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Coolant Temperatures

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Site last updated on 15/08/08

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   A lot of people ask what temperature the coolant ought to be in their conversion, especially when they first get it up and running, and tend to be a little paranoid of airlocks and overheating. Obviously it is a good thing to be a little over cautious when you first fire up your conversion, but a lot of the worry is probably caused by on line misinformation. The real cause if this is likely to be the fact that the VW gauge is not directly compatible with the Subaru sensor, which can result in an over high gauge reading when the coolant is at normal running temperature(see below).
   The following figures give some accurate info on what you can expect if your T25 conversion is cooling properly:

    ‘Low’ ambient:
       In a 15 degree ambient temperature, the RJES Syncro runs a coolant temperature of around 84 degrees C. That’s measured with a datalogger measuring direct sensor info from the ECU.

    High ambient:
       Thanks to Andy in Australia some data on coolant temperature in high ambient temps. On a 500 mile trip, in around a 39 degree ambient, his coolant ran at 90-91 degrees. He has also driven in up to 47 degrees with no problems. His coolant circuit is slightly different, but essentially the same. Not sure how he measured it - probably a k - type thermocouple.


VW Temperature Gauge Reading Correction:

   It is very likely that a lot of the misinformation about Subaru engines running hot in T25’s is due to fact that that can appear to be what the VW temperature gauge is telling you if your conversion has not been done properly. Many people just connect the wire from the Subaru sensor up to the with to the VW temperature gauge when doing a conversion. However, the two sensors are not directly compatible. The result of this is a gauge which reads very near to the top of the scale when the engine is at normal operating temperature. To people who don’t understand the system, especially if they did not install the conversion themselves, this can (not surprisingly) look just like the engine is running too hot, when it is not.
   The ideal fix would be to swap the Subaru sensor for the VW one. The two look very similar. However, the two are not interchangeable, as the treads are different. See the bottom of the Reversed Coolant Manifolds page for more info on the threads. A much neater fix is to alter the signal from the sensor so it moves the gauge needle back to the centre of the gauge when at normal running temperature. This is very easily achieved by adding a resistor in the wire between the gauge and sensor. A 27 ohm resistor is about right, although you could use a variable resistor to allow you to adjust the ‘normal’ reading to wherever you are used to seeing it with a VW engine. Whatever resistor you use, it should be a low wattage type. 1 watt will be fine. Note - electronically this is a very crude fix, as it alters the linearity of the gauge as well as the centre point. However, as the gauge has no degree readings, and is just for indication, this does not matter too much.
   If you want to correct the reading of your VW temperature gauge without using a resistor and having the side effect of altering the linearity of the gauge, you will need to fit a VDO coolant temperature sensor. There are many types available, and they all have the same output. Any intended for the aftermarket range of ‘Cockpit’ gauges will do, and suitable sensors were also fitted to many 1980’s cars with VDO instruments too, such as VW, Audi, Porsche, Vauxhall / Opel, etc. Be aware of the thread compatibility issue mentioned above. The sensor should go in the coolant manifold in top of the engine, after the flows from each half of the engine have joined up, but before where the coolant splits off to heat the thermostat (via the heater matrix in most cases, including the RJES coolant circuit. Fitting it anywhere else will either give a temperature reading of one half of the engine, or only when the thermostat is open (if the sensor were in a radiator hose, etc), for example.


    Thinking of not fitting a thermostat to your Subaru conversion?

         It seems to have become a bit of a trend to install Subaru engines here in the UK with the thermostat removed. There is no logical reason for this, other than bad engineering. If the thermostat were not needed, Subaru (and all other manufacturers) would have cost saved them out of their vehicles many years ago.
         The logic behind the misinformation about running no thermostat is unclear, but it is likely to be to do with removing the problem of how to ensure adequate coolant flow to open the thermostat regardless of heater settings in a T25. This is caused by the in compatibility of the               VW heater with the Subaru coolant system (see Very Important). Removing the thermostat will solve this problem, but in the crudest way possible, and at the expense of lower fuel consumption and accelerated engine wear.
         A couple of people have contacted is having run their T25’s through summer with no problems. In winter they have found no thermostats fitted when investigating poor heater performance. One in particular fitted a thermostat purely to get the heater working properly, and very quickly noticed a welcome improvement in fuel consumption too, despite not looking for or expecting it. He also fitted one of our reversed coolant manifolds at the same time, and filled the system using our filling procedure. He was very pleased with the results.
         If you have a conversion which is currently running with no thermostat, fitting one is highly recommended. Make sure you have adequate flow of hot coolant to it though, otherwise it will never open, and your engine will overheat very quickly. This means installing a bypass between the heater feed and return pipes. See Coolant Circuit Design for more details.

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