Even with a failed viscous coupling on the fan, it shouldn't overheat when idling. The fan is still turning and the viscous will only lock up when it gets very hot. Overheating at idle is usually due to a failing water pump not shifting the coolant fast enough or a partially clogged radiator not allowing it to circulate fast enough. When it starts to get hot, poke the Prog button on the HEVAC. If that brings the temperature down, the radiator isn't doing its job.

Not sure when the fans will cut in. They should run at low speed in series whenever the AC compressor kicks in and when it gets really hot they should come on in parallel so run at high speed. I've only ever heard mine come on at high speed once and that was when I was calibrating the suspension so it was running at idle for maybe an hour.

 

Thank you for the reply.

My water pump is new, so that leaves only the radiator as a failure point if it isn't the fan clutch.

As soon as I start driving, the temperature drops.

I had this happen with other cars with a fan clutch.

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you may have air pocket in the cooling system.

 

You'll know when the clutch engages as it will sound as though you have a jet engine under the bonnet. You may notice it for the first 30 seconds or so on a cold start as the clutch will lock up until the silicone filling gets spread around evenly. It will get hot if there is insufficient airflow through the radiator to keep it cool. The fan will be turning at the same speed at idle if the clutch has engaged or not, so that just means the radiator needs more air than it should. Considering the other faults you have had on this car, unless you have replaced the radiator you can't rule it out. An air pocket would cause pressurisation and it will overheat then as it blows the coolant out. There are only two causes of overheating, insufficient flow (either coolant or air) and insufficient coolant. An air lock will cause excess pressure so it blows coolant out, then you suffer from the latter.

 

When you say "New water pump" was this a new expensive pump with an iron impeller, or an inexpensive pump with a plastic one? (I believe the ones I have used are made in Spain and cost double the price of a pattern part)
The cheap pumps can last as little as 1 month, where the quality parts are good for a couple hundred thousand miles.....So, it could still be the pump.

 

When my water pump failed a cheap pattern replacement was all that I could find. It did have a steel impeller but subsequently when the engine was rebuilt I replaced it with an Airtex pump. The Airtex was much heavier.

 

Do the newspaper test on the fan. . . . . i.e. If you can stop the fan at idle with rolled up newspaper, then the viscous needs replacing. If you get lots of shredded paper, it's probably ok.

As Richard says the electric fans are either low or high speed for AC depending on temperature. The separate mode for engine cooling is somewhat vague:

 

I have got a manual override switch for the A/C fans on my diesel. The fans are two speed. On slow they are wired in series and on fast they are wired in parallel.
Like all other electrical connections on the p38, there is a permanent +ve supply and the switching is done by earthing the -ve side.
You could just splice into the cable with a separate earth connection and switch, or to save butchering the harness, make up some proper econoseal connectors and run the earth through a relay.
With the factory setup, fans do come on when the engine overheats. From experience it is just at the start of the red. It bit more into the red and the (diesel) engine ecu then reduces the power as a failsafe.
The fan override certainly helps but is no substitute for a viscous fan that works. The blast from the fan is a lot more. The fan off a 300TDi fits, ERR2266, and is a lot cheaper.

 

Great, thank you for all the suggestions.

With regards to the water pump, it had a metal impeller - and if it were in fact the pump, it would consistently overheat, not only at idle.

I have had failed fan clutches are many other cars, 1980-1990s V8 and inline 6 Mercedes-Benz, and ALL started overheating when the fan clutch starts to "slip".

I have also experienced on many other car that have electrical fans, if the electric fans fail, none of the radiators can cool the engine enough at idle - it will invariably overheat.

The radiators can only really "radiate" if there is sufficient air going over the surface of the radiator.

Yes to the "no substitute" to the viscous fan - the electric fans cannot pull the same amount of air.

 

The radiators can only really "radiate" if there is sufficient air going over the surface of the radiator.

For car radiators to really "radiate" they need both sufficient air going over the surface of the radiator & sufficient volume of coolant going through the radiator.

 

Some of the cheap aftermarket radiators also are made with less heat transfer area. The depth front to back is shorter. As well as quality control problems with the internal baffles causing the coolant to bypass some of the radiator tubes. You get what you pay for. I would only buy either OEM or a decent all alloy radiator.

 

Some of the cheap aftermarket radiators also are made with less heat transfer area. The depth front to back is shorter. As well as quality control problems with the internal baffles causing the coolant to bypass some of the radiator tubes. You get what you pay for. I would only buy either OEM or a decent all alloy radiator.

Yes, I have seen that full alloy radiators are available at a reasonable price.

