Getting hot? Check thermostatic spring

Hey there, time traveller!
This article was published 14/10/2005 (7560 days ago), so information in it may no longer be current.

QUESTION — I have a 2002 Chev 2500 HD 4 x 4 6-litre with a 4:10 rear axle ratio. I pull a 28′ fifth wheel RV that is within the GVRW. I bought the truck used. It has 60,000 km. Under normal trailering, it seems to be a great combination, but when I get into hilly driving, long grades or steep hills, the truck overheats.

I have changed the thermostat and had the fan clutch checked with no change. Other RV’ers with similar units say they can hear the fan cut in during heavy pulling. Mine doesn’t. The mechanics say there doesn’t seem to be anything wrong with it. My question is what triggers the fan to speed up? I’m told it is activated by coolant temperature. Is it possible the switch mechanism could be at fault? Any suggestions or help would be appreciated.

ANSWER — Your truck uses a belt driven thermostatic cooling fan. Inside the ribbed fan hub, there is a thick silicone fluid. During normal temperatures, this silicone sits in a reservoir in the hub, but as temperatures go up, the fluid is directed into vanes between part of the fan hub connected to the water pump and the other part of the hub connected to the fan blades. The silicone fluid drives the fan blades to draw air through the radiator and cool the engine.

There is a thermostatic spring on the front of the fan hub that senses the air temperature from the radiator. If the engine gets hot, the thermostatic spring opens passages for the silicone fluid to engage the fan. If the thermostatic spring is not operating correctly or the silicone fluid has leaked out through a faulty seal (not a common fault), then the engine can overheat.

Normally, when you start a cold engine, the fan will “roar” because the fan is being driven by the silicone fluid for a few seconds until it can be distributed in the hub. Then the fan will become quiet. The change in sound is quite noticeable. To test for fan operation, cover the truck grille and operate the engine at 2000 rpm. As engine temperature goes up to between 85-96°C (185-205°F), the fan should start to “roar” again. Don’t operate the engine warmer than 120 C during the test. If the fan does not engage, you need a new fan clutch. Finally, shifting to a lower gear will decrease engine load and heat. Under less load, the transmission torque converter can also lock up. A torque converter is a major source of heat when it is working, so letting it lock up can reduce heat transferred to the radiator as well as prolong transmission life.

QUESTION — I have a 1993 Toyota T100 with a 3.0 V6 which keeps losing antifreeze. Here is what we have done so far, but we are still losing it, as a matter of fact, I think we are losing it faster. Hopefully you have some ideas: we have flushed and pressure tested it, found a small leak and replaced a hose, checked the oil and found no antifreeze, checked the spark plugs for a whitish colour which we were told would indicate burning of antifreeze. They were clear.

There is no antifreeze under the truck after driving and parking it. Someone indicated it could be leaking in the automatic transmission housing but sooner or later that would fill up and start leaking somewhere or it would quit losing it out of the coolant system. My son had the truck before me and he said it has been doing this for about a year but never was an issue. He added a couple a litres now and then. It appears to be losing it faster now. Any ideas?

ANSWER — It is possible for coolant to leak into the automatic transmission through the transmission cooler in the radiator, but this should be noticeable as pink foamy fluid on the transmission dipstick. Your coolant leak is more likely an external one, even though you can’t see one.

A small leak can allow steam to escape so coolant levels drop but never leave a puddle. Often, there will be a stain around the leaking area, but this can be difficult to see under manifolds or on a dirty engine. Pressurizing the cooling system with a pressure tester may enable you to find the leak, but small leaks are often best found by adding a special dye to the coolant and inspecting with an ultraviolet light. The dye will show up, pinpointing the leak. If your dealer doesn’t have the dye and equipment, many radiator shops do.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net