Land Rover Repair: Stange behavior on 98 Disco, fan clutch, question thanks

Question
QUESTION: John,

My low mileage Discovery had a couple of incidents of near overheating this summer - which I always caught quickly, never letting the needle pass the top of "normal" range. This was in brutal 100+ degree heat, though and after replacing the thermostat, top hose (old one was swelling and just worn out) and flushing the system, have generally had a cool running truck. This weekend, I was driving on divided highway, 60+ MPH, light rain, etc and noticed the needle sitting at the top of the normal range. Turning off the AC and cruising as I came in to town and 40 MPH traffic did not help, but when I stopped, and turned off the lights before cutting the engine off, I noticed the needle dropped a bit. Lights back on, needle climbed, lights off, it dropped again. I tested once home and found 12.60v at the battery, then 14.18v once the engine was running. Tonight this happened again - running hot, but after a decent hill climb on 50 MPH highway, then a drop as I descended the hill. Lights, AC, etc on. On a hunch, I removed the fan/clutch and found the thermo coil on the clutch gunked up pretty badly. I cleaned this to nearly new condition, but do not need to drive again until morning commute, so I'll be watching things tomorrow. Do you have any thoughts?

Thanks,
Michael

ANSWER: I have written a number of articles on overheating in Land Rovers like yours.  Look on the Robison Service website under service > rover > advice.  Also look at the Robison Service blog.  There is no single answer to your question but I will say two things:

1 -  gunk on the fan clutch will not cause overheating but if the gunk means the fluid has leaked out of the clutch, then it will spin free and that will cause overheating.
2 - the next thing I'd look at - given the truck's age - is the condition of the radiator.  It's surely due for change if its original

---------- FOLLOW-UP ----------

QUESTION: Thanks John,

I could have perhaps organized the question a little better, but I am still a little confused at the temp gauge changing slightly with the lights being switched on/off since the battery is 13 months old and the alternator is still showing strong. I studied the behavior though, and found that rise in temp came after episodes of heavy load, such as ascending a steep hill at highway speed, in 90+ degree temps (east TN here) My thought on the thermo coil on the front of the fan clutch was that the gunk was maybe causing it to not respond effectively to temperature, and even so, not have appropriate free movement. After cleaning and testing, and now having commuted for a few days, I can say I hear it running like I was used to on my previous Disco, this being my second, and temp stays at about 45% of the gauge no matter what driving condition now. I am sensitive to this issue, and had read your info a few years ago. I am a home mechanic and had replaced a cracked intake, water pump, hoses, stat, etc on the previous disco at any sign of trouble. I will not cook my Disco and ask "WTF?"... I do not particularly take joy in engine rebuilds. I just happen to be seeing unprecedented behavior from this current example of the model.

Thanks again,
Michael

Answer
If you observe a change in the performance of the fan clutch after removing and cleaning it, then the clutch is almost certainly going bad.

The external appearance of the clutch is not going to have any effect on its functioning.  The clutch works by spinning two turbines with a viscous fluid between them.  When the fluid gets hot it thickens and the clutch locks up.  So logic tells us a dirty clutch would get hot quicker and hence be locked up more.  So if anything your problem should be worse with it clean!

If there is an audible change in its operation following removal, handling, cleaning . . .  I suggest that is a sign it's going bad and it has responded to your removal, manipulation and refit.  I would swap it for another one.

Your description of heating on hills sounds more like a radiator problem.  The clutch should not matter much (unless it totally freewheels) when the truck is going 50 or 60.  It is also the kind of behavior one sees in early stages of head gasket failure or liner failure.

As to the gauge moving when you switch the lights on . . . I would use a temp gun to see if the engine temp really changes at the sensor location when you switch lights off and on.  My guess is you are seeing a voltage stabilization problem in the instrument cluster that causes the gauge to change, or else there is an alternator problem causing the same result.

If the alternator were taking enough power to heat the engine more you would hear it stagger as the load kicked in.  That happens on trucks with 4-5 sets of off-road lights, for example.  You switch on 100A of load and you can hear the effect on the motor. Ordinary headlamps on a Disco should not do that.