging to test whether it overheats regardless of the fan being on or off.
Ofcourse...I have been looking for an excuse to put a 6.5 Turbo with 4L80E in it so...there's that. Anyone have a line on a good takeout??

 

[911joeblow]

Unfortunately these engines have a poor coolant flow to the rear of the block which causes the rear to run as much as 50 degrees hotter than the front. The cooling system overall temperature will not necessarily show this as it it looking at overall system temperature which is being regulated by the thermostat and radiator/fan combo. The localized overheating at the rear causes the head gasket and cylinder issues common in these. There are also rear crankshaft bearing and rod bearing issues from this overheating too.

The fan cycling issue is due to steam pockets in the cooling system. These occur normally due to pump cavitation and abnormally due to gasket/cylinder failures pushing combustion gasses into the cooling system. To make matters worse the bleeding of the air/gases back to the overflow tank is really poor. All of this leads to occasional system 'burps' which suddenly move overheated rear block coolant to the front which triggers the fan on, or other times hides the overheated coolant by not allowing it to flow properly through the steam pockets that form.

We developed our cooling balance kits to solve these issues. Unfortunately if you do have blown head-gaskets and or cylinders these kits will NOT do anything for you until fixed.

Best of luck!

 

[ken]

OK If you unplugged the switch and the fan did not engaging then you found your problem. It will overheat easily. The clutch should be engaged by default for a fail safe. There isn't enough air flow through the rad while driving to cool it properly. As with most MV's the mileage really doesen't mean much. There is know way to know how much of it's life was spent off road in 1st gear. Or how many hours of run time were wasted in a column of vehicles waiting on the side of the road because the 2ND LT can't read his map. So while the engine could have a lot of hours on it, the truck its self could have only traveled a few thousand miles.

 

[ken]

If you need to replace your switch on the crossover You might consider using. Index Sensors & Controls Control Switch #8037024. Along with Milodon High-Flow Thermostat Part number 16406. The 6.2 was originally designed for 180deg. The 8037024 switch turns the fan on at 195 deg and off at 183. This keeps the coolant below the boiling point and helps prevent air pockets. But with your fan not coming on at all I bet your caddy valve has gave up. Check it first and see if you can get the fan to come on again.

 

[HUNvee]

That was my problem.
But the devil was my kickdown switch. It stays connected and the TDM did not release the fan clutch.

 

[ken]

Or you could scrap the whole thing and go with a regular fan clutch. I got a extra WP pulley from flea bay and bored a 1/4 inch deep pocket in my lathe. I then used a fan clutch for a CUCV and a 20 inch 6 blade fan. Along with the 180 Mildon thermostat. It now runs at 180 and I no longer have any more leaks or expensive parts to replace. This is the way the CIVI H1 is set up and is much smoother and quieter.

 

[avman]

Had the same issue with the fan not engaging. Finally replaced the friction pad and everything works great