Hey all, I know this is an old-ish thread but I wanted to add my experience with this same issue I had with my 85 Sun Land Express.
(btw I’m a long time listener and finally signed up.. going to make a new post soon all about my rig and the adventure it has been fixing it up!)
Anyhow, I rebuilt the engine last winter. Spared no expense. All OE/OEM parts in the cooling system with the exception being a CSF 2 row radiator. Had the same issue with “almost” overheating with extended engine load. Highway driving, hill climbing etc. Swapped the rad out for the 3 row CSF.. same thing, maybe slightly better or took longer for the heat to build. Checked thermostat on the stove, tried another one, tried drilling a 1/8 hole. Nope.. Pulled hoses and water pump and scoped as much as I could with a WiFi endoscope camera. Made sure water pump impeller was pressed properly and not free wheeling, even at boiling temps. (Btw the water pump is a NPW for a 84 because I upgraded to the dual row timing chain kit from 22RE Performance. The timing cover for this uses the 84 style pump. Anyway this NPW pump has a cast iron impeller which is supposed to be superior to stamped steel..)
None of this helped. Rad seemed to flow fine, no cool spots.
To check for running lean condition I checked EGR operation.. Checked O2 sensor output while driving on highway, as well as vacuum to the EGR. All checked out. O2 output was rich side because of high engine load. Adjusted AFM a couple notches rich. Again, no luck.
Installed a mechanical temp gauge by pulling the AC shutoff sensor on the intake manifold and using an adapter. Before installing I tested it on the stove and determined it showed about 6* cooler than actual. When the stock gauge was up close to the white line the mech gauge would read 220-225 (corrected). 60 down the highway temp would be 200-215. Not horrible but didn’t make sense to me with ambient temps being even as low as 60*. Before the engine rebuild the stock gauge would never go higher than slightly over mid. Granted, the stat in at that time was def not a 190* OE unit. But the cooler t-stat I tried trying to resolve this issue did nothing to stop the temp from climbing post rebuild.
After going through all of this I was dreading a possible head gasket failure.. or cracked head.. it baffled me though. The block was decked, brand new aftermarket head, new oe bolts and gasket. How could it be?!
To rule this out I bought a combustion gas test kit and it passed..
On our most recent trip this September I had a bit of a eureka moment when climbing a hill up to Lake Placid. The mech gauge got up to 225, but it didn’t seem like the fan was reacting to this. It was a slow climb, 30-40mph so with the window open I could hear things well. Toward the top of the hill there was a one lane section with a stop light. Still running hot we stopped there for a couple minutes. After a minute or so I shifted into neutral and gave it a rev.. the fan was much louder. So without much ambient airflow through the radiator, the bimetallic coil on the fan clutch must have opened all the way up finally and locked up the clutch more. With the fan engaged the coolant temp went down quickly. This got me thinking that the clutch just isn’t engaging enough/early enough. With this in mind I did some more googling and found a forum that talked about taking the clutch apart and changing the silicone oil to make the fan more effective. The normal stock clutch comes with 4,000 cst oil. Rock crawlers in the desert like to run 8 or 10,000 cst. One suggestion was to simply add some 30,000 cst oil. Others commented that sometimes these clutches would come with too little fluid straight from the AISIN factory! Also they used to sell the clutches with different weight fluid (possibly a regional thing), or sell them dry and you would order the fluid you wanted. Anyhow, AFAIK there’s no way to buy the AISIN clutch with a heavier weight fluid, which you’d think would be a good idea for a 22RE hauling a motorhome!.. I digress..
I added ~15ml of 30,000 cst fluid and closed it back up and put everything back together. Immediately noticed the fan being more engaged, even after the initial warm up of the clutch. Road tested on the highway and pushed it as hard as I could with the AC blasting down the highway. Highest temp I saw was about 200! And getting back to a lower load it would cool back to 190 or lower very quickly. Perhaps I put too much oil in! I’m going to see how this affects gas mileage next season to decide whether to take it all back apart and take some of the -now mixed- fluid out. What’s left would be a higher viscosity still. I think perhaps there’s a bit more fluid than can fit in the reservoir so the fan probably isn’t disengaging as much as it should when it’s not hot. Further testing is required..
So, I suppose it’s possible that these OEM AISIN fan clutches are not all created equal, and even a brand new one could give you issues.. Especially if you’re running high loads or high ambient temps with these trucks. I wouldn’t be surprised if these C&C trucks originally came with the higher viscosity clutches. Of course, I had to go through 27 other possibilities with my rig before I ever thought to question a brand new OEM part!!!! ARGHHH 🤨
I hope this post helps someone else having above normal temps in these circumstances. Hopefully it’s as simple as a fan clutch and NOT a head gasket!
Cheers!
