When overheating, at what point on temp guage should fan increase in speed? New unit, it does roar at start up, but not at 3/4 guarge (top of normal range).

 

technically speaking the fan and fan clutch don't increase in speed. To put it very simply, the outer section of the clutch where the fan is bolted is merely going to spin less freely when warm on its center section (clutch hub).

When the engine is hot, the clutch fan runs nearly as fast as the engine. When the engine is cold, the fan runs much more slowly

There is a range of operation to the thermostatic spring on the hub so it is not on or off but varriable based on engine temp.

 

I see you say it's new, but my experience: my old clutch would roar at start up, then that was it. No more 'till next start up. Replaced it, now I can definetly tell when it kicks in.

It should be running well before the top of norm. My new one kicks in before my temp gauge leaves mid-way(norm)

 

I don't have it in front of me, but the clutch should be fully engaged at the 220 degree mark. Have you actually hooked a scanner to the truck to track you temp range?

 

Tried my scanner, I get readings (no fresh codes), but my el-cheapo scanner does not display temp when engine running. Used my infra red thermometer and put the spot on the metal housing of the thermostat, gets to 240, fan increse slightly. Temp of front of clutch in the 150 - 170 range, but temp guage is approaching credit limit. Turn off AC and maintain 1500 rpm and it cools off.

I changed thermostat to an American 180 degree - no change since the factory unit is about the same based on difference between start to open and fully open. New stat in correct position with bleed gizmo at top. Put right side front tire on ramp and ran engine with expansion chamber open, to help "burp", topped off with distilled water.

May try 160 stat, or a "high flow" stat. Other options include swap 9 blade fan with 11 blade from a Range Rover, perhaps there is some domestic car that has an alternate fan.

Wonder what the difference is on the Chevy subsitute fan clutch (non-AC) and one built for AC? When I looked at the Hayden web site (Imperial fn clutch) they say 170 degree to engage all their version of clutch units.

 

The one for a/c is "heavy duty", whatever that is supposed to mean. I used the non-a/c Imperial Chevy clutch, and it definitly does the job.

 

The Chevy a/c fan clutch is a heavy duty clutch, it will come on at a lower temp and run all the time and suck your gas.
The cut in temp is 210, cut off at 200.
I found out the hard way that you must leave the new fan clutch sitting vertical for 24 hours before installing it to allow all of the fluid to settle, kinda like you have to do with a fridge after you move it.
If you do not do this you will ruin the fan clutch.
It is also possible to install the fan backwards on the clutch.

 

Don't go messing with different t/stats, go find the actual problem by, again, using a scanner and get some accurate numbers, then get back to us.

 

Thanks to Disco Mike and others who replied. My overheating problem appears to have been a combination of bad fan clutch (replaced with pre-owned fairly new unit), junk between radiator and condenser (cleaned out), missing fan shrould top cover (replaced with pre-owned one), and radiator clogged up.

I replaced the used fan clutch with a brand new Chevy model, still overheated at idle with A/C on. When I measured the temp of the front of the fan clutch, it was about 150 degrees, so it never would speed up to give extra cooling. Factory says it needs to be 170 to do that. Radiator shop told me that radiator temp should be within about 10 degrees top to bottom with truck fully warmed up. Checked mine and it was over 60 degrees different (reading the fins with an IR thermometer, engine off). So the fan was being "fooled" because the air flow over it was from the cooler part of the radiator.

Removed radiator, laid it flat, poured in a gallon of muratic acid (about $7 at hardware store). Let sit half an hour, with some aggitation. Lots of gunk came out, black, gray flakes, etc. Flushed with baking soda and water, and more water. Then filled it back with CLR brand cleaner, and let sit five hours. Flushed that out with baking soda and water, and flushed some more. Flushed in both directions.

Re-installed, ran it and temp was uniform over the radiator (with in 10-15 degrees). Fan seems to be keeping temp down in normal range. There is increased cooling (a little "roar"), but not the sound level you get at a cold start, and air flow without A/C on is not enough to spin condenser fans. Mine do spin at cold start just from the air flow. So the thermal valve on the fan clutch is not on/off, it is more like a variable speed drill. Will report back if more problems, next step will be rod out radiator.

 

Your making progress, great job.