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They are available for GEMS V8 too, there's one on my car......

55mm High Flow Radiator - Range Rover P38 4.0 / 4.6 V8 94-99 | Gravity Performance

55mm High Flow Radiator - Range Rover P38 4.0 / 4.6 V8 94-99 - shop online today for better performance with interest free finance and 5* customer service.

www.gravityperformance.co.uk

 

I have a Gravity radiator in my Gems fits well and works fine.

 

and now for Thor too High Flow Radiator - Range Rover P38 4.0 / 4.6 V8 98-02

 

They got the diesel one too

55mm High Flow Radiator - Range Rover P38 2.5 TD 94-99 | Gravity Performance

55mm High Flow Radiator - Range Rover P38 2.5 TD 94-99 - shop online today for better performance with interest free finance and 5* customer service.

www.gravityperformance.co.uk

but with oil cooler.

 

Dave, I thought that 300TDI VC is only for diesels

 

Good point Kapila. It definitely fits the diesel, that I know. Not sure if the central viscous clutch mechanism is the same petrol/diesel. I assumed it was.

 

Is the fan clutch on the 1997 GEMS have reverse or normal thread?

Just saw a YouTube video where the Thor has normal thread, counterclockwise to remove.

Another question: When putting the cabin temp to LOW and turning the AC on (light off), the electric fans do not turn on - when I ground them they work - any idea how to diagnose the issue?

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View attachment 310640 View attachment 310641

 

I think you will find they depend on both engine temp and AC pressure.

They have three modes i.e. Off / On Slow (fan in series / On Fast (fans in parallel)

 

The radiator was hot all over the entire surface.

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New fan clutch going in now.

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The fans are controlled by the AC pressure switch in the high side pipework. Normal operating pressure of R134a at 20C ambient, is 2.6 bar (38psi) on the low side and 10.3 bar (150 psi) on the high side. Pressure on the high side will increase above this if the ambient is higher or if it is having to work hard. The pressure switch operates when the pressure reaches 21 bar (305 psi), brings in relays 13 and 14 so the fans run at high speed in parallel. The switch remains operated until the pressure drops to below 17 bar (247 psi).

The fans should run in series, so at low speed, whenever the AC Control relay (relay 18) is operated by the engine ECU when it supplies the AC grant to the HEVAC.

If they have never come on, not even at slow speed when the AC is operating, the most likely cause is one of the fans being faulty, either seized or more likely open circuit.

 

UPDATE: Fan clutch removed and replaced with a new one, still overheats.

The radiator was removed and has very good flow.

Another thing, the temperature is erratic, sometimes it overheats, other times it doesn't.

I am starting to think it may be a failed thermostat, which is also new.

Any recommendations on by passing it as a test?

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Our Thor Rovers have a way overkill cooling system for normal driving. When you feel you reach 190 degrees does the top hose get a solid feel when thermo opens. If it is solid before mabey head gaskets leaking air into system but that would be instant at start. You could of got a bad thermo or water pump but lets start with basics.

 

Hello,

No, not solid at start, and the temperature drops sometimes. It is intermittent.

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Either the new thermostat is faulty (what make?) or you still have air in there.

 

The make was a no-name brand, white plastic casing. What brand/supplier do you recommend?

 

Just take the thermostat out & test it in boiling water. Ideally use a thermometer probe so you can see what temp it opens up at.

But +1 on you've still got airlock(s) somewhere, especially if the temp varies quite a lot.

 

Fitting no name parts in something as critical as the cooling system is asking for trouble. For something that could potentially destroy your engine, at the price I'd go for genuine Thermostat - Complete - V8 | Range Rover P38

But with it varying, it does sound like you have air in there. If you squeeze the top hose, can you hear gurgling in the cooling system? If you can, that's air.

 

Fitting no name parts in something as critical as the cooling system is asking for trouble. For something that could potentially destroy your engine, at the price I'd go for genuine Thermostat - Complete - V8 | Range Rover P38

But with it varying, it does sound like you have air in there. If you squeeze the top hose, can you hear gurgling in the cooling system? If you can, that's air.

From the link you gave me, only Britpart and Bearmatch are available. Are they both that bad?

How can I have the thermostat bypassed?

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I always fill the top hose first with antifreeze then when it gets almost to the top I finish filling with distilled water.
Then fill radiator with a funnel. Connect hose you may get a little water spillage. Then straight anti freeze into the coolant tank until that is to the mark. Never had to burp or anything, heater works right away. Adjust your ratio to get 50/50 mix.