DHallworth wrote:Lexia showed both of the other sensors at 92 degrees in both the Injection and Sensors screen within the parameter measurements, which is far more like what I would have expected to see.
Hi David,
Don't be fooled by the Lexia screen - the ECU (and therefore Lexia) only reads one of the three coolant temperature sensors - the green one at the front corner of the engine under the plastic cover next to the air filter box. The two different readings are "actual" and "substituted" values for the same sensor. (R and S)
Normally these two values will be the same, however if there is a sensor fault such as an open or short the actual value will show the "true" reading from the sensor (a crazy high or low value due to the fault) but the substituted value will be a "nominal" value that gets substituted when the ECU realises there is a fault.
The same applies to the air intake temperature sensor which has two readings but in reality there is only one air temperature sensor. (Yes I've looked for the "other" one just to be sure
) In the case of the air temperature sensor if you unplug it the actual reading will be about -40 degrees C but the ECU realises this is a fault and substitutes +39 degrees C as a "guess" to let the engine run after a fashion. So actual is the reading that is coming from the sensor, substituted is the value that is actually used by the ECU for its fuel mixture calculations.
Regarding the other two sensors, one runs the fans and the other the dashboard gauge. On my new V6 the temperature gauge reads fairly consistently - and reads about 10-15 degrees lower than the Lexia reading typically. As others have pointed out here and elsewhere the two sensors read in very different parts of the cooling circuit so I think a reading 10 degrees or so lower on the dashboard is quite normal - typically I see about 80 or slightly less on the dashboard.
On my first V6 the dashboard temperature gauge was very quirky, like yours. For quite a while the reading would suddenly drop by 10-15 degrees in the middle of driving on the motorway then come back again, sometimes this would only last 10-30 seconds. I distinctly remember one night where I was on the motorway going up a steep incline and every time I accelerated the reading dropped 15 degrees then the moment I let my foot off the throttle the reading came back up again.
Although I never did track that problem down, IMO there could only really have been one of a couple of things causing that, either:
a) A corroded connection on the plug to the sensor or a bad wire, such that when the engine flexes on its mount with the application and removal of torque the corroded wire moves in the plug slightly causing a resistance change. This is the most likely I think, especially when the sensor for the dash is around the back of the block on the gearbox side and subject to any coolant leaks from the coolant expansion chamber...
b) The dashboard gauge sensor, which is on the back of the block may have a small pocket of air trapped near by and changes in engine RPM or inclination of the engine might move the air bubble around. I noticed the night that I had this issue that it was particularly bad when I was climbing a hill then went away when I was on the level again. If the sensor tip sits in a pocket of air not actually immersed in the water the reading will drop dramatically...
I would try cleaning the connector and/or swapping the sensor over, but also go through a proper bleeding process with a header tank to make sure there are no air pockets...
