Rover SD1 Forum Snippet #22 - What Temperature is Best for Rover V8

Question 1: What temperature should an auxilliary electric fan be set at to assist my standard Rover SD1 Efi engine cooling system?

Question 2: What is the best running temperature for my Rover SD1 engine in the UK climate?

Regarding the first question, the best setting for an auxilliary electric cooling fan would be about 5 deg C. above engine running temperature so that the engine Thermostat and all other cooling functions, Radiator, Viscous, Ram Air, etc, can be allowed to operate normally.

As for the second question, one must bear in mind a correctly working cooling system has more than sufficient capacity to over-cool an engine and, by fitting the correct thermostat, the temperature will be generally controlled at or just above the specification of the thermostat.

I'm always puzzled why some Efi RV8 owners choose a low temperature thermostat, hoping perhaps, or even wanting the engine to run cooler than (say) the optimum 90 to 95 deg C. operating temperature, a figure supported by several factory documentation references.

First: The Rover SD1 Vitesse workshop manual specifies an 88 deg C. thermostat, so, given that there will be a small hysteresis as the unit opens plus the thermal delay between the thermostat function and the coolant circulating away from the radiator through water pump/engine block, it's clear the intended Vitesse operating temperature will be a bit higher, in the range 90 to 95 deg C.

In cold ambient conditions and/or at higher road speeds the natural radiator cooling capacity will try to overcool and when this happens the thermostat responds, by closing a little, to restrict coolant flow through the radiator again. Thus even in adverse cold conditions an 88 deg C. thermostat will help maintain engine temperature at the system design level, but as mentioned, choosing a lower thermostat (say 82 deg C.) would be disadvantageous allowing the coolant temperature to drop to around 82 deg C. before closing. Not much help for a system designed to run at or above 90 deg C. as we shall see.

Second: Most Rover SD1 Efi owners are familiar with the coolant temperature sensor resistance profile (click the chart to enlarge the image) which indicates the sensor will signal 300-400 ohms at 80 deg C. and 150-200 ohms at 100 deg C. In line with this data, the Rover SD1 Efi technician training manual recommends a "get you home" fix substituting a faulty coolant temperature sensor with a 200 ohm resistor to allow the engine to run properly. Such a resistor simulates a temperature in the range of 90 to 100 deg C and sets the ECU fueling map accordingly.

Third: The specified Rover SD1 Efi viscous fan unit with 11 blade fan ('Tempatrol 185E' - ERC8007A ), according to the manufacturers profile, starts to engage at approximately 85 deg C. and will become fully engaged about 10 - 15 deg C. higher than that. This means that its control range is 85 to 100 deg C. For comparison, the control range for the viscous fan unit with a 7 blade fan (Tempatrol 180E' ERC8660) fitted to an RV8 carburetter car is typically a few degrees lower.

My conclusion: This collection of information is sufficiently compelling to indicate the intended operating temperature for the late Rover V8 Efi engines being firmly in the range 90 to 95 deg C. and choosing to run at a lower temperature compromises engine performance as follows:

* Poor fuel economy due to continuous over-rich mixture.

* Increased engine wear because the persistent rich mixture washes lubricant from the bores.

* Excess hydrocarbons due to inefficient combustion.

On the other side of this argument, people point out, in extremely hot weather or in hot climates there is a mythical option to fit a so called "Summer" thermostat at 82 deg C. giving a safety margin to allow the engine the opportunity of NOT overheating in adverse (hot) conditions/climates and thereby mitigate the chances of overheating in case of cooling system malfunction.

In actuality, choosing a lower temperature thermostat does not necessarily restrict higher engine operating temperature, it does however open at a lower temperature so in reality, the engine simply takes longer to warm up as the radiator cooling effect is triggered sooner in the heating cycle. But that's not the whole story! To be properly effective, It's more likely that if the lower thermostat could be coupled with a lower temperature viscous fan unit then the engine operating temperature will be reliably lower in all climatic and adverse driving conditions.

However, for the Efi engine there is no specified low temperature viscous fan unit with correct front fixing nut, so whilst fitting a lower temperature thermostat may seem to satisfy a desire to run a cooler engine, all the above mentioned disadvantages still apply, plus this new one:

* Unstable operating temperature due to mismatched thermostat and viscous fan unit.

This brings us to carburetter cars, where the fuelling system and its greater flexibility is much more suited to lower engine operating temperatures, if the owner desires. Indeed it's not uncommon to see the thermostat specified at 82 deg C which would give an engine operating temperature a few degreers higher, at say 85 to 90 degrees C. Similarly, as already referenced, examination of the specification for the Carburetter RV8 viscous fan units indicates it engages at a slightly lower temperature than the specified Efi RV8 unit.

Examining the differences between the Efi issues encountered when running at too low a temperature, particularly the resulting rich mixture due to the action of the coolant temperature sensor, such issues may not occur with Carburettor RV8's. Mixture control can be adjusted at the carburettor itself, using appropriate needles and jets, so no problems need occur with a rich mixture, lubricating oil being washed from the bores nor control over emissions. Its all down to proper carburettor adjustment independent of coolant temperature.

Those advantages do not readily exist wiith the fixed fuel mapping of the standard RV8 Efi system. Sure, the system can be fettled by adjusting certain components such as the air flow meter, the fuel pressure regulator and ECU, but this is generally outside the scope of the average DIY home enthusiast and certainly not included in the maintenance schedules for the Efi system.

In summary: The simple answers to the second question are as follows:

* For the Efi RV8 engine the best operating temperature is in the range 90 to 95 deg C.

* For the Carburetter RV8 engine the best operating temperature is in the range 85 to 90 deg C.

-- as borne out by generally available documentation and reviewing the original fitted components.

Please contact me regarding Errors and Omissions.

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Ramon Alban


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