I don't have all the answers, but as a shop owner I can relate experiences.........keep in mind that as you are cruising at higher speeds, air is deflected mostly around and under the vehicle...if the fan is not "locked in" then it is just along for the ride. Also if you look at how much of an opening the radiator receives for air coverage, in most cases 30 percent or more of the radiator is obstructed. With that being said, I do believe the OP may be looking at a head gasket issue, but one should never rule out other possibilities.

 

Only slight overheating problem I've ever had was at highway speeds. Replaced the clutch on the fan and the problem went away. I know it defies logic but as I've said before these clutch fans are a mystery in themselves.

 

There are several youtube videos that explain exactly how viscous fan clutches work, and they are not mysterious. They are very simple, and relatively few moving parts. Probably the biggest misconception is that they "engage" and "disengage" like an AC compressor clutch. The fan clutch is always engaged, but to varying degrees depending on the heat of the radiator. Internally it is similar to a torque converter (a fluid coupler that couples based on how much the fluid is allowed to move internally).

 

Same here replaced it b4 jumping into head gasket. But also my coolant cap was not looseing pressure.

 

10-4 on that volvo rad cap - it works

 

When I say the clutch fan is a mystery in itself I'm not referring to how it operates. GOOD explanation though. I'm meaning it's a mystery in determining if it by itself or it along with other components is the cooling system problem. There may be some that go directly from good to bad but I'd venture to say that the majority of them fail very gradually. Some so bad that it causes overheating at an idle but some gradually weakening to the point they cause a problem at highway speeds.

 

I was questioning my fan clutch on a thread a few days ago. Similar situation as Bad Idea. I got the same responses. "bad fan clutch only causes issue at idle / crawling etc" I still haven't ruled it out even though it "appears" to be functional.....


With AC on "NORM" at 75mph I consistently am running ~220, give or take 5 on "MAX" it seems to continue the creep but I haven't tested to see how high it will go. lol


I may break down and swap the clutch this weekend...

 

Fan speed can be checked with an optical tachometer, by marking one of the fan blades with chalk and using a timing light to observe speed changes, and/or listening for changes in fan noise as engine speed changes. Basically as engine speed increase so should fan speed when engine is fully warmed up, if fan speed is slow to respond or maintains a slower rpm, then it has failed.

 

I'm having this same problem too and I've tested for combustion gases in the coolant twice and I'm pretty sure now that the head gasket is not the issue. You can find my thread about it here in the OEM forum (currently front page)

Yeah I don't get these stupid clutch fans either. I replaced mine because it was spinning with the same amount of resistance (not freely spinning but not fully locked up either) cold as it was hot. Now the new one feels exactly the same!

Could they be non-functioning out of the box??

 

some helpful info about fan clutches I found on another forum:

"These clutches are never supposed to "lock in" they become more positive as the air temp increases and less positive as it cools in a range based on the viscous fluid in the clutch.

When they sit the fluid settles to the bottom and causes them to be much more positive and thats when they roar at start up. once the rotational forces work on the fluid it flows back out evenly and the clutch becomes less positive and starts to work correctly.

If the engine rpm increases past a point the same rotational forces push the fluid out farther and the clutch releases even more so the that the fan is not spun at too high of a rpm. when they are spun too fast they can cause the fan to explode and that can cause damage to the hood as well as all kinds of other things under the hood.

Another way they fail is they just stop cooling the engine even under light driving conditions. This is the most common and it will often show it self first at low speeds under load like in sand."

 

I don't believe a properly functioning fan clutch on your XJ will ever freely spin, either hot or cold. If it freely spins with no resistance, it would be a sign that the fan clutch needs to be replaced (likely because the fluid inside has leaked out).

There is a small bi-metal clockspring on the front of the clutch (that you can see with a mirror). It rotates as it heats up and lengthens. It's sole purpose in life is to rotate a small shaft, that internally will increasingly cover a small passage hole between the two primary sections of the fan clutch. The more this hole is covered, the closer the mechanical fan will spin to belt speed. The less this hole is covered, the slower the fan will spin in relation to belt speed.

So the most likely failure mode, provided the fluid has not leaked out, is that the small spring on the front gets rusty, or that the shaft that is moved by the spring starts to have problems rotating smoothly. A shaft that is having trouble moving can result in your temperature constantly dropping low and then building up to a high point, and then dropping back low again. (because the spring is trying to turn the shaft but the shaft is resisting the movement, but once the spring is really forcing hard, the shaft finally makes one large motion instead gradually moving with the changes in the spring).

I recently had this symptom, and even ordered a new fan clutch. But after inspecting the front of my factory fan clutch with a mirror decided one thing to try first was simply giving the spring and shaft a little spray of oil (I used Kroil, but some PBblaster, or the like would work). Sure enough, the temperature swings went away within the next 10-20 miles as the shaft and spring got back to smoothly moving instead of jumping from extreme to extreme. I never even had to take the clutch off the vehicle. Though reaching the spring on the front with a spray straw was challenging, and I had to use an extra long straw that I curled at the end, and a mechanic's mirror to see where it was going.

If the small shaft gets fully locked up, the fan will either be spinning too fast all the time, making more noise, robbing gas mileage, and causing strange under-temps (like driving around at 205-208 on hot days). Or it might lock up in the cold position, which will cause over-heating at higher engine loads or during extended idles. Meaning the fan never speeds up as it should under heavier engine loads and rpms. This will be particularly evident when high engine load, but relatively low speed (like going hard up a long hill at 45-50 mph). But as the hill ends, and speed increases, the temp will probably come down again because of the increased air flow caused by the vehicle's speed, and the reduced load on the motor at the same time.

 

Interesting. So you're saying it's magic!!

Seriously though I do appreciate the thorough explanation

 

Yes, it's magic!

I'm actually impressed by components that "do stuff" but have no wires going to them. I think that is pretty neat.