Oil Temp Sensor on Acura Legend and Sludge.

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May 13, 2003
Toronto, Canada
In my 1989 Acura Legend V6, there is an Oil Temperature Sensor on the rear bank of the V6. It is located on the top of the rear valve cover that is close to the firewall. Only North American versions of the engine have this sensor and Japanese market versions of the engine do not have this. The Japanese version of the engine is slightly different with different ignition timing, higher compression and no EGR. Otherwise they are pretty much identical. The other odd thing about this sensor is it is always called the Oil Temperature sensor but is never immersed in the path of the oil but depends on the spray or ambient temperature inside the valve train. This sensor activates the AC condenser fan to go into low speed when the temperature of the sensor exceeds 221 degrees after the engine is shut off. Initially I could not figure out what the reasoning behind this sensor was all about. It was always shown in the PGM-FI ECU diagrams but did not have any inputs to alter PGM-FI. But I have come to some "guesses". First point of interest is that these motors are prone to sludge building up in the lifters and then becoming noisy. This has pretty much become a normal aging process for these motors unless they have seen meticulous oil maintenance. Despite this, I have noted that there is a slight film of sludge that eventually builds up on the valve cover of these engines despite the oil maintenance of 3k/3mos with dino. My guess is that Honda is aware of the remaining heat that can cause oil to break down after the engine is shut off. After all why locate an oil temp sensor where there is no oil to speak of? If they truly wanted a true oil temperature they would have mounted it immersed in oil somewhere near or after the standard oil cooler that is cooled by the coolant system. It definitely addresses a heat related oil issue after the engine is shut off. This latent heat after the engine is shut off is sufficient to start oil breakdown. Maybe this is what Toyota neglected in their application of the 1MZF3 V6 in the North American Market. Maybe it is this and not the shearing that causes sludge on the Toyota V6? What do you think? Am I onto something?
Would turning on the radiator fan cool the valve banks, since coolant doesn't circulate much once the engine is shut off?
The purpose of the set up is to reduce underhood temps after shutdown. VW was using this setup in some of its cars. My scirocco has an sensor installed in the head that just measures the head temps but has no wiring. Other VW had the sensor with lower temp rating installed on top of the valve cover exposed to air. Unfotunately the sensor would get stuck and run the fans untill the battery is flat. Reducing the underhood temps helped eliminating heatsoaking the injector lines which caused warm start problems. VWs VR6 engines would use an auxiliary electric pump to circulate coolant too after shutdown. If you need to eliminate oilk coking , get a colder fan switch.
That's a good suggestion but Japanese engines would have this heatsoaking problem too. Their engines don't have it. I got my feet wet with higher head temps due to emission controls with VW water cooled engines in the 70s. Result was that I had to replace valve seals after three years, like clockwork.
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